Two kinds of operational methods (A and B) are proposed to a solar-heat/electricity hot-water supply system. These operational methods are based on the forecasted insolation of next day, i. e., after the insolation of next day is forecasted, the heat storage tank of the system is heated with midnight-time electricity (22 o'clock at previous day_??_8 o'clock at next day). In operational method A, the maximum power Pmax is set to the midnight-time electricity. In operational method B, the minimum temperature TSmin is set to the heat storage tank at 8 o'clock of next day in addition to Pmax. The value of Pmax and TSmin, are determined using the simulation with meteorological data from 1991 to 1994. Using meteorological data of 1995, the operational methods A and B are compared with the normal operational method which does not consider the insolation forecast. The results show that the operational method A is economical when the forecast error is lower than 15%, while the operational method B is economical when the forecast error is higher than 15%.