This research is aimed to propose the method of calculating COP of the residential air conditioner, and to clarify an energy conservation performance and the best operation method. Because the information on the performance of residential air conditioners that we can obtain is usually limited, we propose the calculation method of COP which can be predicted from general catalog information. First, the change in the refrigerating cycle by the outside temperature and the amount of the heat supply was analyzed. Next, the characteristic of COP change was clarified by the theoretic refrigerating cycle COP. Finally, we led the prediction method of COP based on the similarity of the relation between power consumption and the heat supply.