The present invention provides a power supply device that supplies power to an external load, comprising: a generator configured to generate electric power by power of an engine; a housing container configured to house a battery; a state determination unit configured to determine a state of the power supply device; and a control unit configured to control charging of the battery housed in the housing container and power supply to the external load, wherein the generator includes a tank that stores fuel for the engine, and in a case where the state determination unit determines that a remaining amount of fuel in the tank is less than a threshold, the control unit automatically transitions to a control mode in which power supply to the external load is controlled by the power from the generator and the power from the battery.