The power supply system includes an alternator that generates electric power according to the number of revolutions of an engine mounted in a vehicle and charges a predetermined battery; an output unit for outputting the electric power from the charged battery to the outside; a power generation control unit that detects the current amount of the electric power transmitted from the battery to the output unit and sets the number of engine revolutions according to the detected current amount; and an engine control unit that controls the number of revolutions of the engine in accordance with the number of engine revolutions set by the power generation control unit.