Three kinds of operation methods are proposed for a solar / electric power hybrid hot-water system. The operation method A is a conventional operation in which the storage tank is heated with the night-only electricity without forecasting insolation of a next day. In the operation method B, solar energy of the next day is forecasted, and if the energy is not enough for next day hot-water load, the storage tank is heated with the night-only electricity. In the operation method C, insolations until a few days later are forecasted. FIrst, assuming that the insolation forecast has no error, the simulation is conducted using hourly data of insolation and air temperature in Shizuoka city, 1996. The result shows that the electricity charge reduces to 64% in the operation method B and to 55% in the operation method C, compared with in the operation method A, and the insolation forecast is enough for until 2 days later. Next, assuming that the insolation forecast has some error, the simulation is conducted under the operation method B and C, and the electricity charge is calculated as a function of the error ratio (10_??_50%). The result shows that the electricity charge in the operation method C reduces to 71% compared with the operation method A, even if the error ratio is 50%.