A power supply unit that allows a main battery and an auxiliary battery to be charged by a system power supply is disclosed. The first and second bridge circuits of the power supply unit are each formed of four switching elements. The transformer of the power supply unit has a primary winding connected to the first bridge circuit, and a secondary winding connected to the second bridge circuit. The DC/DC converter of the power supply unit allows the auxiliary battery to be connected to the first and second circuits. The controller of the power supply unit controls the switching elements of the first bridge circuit, the switching elements of the second bridge circuit, and the switching element of the DC/DC converter such that power that has been charged to the main battery is output as an AC voltage having voltage and frequency for electric appliances.