Heating and cooling are one of the fundamental research area for the engineers. Due to increment of the energy demand and decrement on the sources leads the researchers to develop the recent systems and find out new systems. In the study, performance of a commercial heat pump water heater is studied. The performance of system under various conditions is analyzed with a program prepared in MATLAB 7.3 software by using exergy analysis method. The exergy destruction of the system is calculated by using collected data for each component of the system as well as considering the whole states of entire system. The analyses represents effective conditions and components on the destruction of the performance. It is found that performance of system decreases for low temperature and high humidity ambient conditions.