A power supply and demand control method for a power storage system includes predicting a consumption power of a customer for each of time slots divided by a specific time interval, assigning one of a frequency control or a peak shaving control to each of the time slots based on the predicted consumption power, controlling the storage battery based on the assigned frequency control or the assigned peak shaving control in each of the time slots, wherein the peak shaving control is assigned to each of the time slots in which the predicted consumption power is higher than a contract power, and the frequency control is assigned to each of the times slot in which the predicted consumption power is equal to or lower than the contract power.