In order to construct an efficient photovoltaic (PV) power-generation system, we have developed a new system equipped with a cooling device utilizing siphonage. The major components of the system are an array of PV modules and cooling panels attached to the backside of the PV modules. The PV modules are cooled with cooling water flowing through a narrow gap in each cooling panel, and hot water discharged from the cooling panel can be reused. In order to save energy for introducing cooling water into the panel, siphonage from an upper level of a building to the ground level is utilized. From long-term monitoring tests in summer for the PV system, we confirmed that the cooling of the PV modules increases the electric power and that the reuse of hot water from the cooling panel contributes very much for saving energy consumed in a hot-water-supply system.