Evolution of the Payback Period for Energy-Efﬁcient Residential Buildings in Romania in the Last Decade

: The European Union set ambitious targets to achieve climate neutrality by 2050, and one of the measures taken towards this goal was the implementation of nearly Zero-Energy Buildings. Despite the commitments of the EU member states, many householders and investors had a disposition to incredulity regarding the energy efﬁciency of the buildings due to the higher cost of the investment and the relatively long payback time. However, at the end of 2021, the energy crisis signiﬁcantly rewrote the circumstances, and energy prices and the costs of construction materials began to rise. In this situation, it was necessary to reconsider the importance of energy efﬁciency for buildings due to the maintenance costs. This article aims to assess changes in payback periods over the past 13 years and conduct life cycle cost evaluations by comparing energy-efﬁcient residential buildings with traditional houses. The analysis considers variations in construction materials and labor costs in Romania, as well as energy price changes during the building’s operational phase. Through these methodologies, it has been demonstrated that the implementation of energy-efﬁcient buildings offers a cost-effective solution already in the medium term, providing incentives for investors and future houseowners to reduce their dependence on energy and pursue long-term decarbonization.


Introduction
The European Union (EU) aims to become a leader in the decarbonization process by gradually reducing greenhouse gas emissions, making the continent, along with other European countries, the first carbon-neutral region in the world by 2050 [1]. However, this strategic goal must not affect the competitiveness and economic growth of the member countries [2], so initiatives were needed that financially support the next generation of renewable and innovative technologies and promote the decrease in energy demand in such a way that the consumers' sense of comfort remains at the same level. Ambitious targets have met expectations until recently: between 1990 and 2020, the European Union's GDP grew by 54%, while greenhouse gas (GHG) emissions fell by 34% [3]. Given the EU's challenge to ensure greater sustainability and energy security for households and businesses, the European Commission published the Energy Union strategy [4], with a strong focus on energy efficiency.
Since the building sector accounts for 43% of the final energy consumption of the European Union, and nearly two thirds of this can be attributed to residential buildings [5], the EU has developed a comprehensive regulatory framework based on energy efficiency measures targeting buildings [6]. Between 2000 and 2014, significant reductions in energy consumption were observed as a result of these directives and laws. However, the expected continuous decline in energy demand did not materialize in all member states during the period of 2014-2019. In fact, in some cases, there was even a reversal in energy consumption in households. Romania is among the member states where this trend occurred. The average energy consumption per residence in a normal climate in the EU was 1.30 toe/dwelling in 2019. Even though Romania still has below-average consumption in the building sector, with 1.09 toe/dwelling, the energy demand for buildings has increased by almost 5% since 2014, when the value was 1.04 toe/dwelling [7]. Low energy prices played an essential role in the reversal of the trend. The continuous change in occupants' behavior, the increase in comfort, and the expansion in the built surface of buildings due to economic prosperity also influenced the slowdown in energy efficiency seen at the EU level. Last but not least, those investors or customers who built houses amid adequate economic growth between 2011 and 2019 did not pay enough attention to energy efficiency, as such investments entailed additional costs [8]. The neglect of energy efficiency is even more noticeable in the countries of the former Soviet Bloc since, in this period, the economic catch-up with Western European countries received more attention than sustainability and decarbonization [9]. However, the energy crisis that unfolded at the end of 2021 and the subsequent political processes overrode the calm and optimal conditions [10]. The price of construction materials began to increase drastically, and the supply of materials at a given moment became unpredictable. As a result of the increase in energy prices [11], EU member states had to intervene in the free market to partially regularize prices and somewhat take over the costs of consuming society. As a result, many homeowners began to reduce consumption for financial reasons. They reduced their previous needs for comfort, which developed society's EU citizens experienced negatively. On the other hand, those property owners who committed themselves to the energy efficiency of their buildings before 2021 did not feel the rise in energy prices at such a level in their use of the building.
Considering these facts, the importance of nearly Zero-Energy Buildings (nZEBs), whose implementation has been required by Romanian laws since 2016 [12], has come to the forefront more than ever. Although the state continuously promoted and financed the installation of renewable energies and the construction of nZEBs, the Romanian construction industry did not receive adequate education and was not fully ready for their application [13,14]. To ensure the appropriate level of trustworthiness for this type of building, the customer must be motivated not only by energy efficiency and low maintenance costs but also by the payback period for the additional investment [15]. Therefore, the payback time of the additional investment compared to traditional buildings is an essential aspect. For those who look at the construction and maintenance of the building as a long-term investment, the return on the invested capital is already outlined in the planning phase, providing additional certainty to the doubting customer.
In the first step, this paper studies the evolution of the construction cost per square meter in the case of residential buildings over the past 13 years, during which the implementation costs of both traditional buildings and nZEBs per unit built-in area are determined, thereby highlighting the additional cost of energy-efficient residential buildings. Knowing the additional costs, in the second phase, the payback period of residential nZEBs built in different periods is determined, during which it is also revealed what influence the past year's events had on the importance of energy efficiency.

Nearly Zero-Energy Buildings
nZEBs are constructions with a very high energy performance, where the energy required to ensure the demand is almost equal to zero or very low, and renewable sources cover at least 30% of the energy needed and is produced on-site or nearby within a radius of 30 km [16]. The required energy efficiency can be achieved by taking into account the following principles: suitable thermal characteristics of building elements, a thermal bridge-free building envelope, compact design and optimal orientation of the building, high-performance windows and doors, good airtightness, mechanical ventilation with heat recovery, energy-saving equipment, energy-efficient heating and air-conditioning systems, the use of renewable energy, built-in lighting, passive solar systems, shading, indoor climatic conditions, internal heat loads [17], and quality control in the implementation phase. According to Law no. 372/2005, all new constructions for which reception on completion of works is carried out under the building permit issued from 31 December 2020, onwards will be nZEBs [12]. As a result, the requirements for new buildings have been in effect since 2021. Furthermore, the EU has proposed to move from the current nearly zero-energy facilities to zero-emission buildings by 2030 [18].
The annual primary energy consumption and the CO 2 emissions from use determine the energy performance of a building. The EU member states establish the minimum requirements for these energy performance indicators, considering the building's purpose, life span, geographical location, and indoor and outdoor climatic conditions. According to the Romanian Technical Regulation Mc-001/2022, the maximum allowed limit values of total primary energy consumption (from renewable and non-renewable sources) and CO 2 equivalent emissions for new individual residential nZEBs are 120.1-147.9 kWh/m 2 per year and 14.7-19.9 kgCO 2 /m 2 per year, depending on climatic zones [19]. In Romania, buildings' annual primary energy consumption per square meter of floor area is approximately 250 kWh/m 2 year [20], 25% higher than the EU average. This difference is basically due to the predominance of buildings with low energy efficiency built before 1989, which account for around 82% of the real estate stock [21]. Therefore, the EU and the government provide a lot of financial support to encourage owners to increase the energy efficiency of existing buildings.

The Construction Cost of the Residential Buildings
Rising wages and fluctuating construction material prices primarily impact the construction costs of residential buildings. Additionally, nZEBs incorporate a third component consisting of renewable energy sources and innovative energy-efficient technologies. To determine the evaluation of the construction cost per square meter of residential buildings, it is necessary to analyze the development of those three influencing factors over the past 13 years. In this way, even skeptical customers and real estate investors who find it difficult to understand why it is impossible to carry out a suitable construction with an average cost of 400 euros per square meter, which was applied in the 2010s, can obtain an accurate picture of the present situation.

Labor Cost Evolution
The importance of the labor force in construction is indispensable, even in a situation where a significant change regarding the digitization and automation of the industry is expected. The number of employees in construction and their level of qualification directly influence the development and quality of work in this sector. Construction activities contribute to the formation and growth of a country's Gross Domestic Product (GDP), and their optimal, balanced operation can be achieved in conditions where the living quality of employees and employers is reasonable. From this perspective, stimulating the workforce with a decent salary is essential in this field. Neglecting it can initiate processes with a long-term negative impact, such as a labor shortage and a decrease in skills to complete the work on time and at the appropriate level.
The countries of the former Eastern Bloc, including Romania, had a significant disadvantage in terms of the degree of development and the standard of living compared to the countries in Western Europe. Over the years, despite the efforts made by the state, the tendency to raise wages at the level of developed countries has met political and economic obstacles [22]. The evolution of average salaries in the economy and construction industry between 2010 and 2023 Q1, according to the Romanian National Institute of Statistics data [23], is represented in Figure 1. The recession caused by the 2008 economic crisis ended in September 2010, when the Romanian economy noted consecutive increases in GDP in the fourth quarter of 2010 and the first quarter of 2011. The positive effect of the economic recovery did not appear immediately in the construction market because financing and implementation of the projects required a long process. On the other hand, the investors from the private sector were cautious, which is why the average net monthly salary in this field increased by only 4.40% between 2010 and 2014, from 267 EUR to 279 EUR. Additionally, during this period, the average net salary in the economy increased by 15.56%, 3.5 times higher compared to the construction sector. In the following four years, there were decent increases in the specialized field. The annual salary increase was over 12%, reaching an average net income of 413 EUR, which represented an increase of 54% compared to 2010. However, from the 2010s to 2018, a gradual increase in the gap was noted between the average incomes in the economy and the incomes from construction, and the difference in 2010, which was 23.6%, reached 37.3% by 2018.
This trend manifested itself through the emigration of qualified labor to developed countries for more attractive remuneration and through professional reprofiling. Following the entry into force of the Romanian Government's Emergency Ordinance no. 114/2018, which provides for reductions and exemptions from labor taxes in the construction industry, respectively, a minimum gross salary of 645 EUR (3000 RON) was imposed [24]. The positive effect of the act regarding the sharp increase in incomes in the coming years can be seen. However, even this monetary stimulus failed to reverse the rise in the labor shortage in the construction sector.
Considering the evolution of salary income in the last four years, an increase of approx. 96% in construction reached an average net salary of 809 EUR in the Q1 quarter of 2023, slightly below the net average for the economy of 860 EUR [23]. As a result, an increase in salary income of 202.7% net (204.6% gross, due to the reductions and exemptions of labor taxes) can be observed in the construction sector since 2010. Following the entry into force of the Romanian Government's Emergency Ordinance number 168/2022 [25], which changes the minimum gross basic salary to 814 EUR monthly (4000 RON), the working conditions for this specialist field will be raised from a financial point of view.  The recession caused by the 2008 economic crisis ended in September 2010, when the Romanian economy noted consecutive increases in GDP in the fourth quarter of 2010 and the first quarter of 2011. The positive effect of the economic recovery did not appear immediately in the construction market because financing and implementation of the projects required a long process. On the other hand, the investors from the private sector were cautious, which is why the average net monthly salary in this field increased by only 4.40% between 2010 and 2014, from 267 EUR to 279 EUR. Additionally, during this period, the average net salary in the economy increased by 15.56%, 3.5 times higher compared to the construction sector. In the following four years, there were decent increases in the specialized field. The annual salary increase was over 12%, reaching an average net income of 413 EUR, which represented an increase of 54% compared to 2010. However, from the 2010s to 2018, a gradual increase in the gap was noted between the average incomes in the economy and the incomes from construction, and the difference in 2010, which was 23.6%, reached 37.3% by 2018.
This trend manifested itself through the emigration of qualified labor to developed countries for more attractive remuneration and through professional reprofiling. Following the entry into force of the Romanian Government's Emergency Ordinance no. 114/2018, which provides for reductions and exemptions from labor taxes in the construction industry, respectively, a minimum gross salary of 645 EUR (3000 RON) was imposed [24]. The positive effect of the act regarding the sharp increase in incomes in the coming years can be seen. However, even this monetary stimulus failed to reverse the rise in the labor shortage in the construction sector.
Considering the evolution of salary income in the last four years, an increase of approx. 96% in construction reached an average net salary of 809 EUR in the Q1 quarter of 2023, slightly below the net average for the economy of 860 EUR [23]. As a result, an increase in salary income of 202.7% net (204.6% gross, due to the reductions and exemptions of labor taxes) can be observed in the construction sector since 2010. Following the entry into force of the Romanian Government's Emergency Ordinance number 168/2022 [25], which changes the minimum gross basic salary to 814 EUR monthly (4000 RON), the working conditions for this specialist field will be raised from a financial point of view.

Building Material Prices
Another important aspect that reflects the evolution of the investment cost for a project is the price of construction materials. The dynamics of the market are primarily guided by the forces of supply and demand, and this principle applies to building materials as well. However, there are external factors beyond the control of both distributors and customers. These factors include the availability and quality of raw materials as well as the production cost of the final product, which is notably influenced by labor and energy costs. To have a comprehensive picture of the changes in the price of construction materials over the past decade, some primary construction products were analyzed, which are essential parts of a residential building [26], namely: concrete, reinforcement steel, ceramic bricks, autoclaved aerated concrete (AAC) blocks, cement, mortar, cement-based adhesive, plasterboard, gypsum plaster, timber and wooden beams, oriented strand board (OSB), EPS polystyrene thermal insulation, and PVC doors and windows.
The construction process has not changed significantly in the last ten years. Only the technical regulations, such as Law no. 372/2005 and Order no. 2641/2017, have become stricter, so it can be concluded that the change in material expenses is directly proportional to the difference in the costs of the building [12,27]. The changes in the prices of the mentioned materials and final products can be found in Figure 2. Their annual percentage differences in various types of materials or products will be included in determining the construction costs of the buildings.
Summing up the changes in material prices, it can be concluded that the average cost of products has increased by 101% over the past 13 years, which can be broken down into three stages. Between 2010 and 2018, in 8 years, an increase of nearly 15.2% was noticeable, representing an average annual growth of 1.9%. The apprehension after the economic crisis was still felt in the first half of this period. The desire to invest in the construction industry only started to return to normal after 2014. In the case of some construction products, a moderate price decrease can also be noticed between 2014 and 2017. Between 2018 and 2020, when the world economy performed excellently, prices increased by 9.1%, which means 4.5% annually. At that time, it was already felt that the position of the investors was becoming worse and worse in the market and that it was difficult for them to keep pace with the prices due to the high demand. Because of the intermittent supply, the lack of unpredictable long-term investments, and partly as a result of economic speculation, in 2020-2022, construction material costs increased by 55.3%, which is an annual 27.6% increase in price.

Building Material Prices
Another important aspect that reflects the evolution of the investment cost for a project is the price of construction materials. The dynamics of the market are primarily guided by the forces of supply and demand, and this principle applies to building materials as well. However, there are external factors beyond the control of both distributors and customers. These factors include the availability and quality of raw materials as well as the production cost of the final product, which is notably influenced by labor and energy costs. To have a comprehensive picture of the changes in the price of construction materials over the past decade, some primary construction products were analyzed, which are essential parts of a residential building [26], namely: concrete, reinforcement steel, ceramic bricks, autoclaved aerated concrete (AAC) blocks, cement, mortar, cement-based adhesive, plasterboard, gypsum plaster, timber and wooden beams, oriented strand board (OSB), EPS polystyrene thermal insulation, and PVC doors and windows.
The construction process has not changed significantly in the last ten years. Only the technical regulations, such as Law no. 372/2005 and Order no. 2641/2017, have become stricter, so it can be concluded that the change in material expenses is directly proportional to the difference in the costs of the building [12,27]. The changes in the prices of the mentioned materials and final products can be found in Figure 2. Their annual percentage differences in various types of materials or products will be included in determining the construction costs of the buildings. Summing up the changes in material prices, it can be concluded that the average cost of products has increased by 101% over the past 13 years, which can be broken down into three stages. Between 2010 and 2018, in 8 years, an increase of nearly 15.2% was noticeable, representing an average annual growth of 1.9%. The apprehension after the economic crisis was still felt in the first half of this period. The desire to invest in the construction industry only started to return to normal after 2014. In the case of some construction products, a moderate price decrease can also be noticed between 2014 and 2017. Between 2018 and 2020, when the world economy performed excellently, prices increased by 9.1%, which means 4.5% annually. At that time, it was already felt that the position of the investors was becoming worse and worse in the market and that it was difficult for them to keep pace with the prices due to the high demand. Because of the intermittent supply, the lack of unpredictable long-term investments, and partly as a result of economic

Costs for Energy Efficiency Features
The additional cost of energy-efficient features in buildings to ensure that the building's total primary energy demand meets the minimum standards of nZEBs mainly consists of three components: (a) the development of a higher thermal resistance for the building envelope and adequate airtightness in order to reduce the heat loss; (b) the installation of high-performance heating, ventilation, and air conditioning systems to reduce the specific energy consumption; and (c) the use of renewable energy sources to ensure that these provide at least 30% of the building's energy demands.
Providing a high thermal resistance factor for the building envelope depends primarily on the thicker thermal insulation of the building structures and using materials with good thermal conductivity. However, minimizing heat bridges also reduces energy loss, as manifested by using additional thermal insulation at connection points with higher heat flux. Installing modern doors and windows with the lowest possible thermal transmittance value represents a higher-priced category of product that has grown continuously over the years. Regarding doors and windows, it is important to consider the minimization of heat bridges by correctly positioning them within the wall's thickness and using additional thermal insulation around the frame edges. Furthermore, ensuring the building's airtightness is essential. This can be achieved by correctly installing building materials, sealing gaps that may be penetrated by HVAC systems or electrical installations, and utilizing sealing tape around doors and windows.
The performance and efficiency of heating, ventilation, and air conditioning (HVAC) systems predominantly depend on the type of equipment used. However, optimization during design is also essential; for current standards, the dimensioning parameters offer the chance of extensive, oversized installation systems. This also results in a percentage difference in energy-efficient buildings where, after optimization, the investment cost is a few percent, up to 3%, lower than the initial cost. Exploiting highly efficient systems is also essential, which is why using heat pumps for heating and cooling is becoming more relevant. This heating system, in most cases, is based on geothermal energy and has fewer energy requirements than conventional gas-based heating equipment. The heat recovery system used for mechanical ventilation is a highly efficient solution where the outflow of thermal energy from the air is recovered. However, the use of these new technologies is generally more expensive than that of conventional systems.
In the case of residential buildings, renewable energy sources are mainly used by installing solar photovoltaic (PV) and concentrated solar power (CSP) systems and implementing installations using geothermal energy. However, there are cases where wind energy and hydropower infrastructure exist. If the production point falls within a 30 km radius of the building, renewable energy sources are considered to be provided for nZEBs.
In recent years, the technology associated with these energy sources has been consistently advancing. As a result, the production costs and the expected energy generation over their lifetime, as indicated by the Levelized Cost of Energy (LCOE) indicator, have become considerably more favorable compared to the majority of other sources [28,29]. The changes that have occurred in the last decade for renewable energy sources are shown in Figure 3.  The most significant advances have been made in the use of solar energy; in 2011, it cost 0.381 USD to produce one kWh of electricity, and by 2021, this cost had dropped to 0.048 USD/kWh, representing an approximate 87% drop in the case of solar photovoltaic [30]. The same trend is valid for concentrating solar power (CSP), where the 0.358 USD/kWh in 2011 decreased to the level of 0.114 USD/kWh in ten years, which means a reduction of 68%. Looking at its proportions, the trend is similar in the case of wind energy; an improvement of between 60 and 68% was observed, where the LCOE value of onshore wind turbines fell from 0.102 to 0.033 USD/kWh, and in the case of offshore wind turbines, it decreased from 0.188 to 0.075 USD/kWh in 2021. Over the past years, geothermal and hydropower LCOE indicators have not shown any profound changes. However, from the point of view of availability, they are still considered favorable, cost-effective investments, with values of 0.68 USD/kWh and 0.48 USD/kWh, respectively. Overall, an average decrease of 65% can be observed between 2011 and 2021 in the case of the aforementioned renewable energy sources. When calculating the additional cost of energy-efficient features of buildings, the average annual changes in renewable energy in the LCOE indicators are used, whose average yearly value varies between −23% and +6%.
The energy requirements of buildings have continued to improve in recent decades, which has been materialized by enhancing the performance of the building envelope and  The most significant advances have been made in the use of solar energy; in 2011, it cost 0.381 USD to produce one kWh of electricity, and by 2021, this cost had dropped to 0.048 USD/kWh, representing an approximate 87% drop in the case of solar photovoltaic [30]. The same trend is valid for concentrating solar power (CSP), where the 0.358 USD/kWh in 2011 decreased to the level of 0.114 USD/kWh in ten years, which means a reduction of 68%. Looking at its proportions, the trend is similar in the case of wind energy; an improvement of between 60 and 68% was observed, where the LCOE value of onshore wind turbines fell from 0.102 to 0.033 USD/kWh, and in the case of offshore wind turbines, it decreased from 0.188 to 0.075 USD/kWh in 2021. Over the past years, geothermal and hydropower LCOE indicators have not shown any profound changes. However, from the point of view of availability, they are still considered favorable, cost-effective investments, with values of 0.68 USD/kWh and 0.48 USD/kWh, respectively. Overall, an average decrease of 65% can be observed between 2011 and 2021 in the case of the aforementioned renewable energy sources. When calculating the additional cost of energy-efficient features of buildings, the average annual changes in renewable energy in the LCOE indicators are used, whose average yearly value varies between −23% and +6%.
The energy requirements of buildings have continued to improve in recent decades, which has been materialized by enhancing the performance of the building envelope and HVAC systems [31]. However, no single technology or category can determine the extra cost due to the various types of buildings in diverse climate zones with different requirements. Based on numerous case studies [32], the breakdown per cost category for energy-efficient buildings is between +1 and +11% for the building envelope and −3 and +7% for HVAC systems. Regarding renewable energy sources, the breakdown cost is between +2 and +22%. Based on the average breakdown per cost category for the nZEBs, a +20% investment cost has been determined for the 2010s, which consists of the following categories: +6% for the adequate envelope, +3% for the high-performance HVAC, and +11% for renewable energy systems. An annual change in this 20% extra investment cost has been determined between 2011 and 2023 Q1, taking into account the average yearly percentage modification of construction material prices and the change in the LCOE for renewable energy. The percentage evolutions are collected in Table 1. As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level.
Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years. 2012 18.2% As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level.
Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years. 2013 16.8%  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years. 2014 16.3%  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years. 2015 15.3%  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years. 2016 15.4%  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level.
Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the   As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years.   As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years.  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the   As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years.  As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593   As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years.   As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level. Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will As a result of the continuous change in these factors, the initial additional cost of +20% gradually decreased and approached +14.4% by 2020. However, the increase in energy prices in this case also ends the favorable trend, and by 2021, it had risen back to +16.6% and then continued to decrease by the first quarter of 2023, reaching 16%.

Investment Cost Evolution of nearly Zero-Energy Buildings
In order to determine the investment expenses, the cost change in three factors over time was defined: construction material prices, labor costs, and energy efficiency costs. The annual percentage changes in all three influencing factors were considered to calculate the following year's global cost of the building. As mentioned in the first paragraph of Chapter 3, the starting point is the average investment cost of the 2010s for the construction of a traditional residential building, which was 400 EUR/m 2 , and applying the 20% additional cost to increase energy efficiency to achieve the nZEB requirements, the investment cost was 480 EUR/m 2 . The diagrams in Figure 4 show why it is a severe challenge in the current circumstances to construct buildings at the prices of a decade ago without significant compromises. It can be concluded that the global cost per square meter of built-in area of nearly zero-energy buildings did not increase significantly between 2010 and 2017. Only a 23% growth was observed, which roughly corresponds to a cost of 593 EUR/m 2 . On the other hand, in the following six years, this price range reached a double investment cost, and by the Q1 quarter of 2023, it had approached a 1233 EUR/m 2 level.

Global cost per category
Total global cost

Comparison between Nominal and Real Global Costs
The calculations above represent the change in the nominal values of the global costs. To obtain a more accurate picture, it is necessary to consider the evolution of the inflation rate in the studied period, in which real wage growth and real investment cost change will be defined. These adjusted values show how much the real value of the service has increased or decreased compared to a specific time after the depreciation factor has been applied. The base year for regulating the inflation rate required for calculations is 2010. Starting from this period, considering the annual inflation reported by the National Institute of Statistics [23], a depreciation of 58.6% can be determined until the first quarter of 2023. Romania's inflation rate fluctuated widely around 3% in the last decade. Its lowest depreciation was in 2014-2017, with negative inflation for two consecutive years. However, in the previous two years, the inflation rate rose again to 15.3%, and based on forecasts, it will hover around 10% in the coming period as well. The percentage change in inflation over the past 13 years compared to the previous year can be found in Figure 5. Applying the depreciation adjustment to average net wages and construction costs, the increase shows a more moderate growth than nominal values. Moreover, in some cases, the conversion to real values resulted in a decrease. If the economic net average wages are considered, the absolute change resulted in a 160% increase between 2010 and 2023, during which the average salary jumped from 330 euros to 860 euros per month.  Two types of buildings will be analyzed to obtain a comprehensive picture of the global costs: nZEBs and traditional buildings. These theoretical constructions are built in four different periods, 2010, 2014, 2018, and 2023, for which, in addition to the implementation costs, it will be necessary to evaluate the running costs as well. Furthermore, the payback period for the energy efficiency of the buildings implemented in 2010-2023 will be determined and broken down into years.

Comparison between Nominal and Real Global Costs
The calculations above represent the change in the nominal values of the global costs. To obtain a more accurate picture, it is necessary to consider the evolution of the inflation rate in the studied period, in which real wage growth and real investment cost change will be defined. These adjusted values show how much the real value of the service has increased or decreased compared to a specific time after the depreciation factor has been applied. The base year for regulating the inflation rate required for calculations is 2010. Starting from this period, considering the annual inflation reported by the National Institute of Statistics [23], a depreciation of 58.6% can be determined until the first quarter of 2023. Romania's inflation rate fluctuated widely around 3% in the last decade. Its lowest depreciation was in 2014-2017, with negative inflation for two consecutive years. However, in the previous two years, the inflation rate rose again to 15.3%, and based on forecasts, it will hover around 10% in the coming period as well. The percentage change in inflation over the past 13 years compared to the previous year can be found in Figure 5.

Comparison between Nominal and Real Global Costs
The calculations above represent the change in the nominal values of the global costs. To obtain a more accurate picture, it is necessary to consider the evolution of the inflation rate in the studied period, in which real wage growth and real investment cost change will be defined. These adjusted values show how much the real value of the service has increased or decreased compared to a specific time after the depreciation factor has been applied. The base year for regulating the inflation rate required for calculations is 2010. Starting from this period, considering the annual inflation reported by the National Institute of Statistics [23], a depreciation of 58.6% can be determined until the first quarter of 2023. Romania's inflation rate fluctuated widely around 3% in the last decade. Its lowest depreciation was in 2014-2017, with negative inflation for two consecutive years. However, in the previous two years, the inflation rate rose again to 15.3%, and based on forecasts, it will hover around 10% in the coming period as well. The percentage change in inflation over the past 13 years compared to the previous year can be found in Figure 5. Applying the depreciation adjustment to average net wages and construction costs, the increase shows a more moderate growth than nominal values. Moreover, in some cases, the conversion to real values resulted in a decrease. If the economic net average wages are considered, the absolute change resulted in a 160% increase between 2010 and Applying the depreciation adjustment to average net wages and construction costs, the increase shows a more moderate growth than nominal values. Moreover, in some cases, the conversion to real values resulted in a decrease. If the economic net average wages are considered, the absolute change resulted in a 160% increase between 2010 and 2023, during which the average salary jumped from 330 euros to 860 euros per month. Despite this fact, the purchasing power of earnings only shows an increase of 64% in the last 13 years, which means a monthly real net salary of 542 EUR in retrospect to the 2010s. The same ratio applies to the construction costs of energy-efficient residential buildings. The 157% nominal increase converted to real investment costs means only a 62% growth, which means 778 euros per square meter of built-in surface real costs in 2023 with values from the 2010s. As seen in Figure 6, the real average incomes and the real construction costs of nZEBs have changed in direct proportion over the years. Despite this fact, the purchasing power of earnings only shows an increase of 64% in the last 13 years, which means a monthly real net salary of 542 EUR in retrospect to the 2010s. The same ratio applies to the construction costs of energy-efficient residential buildings. The 157% nominal increase converted to real investment costs means only a 62% growth, which means 778 euros per square meter of built-in surface real costs in 2023 with values from the 2010s. As seen in Figure 6, the real average incomes and the real construction costs of nZEBs have changed in direct proportion over the years. From Figure 6, it can be seen that, in terms of wages, in the years 2011, 2012, and 2023, the purchasing power decreased by an average of 3% even though there was an increase in nominal real wages. Similarly, in the case of construction costs, it can be noticed that the real price decreased by almost 7% in 2012, by 2% in 2013, and by 3% in 2023. In the second case, however, the reduction in real costs positively affected the realization of energy-efficient buildings. Comparing the evolution of real average wages and investment costs, when in 2010 it was possible to realize 0.69 m 2 for an energy-efficient building from one monthly salary, a continuous improvement can be observed in the period 2011-2017; thus, in 2017, it was already possible to realize 0.86 m 2 from an average monthly salary, increasing by more than 25%. However, this favorable situation for investment has continuously deteriorated over the past five years and almost dropped back to the level of 2010, as currently only 0.70 m 2 can be constructed from an average net salary.

Changes in Energy Prices and Maintenance Costs of the Buildings
Ensuring the supply of energy, whether from conventional or renewable sources, is essential for the proper functioning and growth of countries' economies. After the global economic crisis of 2008, gradual economic growth was observed in most countries. Thus, energy prices were adjusted for inflation and did not increase significantly. On the other hand, if energy consumption suddenly increases or a drastic decrease is observed, the balance of supply and demand is lost, leading to a drastic change in energy prices. This principle has also prevailed in the EU throughout the past few years, primarily because the states obtain a large proportion of their energy needs from imports, so they depend on external factors. That is why the accelerated decarbonization of this region is so vital. Electricity and natural gas prices were analyzed to calculate the annual maintenance costs of residential buildings. Data on average energy prices between 2010 and 2023 Q1 were provided based on the Eurostat database [33]. The average electricity and natural gas prices in the EU and Romania for household consumers, with all taxes and levies included From Figure 6, it can be seen that, in terms of wages, in the years 2011, 2012, and 2023, the purchasing power decreased by an average of 3% even though there was an increase in nominal real wages. Similarly, in the case of construction costs, it can be noticed that the real price decreased by almost 7% in 2012, by 2% in 2013, and by 3% in 2023. In the second case, however, the reduction in real costs positively affected the realization of energy-efficient buildings. Comparing the evolution of real average wages and investment costs, when in 2010 it was possible to realize 0.69 m 2 for an energy-efficient building from one monthly salary, a continuous improvement can be observed in the period 2011-2017; thus, in 2017, it was already possible to realize 0.86 m 2 from an average monthly salary, increasing by more than 25%. However, this favorable situation for investment has continuously deteriorated over the past five years and almost dropped back to the level of 2010, as currently only 0.70 m 2 can be constructed from an average net salary.

Changes in Energy Prices and Maintenance Costs of the Buildings
Ensuring the supply of energy, whether from conventional or renewable sources, is essential for the proper functioning and growth of countries' economies. After the global economic crisis of 2008, gradual economic growth was observed in most countries. Thus, energy prices were adjusted for inflation and did not increase significantly. On the other hand, if energy consumption suddenly increases or a drastic decrease is observed, the balance of supply and demand is lost, leading to a drastic change in energy prices. This principle has also prevailed in the EU throughout the past few years, primarily because the states obtain a large proportion of their energy needs from imports, so they depend on external factors. That is why the accelerated decarbonization of this region is so vital. Electricity and natural gas prices were analyzed to calculate the annual maintenance costs of residential buildings. Data on average energy prices between 2010 and 2023 Q1 were provided based on the Eurostat database [33]. The average electricity and natural gas prices in the EU and Romania for household consumers, with all taxes and levies included in 2010-2023 Q1, are enumerated in Table 2. Although energy prices in Romania were significantly lower than the European Union average until 2019, this difference has decreased due to the pandemic and the energy crisis. While the energy cost for households increased by approximately 47% in 12 years in the EU, in Romania, this growth ratio was greater than 120%, both for natural gas and electricity. Nevertheless, in 2022, the prices in the Eastern European country were still affordable.
Regarding the 2023-2043 interval, it was necessary to estimate the price change to determine the payback time of further investments for residential nZEBs built in the last period. A slowly decreasing inflation environment was used to estimate the electricity and natural gas prices for the next 20 years, which will drop from the 13.8% reported in 2022 to nearly 1.7% by 2043, and the average energy price trends in the previous three years were taken into account in each case. This scenario did not consider the emergence of possible crises in the next period. The specific calculation for the prediction of natural gas and electricity costs in the next 20 years is represented in the following formula: where C i.j = unit price of "i"-type energy source for household consumers in year "j" [EUR/kWh]; I j = annual inflation rate in year "j" [%]; and f i.j = annual growth rate of "i"-type energy cost in year "j" [−]; To achieve a better standard deviation of the energy price evolution, three cases were defined, and the estimated values can be found in Table 3: 1.
An optimistic variant, where energy prices stagnate and even fall minimally, the final value of which is only affected by inflation (f el.j = 1.00 and f gas.j = 1.00).

2.
A realistic version, where energy prices follow the trend in the past period and gradually increase due to high demand, and similarly to the previous case, it also expects a long-term decrease in inflation (f el.j = 1.06 and f gas.j = 1.07).

3.
A pessimistic scenario where the energy crisis persists and the energy price increases at a higher rate due to the lack of supply (f el.j = 1.11 and f gas.j = 1.12). As seen in the above tables and Figure 7, the unit price for electricity for 2043 was determined to be between 0.381 and 1.195 EUR/kWh, and for natural gas, a slightly moderate price of 0.102 and 0.329 EUR/kWh was calculated. Considering the intermediate value of energy price evolution, this means an average annual increase of 5% for both energy carriers. As seen in the above tables and Figure 7, the unit price for electricity for 2043 was determined to be between 0.381 and 1.195 EUR/kWh, and for natural gas, a slightly moderate price of 0.102 and 0.329 EUR/kWh was calculated. Considering the intermediate value of energy price evolution, this means an average annual increase of 5% for both energy carriers. In order to assess the operational expenses and calculate the yearly savings achieved through energy efficiency, two categories of residential buildings were examined: (1) a nearly zero-energy building with high energy performance and an annual energy consumption of 48 kWh/m 2 year and (2) a traditional building with low energy efficiency and an annual energy demand of 197 kWh/m 2 year. In both cases, the space heating of the buildings and the preparation of domestic hot water (DWH) are based on systems using natural gas. The space cooling and lighting, as well as the mechanical ventilation, which In order to assess the operational expenses and calculate the yearly savings achieved through energy efficiency, two categories of residential buildings were examined: (1) a nearly zero-energy building with high energy performance and an annual energy consumption of 48 kWh/m 2 year and (2) a traditional building with low energy efficiency and an annual energy demand of 197 kWh/m 2 year. In both cases, the space heating of the buildings and the preparation of domestic hot water (DWH) are based on systems using natural gas. The space cooling and lighting, as well as the mechanical ventilation, which is implemented only in the case of the nZEB, rely on electricity. The breakdown of annual energy consumption for these two cases is represented in Table 4 below: In the case of the two residential buildings with different energy performances, the operating cost per square meter was determined for each year, first by dividing the energy consumers into two categories according to energy sources, which consist of 30.97 kWh of natural gas and 17.03 kWh of electricity for the nZEB, and 172 kWh of natural gas and 25 kWh of electricity for the low energy efficiency building, and then calculating the annual costs per category. After that, the yearly running expenses for the two case studies were determined from the sum of the natural gas and electricity costs, the values of which can be found in Table 5. These values will be used to determine the annual savings made up of the difference in the maintenance costs of the two types of buildings.

Payback Period of the nZEBs in the Residential Sector
Based on the value of the initial investments for the residential building and the additional cost spent on energy efficiency that is adjusted to the criteria of nearly zeroenergy building requirements and defining the annual financial savings in the case of the nZEBs in comparison with the traditional buildings, the payback time of the additional investment can be calculated for constructions built in each year starting in 2010. Since three variants were defined for 2023-2040 during the estimation of energy prices, a lower and upper limit and a median value also appear when evaluating the payback time. In order to determine these payback periods, it is necessary to solve the following equation: where I (AC) = additional cost of the investment related to achieving nZEB [EUR/m 2 ]; Tpb = payback time of the energy efficiency [years]; and ∆EC s.t = annual energy cost savings provided by energy efficiency in the year "t", resulting from the difference in the yearly running costs of traditional buildings and nearly zero-energy buildings [EUR/m 2 year]. Applying the equation, it is possible to obtain an answer to how the payback values have changed in the last 13 years for buildings with high energy efficiency implemented in different years. The change in these intervals and the evolution of the trend can be found in Figure 8.

Payback Period of the nZEBs in the Residential Sector
Based on the value of the initial investments for the residential building and the additional cost spent on energy efficiency that is adjusted to the criteria of nearly zero-energy building requirements and defining the annual financial savings in the case of the nZEBs in comparison with the traditional buildings, the payback time of the additional investment can be calculated for constructions built in each year starting in 2010. Since three variants were defined for 2023-2040 during the estimation of energy prices, a lower and upper limit and a median value also appear when evaluating the payback time. In order to determine these payback periods, it is necessary to solve the following equation: where I(AC) = additional cost of the investment related to achieving nZEB [EUR/m 2 ]; Tpb = payback time of the energy efficiency [years]; and ΔECs.t = annual energy cost savings provided by energy efficiency in the year "t", resulting from the difference in the yearly running costs of traditional buildings and nearly zero-energy buildings [EUR/m 2 year]. Applying the equation, it is possible to obtain an answer to how the payback values have changed in the last 13 years for buildings with high energy efficiency implemented in different years. The change in these intervals and the evolution of the trend can be found in Figure 8. Examining the graphic above, it can be concluded that energy-efficient residential buildings constructed after 2010 show a decreasing trend in the recovery of additional costs until 2017. Thus, in these seven years, the payback period decreased from twelve to almost eight years, representing nearly a 36% decrease. A minimal change occurred in the case of buildings completed in 2018 and 2019. In this period, wage increases in the construction industry began to catch up with the average net income in the Romanian economy. As a result, compared to 2017, where the payback was 7.8-8 years, the discussed interval grew to 8.1-8.8 years in 2019. Given the circumstances of the pandemic, 2020 was V1 -Optimistic energy price evolution V2 -Realistic energy price evolution V3 -Pessimistic energy price evolution Examining the graphic above, it can be concluded that energy-efficient residential buildings constructed after 2010 show a decreasing trend in the recovery of additional costs until 2017. Thus, in these seven years, the payback period decreased from twelve to almost eight years, representing nearly a 36% decrease. A minimal change occurred in the case of buildings completed in 2018 and 2019. In this period, wage increases in the construction industry began to catch up with the average net income in the Romanian economy. As a result, compared to 2017, where the payback was 7.8-8 years, the discussed interval grew to 8.1-8.8 years in 2019. Given the circumstances of the pandemic, 2020 was not the most promising year for real estate investments. However, in the case of houses built, the payback period for the additional cost invested in energy efficiency is favorable-on average, 7.8 years. This value was primarily influenced by the slight decrease in energy prices. On the other hand, 2020 saw the lowest percentage of additional cost in energy efficiency to achieve an nZEB, 14.4%. In the following two years, energy prices that started to rise partially reshaped the favorable trend in the last ten years for energy efficiency. Thus, in 2022, compared to 2020, the payback period of nZEBs increased by almost two years, from an average of 7.8 years to 9.5 years. Thus, the investors who emphasized nZEBs in 2023 can count on a return rate of 8.4-11.1 years, depending on the change in energy prices in the future.
In supplement to the payback time of the additional costs spent on the implementation of energy efficiency to achieve nZEBs, four life cycle cost calculations are also analyzed, which determine the global prices of traditional and energy-efficient buildings implemented in the years 2010, 2014, 2018, and 2023 for the first 20 years of the buildings. The evolution of global costs for the mentioned cases can be seen in Figures 9-12. As can be seen in the graphs, the curve of nearly zero-energy buildings (golden lines) describes a much flatter arc than the exponentially rising bend of traditional buildings (blue lines). not the most promising year for real estate investments. However, in the case of houses built, the payback period for the additional cost invested in energy efficiency is favorable-on average, 7.8 years. This value was primarily influenced by the slight decrease in energy prices. On the other hand, 2020 saw the lowest percentage of additional cost in energy efficiency to achieve an nZEB, 14.4%. In the following two years, energy prices that started to rise partially reshaped the favorable trend in the last ten years for energy efficiency. Thus, in 2022, compared to 2020, the payback period of nZEBs increased by almost two years, from an average of 7.8 years to 9.5 years. Thus, the investors who emphasized nZEBs in 2023 can count on a return rate of 8.4-11.1 years, depending on the change in energy prices in the future. In supplement to the payback time of the additional costs spent on the implementation of energy efficiency to achieve nZEBs, four life cycle cost calculations are also analyzed, which determine the global prices of traditional and energy-efficient buildings implemented in the years 2010, 2014, 2018, and 2023 for the first 20 years of the buildings. The evolution of global costs for the mentioned cases can be seen in Figures 9-12. As can be seen in the graphs, the curve of nearly zero-energy buildings (golden lines) describes a much flatter arc than the exponentially rising bend of traditional buildings (blue lines).   not the most promising year for real estate investments. However, in the case of houses built, the payback period for the additional cost invested in energy efficiency is favorable-on average, 7.8 years. This value was primarily influenced by the slight decrease in energy prices. On the other hand, 2020 saw the lowest percentage of additional cost in energy efficiency to achieve an nZEB, 14.4%. In the following two years, energy prices that started to rise partially reshaped the favorable trend in the last ten years for energy efficiency. Thus, in 2022, compared to 2020, the payback period of nZEBs increased by almost two years, from an average of 7.8 years to 9.5 years. Thus, the investors who emphasized nZEBs in 2023 can count on a return rate of 8.4-11.1 years, depending on the change in energy prices in the future. In supplement to the payback time of the additional costs spent on the implementation of energy efficiency to achieve nZEBs, four life cycle cost calculations are also analyzed, which determine the global prices of traditional and energy-efficient buildings implemented in the years 2010, 2014, 2018, and 2023 for the first 20 years of the buildings. The evolution of global costs for the mentioned cases can be seen in Figures 9-12. As can be seen in the graphs, the curve of nearly zero-energy buildings (golden lines) describes a much flatter arc than the exponentially rising bend of traditional buildings (blue lines).     On 20-year horizons, the positive impact of energy-efficient buildings on running costs becomes unequivocally evident, highlighting their cost effectiveness through the ratio between operating costs and the additional expenditure dedicated to energy efficiency. In the case of nZEBs implemented in 2010, the financial savings are almost 148% of the extra investment, an average of 118 EUR/m 2 . In the second and third cases, the savings are, on average, 274% higher than the additional investment. Even though construction prices have increased in the last couple of years, in the case of nearly zero-energy buildings that were completed in 2023, it can still be calculated that in the first two decades of their operation, the investment in energy efficiency generates an average of 250 EUR/m 2 in savings, which still exceeds the auxiliary investment.

Conclusions
The past few years' events-the pandemic, the ongoing war, and the energy crisishave fundamentally changed the economic position of the European Union and created uncertainty among social classes in both developed and emerging countries regarding their current level of wellbeing. Thus, the EU's aspirations towards carbon neutrality  On 20-year horizons, the positive impact of energy-efficient buildings on running costs becomes unequivocally evident, highlighting their cost effectiveness through the ratio between operating costs and the additional expenditure dedicated to energy efficiency. In the case of nZEBs implemented in 2010, the financial savings are almost 148% of the extra investment, an average of 118 EUR/m 2 . In the second and third cases, the savings are, on average, 274% higher than the additional investment. Even though construction prices have increased in the last couple of years, in the case of nearly zero-energy buildings that were completed in 2023, it can still be calculated that in the first two decades of their operation, the investment in energy efficiency generates an average of 250 EUR/m 2 in savings, which still exceeds the auxiliary investment.

Conclusions
The past few years' events-the pandemic, the ongoing war, and the energy crisishave fundamentally changed the economic position of the European Union and created uncertainty among social classes in both developed and emerging countries regarding their current level of wellbeing. Thus, the EU's aspirations towards carbon neutrality On 20-year horizons, the positive impact of energy-efficient buildings on running costs becomes unequivocally evident, highlighting their cost effectiveness through the ratio between operating costs and the additional expenditure dedicated to energy efficiency. In the case of nZEBs implemented in 2010, the financial savings are almost 148% of the extra investment, an average of 118 EUR/m 2 . In the second and third cases, the savings are, on average, 274% higher than the additional investment. Even though construction prices have increased in the last couple of years, in the case of nearly zero-energy buildings that were completed in 2023, it can still be calculated that in the first two decades of their operation, the investment in energy efficiency generates an average of 250 EUR/m 2 in savings, which still exceeds the auxiliary investment.

Conclusions
The past few years' events-the pandemic, the ongoing war, and the energy crisishave fundamentally changed the economic position of the European Union and created uncertainty among social classes in both developed and emerging countries regarding their current level of wellbeing. Thus, the EU's aspirations towards carbon neutrality prove to be the correct answer to energy and natural resource addiction and other external factors, ensuring a sustainable economy. This is one of the main reasons for the increasing emphasis on high-energy-performance buildings, such as nearly zero-energy buildings.
Going further, the obligation to implement zero-emission buildings is already planned for the 2030s.
Knowing the factors that influence a building's investment and running costs, it can be concluded that assets in energy efficiency make sense in the long run, as evidenced through the cost analysis in this paper. Over 13 years, the return on the extra cost of energy-efficient buildings has declined from 12.3 years to an average of 9.4 years, which is a 24% drop, even though the indicator started to grow in the last 2 years and increased by almost 1.5 years compared to the absolute minimum in 2017, which was estimated at 7.8-8 years.
The same favorable direction has been shown in the 20-year life cycle cost of residential nearly zero-energy buildings. Compared to traditional buildings, the running costs are much lower, and in two decades, savings can reach 126% of the initial additional costs of the investment. In 2010, the additional cost of enhancing a building's energy efficiency for a square meter was 80 EUR/m 2 . After 20 years, potential savings could range between 103 and 132 EUR/m 2 . By 2023, the extra cost had risen to 170 EUR/m 2 . However, it is anticipated that over the next two decades, the average savings will amount to 250 EUR/m 2 , which continues to demonstrate a favorable cost-efficiency ratio. Overall, it can therefore be concluded that increasing the energy efficiency of buildings can be the right direction, even under the current conditions.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.