The systems which are used for heating and cooling are the main research areas of mechanical engineering for centuries. In this study, performance of a commercial heat pump water heater is investigated by exergetic analysis. The system performance is evaluated experimentally first in an environmental chamber under different climates to simulate the actual weather conditions which are obtained with the change of temperature and humidity values. The performance of the system is then investigated with the calculation of exergy destruction of the entire system by a program using MATLAB 7.3. This study presents the characteristics and performance of the compressor during the charging time for various environmental conditions. It is found that performance of compressor decreases for poor ambient conditions which is obtained by low temperature and high humidity while it is high for the warm environment conditions.