In general, polycrystalline silicon is used as the material of solar module for a general photovoltaic power system. In the summer, the temperature of solar module remains over 60 degrees C for a long time in daytime. It is known that the conversion efficiency of the solar module remarkably drops because of the temperature rise. It is possible to keep the conversion efficiency high by water-cooled equipment in photovoltaic power system. Furthermore, the cooling water can store heat energy from the solar module, so that a large amount of hot water can be produced. The results show that the water-cooled equipment has been effective in photovoltaic power system for a year.