A power supply system for supplying a grid power to a building includes a power generator, a power storing device, and a power controller. The power generator generates off-grid power from a predetermined energy. The power storing device stores the grid power and the off-grid power and supplies the stored power to the building. The power controller controls consumptions of the grid power and the off-grid power. The power controller calculates predicted consumption data related to power consumed in the building and predicted generation data related to power generated by the power generator. The power controller calculates a charging-discharging schedule for the power storing device based on the predicted consumption data and the predicted generation data by formulating the charging-discharging schedule as a mixed integer programming problem.