The thermal design of weatherproof power supply system has been evaluated by experiments and simulation. Nickel-metal hydride batteries developed for stationary use were employed in the system due to their high charge/discharge performance in high-temperature environments. As the weatherproof power supply system has a sealed cabinet, we evaluated the temperature characteristics of the battery in a 40°C environment. In order to predict the temperature of the power system, it was necessary to obtain the rate of heat generation from the battery. For this purpose, we developed an estimation method in which the measured temperature of the battery is approximated as a function of cubic spline interpolation. The temperatures of critical items mounted in the system were calculated on the basis of the experimental results. This paper describes the procedure for estimating the heat generation rate of the battery and the temperature of the system. Our results show that the weatherproof power system is useful for operation in high-temperature environments.