This paper presents an energy performance analysis of an air-conditioning system with heat pump chillers controlled by inverters. The inverter is attached to the chiller's compressor, and controls the number of revolutions of the motor. As a result, the efficiency of the chiller in a partial load can be largely improved more than in a full load. Traditionally, a chiller in a HVAC&R system is operated at a full load from the viewpoints of the energy much. However, if an inverter is attached to the chiller, there is a possibility that the chiller's partial load operation will make the energy efficiency higher. This inverter utilization is one effective solution to reduce the energy consumption. The energy performance is evaluated by the HVAC&R's measurements and a simulation analysis.