Li-ion batteries are modern electrical energy storage systems which are undergoing an intense development in their technological features. Their high specific energy, efficiency and durability, together with the lowering of their production costs, are placing them as one of the most promising actors in the field of electrical energy storage technology used as tool for integrating renewable energy sources. But due to the fact that they are in an early stage of development, their reliability must be trusted. This paper describes a method and the results for testing round trip efficiencies of a modern Li-ion battery based energy storage system integrated in a PV system. According to the related standards, the efficiency of a battery and an inverter/charger has been characterised, under technical restrictions and specific operation conditions for its use with PV systems.