A grid-connected power supply system comprising: a power generation apparatus configured to generate power by using natural energy and connected in parallel to a commercial power grid; an electricity storage device configured to convert AC power, supplied from the commercial power grid, into DC power to be charged with the DC power, and to convert a discharged DC power into AC power and supply the AC power to one or more loads; and a control device configured to control charging and discharging of the electricity storage device, wherein the control device is configured to predict an amount of power to be generated on a subsequent day by the power generation apparatus and a power demand of the loads and to charge the electricity storage device with power in a time zone in which an electricity rate of the commercial power grid is lower than an electricity rate of the commercial power grid in another time zone if a predicted value of the amount of power to be generated is lower than a predicted value of the power demand, or not to charge the electricity storage device with power during the time zone if the predicted value of the amount of power to be generated is not lower than the predicted value of the power demand, wherein the electricity rates with regard to the commercial power grid are set for respective time zones, and characterized in that the control device is configured to compare an amount of power dischargeable from the electricity storage device with a predicted value of the power demand of the loads, and to provide notification if it is determined that the amount of dischargeable power will be less than the power demand of the loads within a predetermined time period, the control device has a function of controlling operations of electric devices as the loads, and is configured to control sequentially the operations of the electric devices in order of priority so that power consumption is reduced if it is determined that the amount of dischargeable power will be smaller than the power demand of the loads within the predetermined time, when the predicted value of the amount of power to be generated is smaller than the predicted value of the power demand, the control device is configured to make a schedule for the power demand of the loads that prevents the predicted value of the amount of power to be generated from being smaller than the predicted value of the power demand, and to present the schedule and a schedule for the power demand for a normal day to a user. a power generation apparatus configured to generate power by using natural energy and connected in parallel to a commercial power grid; an electricity storage device configured to convert AC power, supplied from the commercial power grid, into DC power to be charged with the DC power, and to convert a discharged DC power into AC power and supply the AC power to one or more loads; and a control device configured to control charging and discharging of the electricity storage device, wherein the control device is configured to predict an amount of power to be generated on a subsequent day by the power generation apparatus and a power demand of the loads and to charge the electricity storage device with power in a time zone in which an electricity rate of the commercial power grid is lower than an electricity rate of the commercial power grid in another time zone if a predicted value of the amount of power to be generated is lower than a predicted value of the power demand, or not to charge the electricity storage device with power during the time zone if the predicted value of the amount of power to be generated is not lower than the predicted value of the power demand, wherein the electricity rates with regard to the commercial power grid are set for respective time zones, and characterized in that the control device is configured to compare an amount of power dischargeable from the electricity storage device with a predicted value of the power demand of the loads, and to provide notification if it is determined that the amount of dischargeable power will be less than the power demand of the loads within a predetermined time period, the control device has a function of controlling operations of electric devices as the loads, and is configured to control sequentially the operations of the electric devices in order of priority so that power consumption is reduced if it is determined that the amount of dischargeable power will be smaller than the power demand of the loads within the predetermined time, when the predicted value of the amount of power to be generated is smaller than the predicted value of the power demand, the control device is configured to make a schedule for the power demand of the loads that prevents the predicted value of the amount of power to be generated from being smaller than the predicted value of the power demand, and to present the schedule and a schedule for the power demand for a normal day to a user.