A power supply circuit for powering components of a facility are disclosed. The power supply circuit may include a power transformer, a power input terminal, a power output terminal, a bridge, a relay, and a control circuit. The relay is connected between the power input terminal and the power output terminal. A primary side of the power transformer is coupled between the relay and the power output terminal. The bridge is coupled to the secondary side of the power transformer. In a normal power mode, the control circuit closes the relay and operates the bridge as a rectifier. In an emergency power mode, the control circuit opens the relay and operates the bridge as a pulse width modulator. In this manner, the bridge and the power transformer can be used both to charge a battery during normal operation and generate emergency AC power from the battery.