An uninterruptible power supply for providing power to a load, the uninterruptible power supply comprising: a first input to receive input power from a first power source; an output to provide output power; a bypass input to receive bypass power, wherein the bypass input is selectively coupled to the output to provide output power from the bypass power; an input power circuit, configured to receive power through the first input from the input power source, coupled to the first input and having a DC output that provides DC power having a first DC voltage level; a back-up power source coupled to the input power circuit to provide DC power having a second DC voltage level in a back-up mode of operation; an inverter circuit having an input coupled to the DC output of the input power circuit and an output coupled to the output of the uninterruptible power supply and configured to provide the output power derived selectively from the input power in a normal mode of operation and power from the back-up power source in the back-up mode of operation; a bypass switch coupled between the bypass input and the output of the inverter circuit and controlled to operate in a closed position in a bypass mode of operation to provide bypass power to the load in the bypass mode of operation through the bypass switch; means for controlling the inverter circuit in the bypass mode of operation to convert AC power from the bypass power source at the output of the inverter circuit to DC power at the input of the inverter circuit; means for charging the back-up power source in the bypass mode of operation; and a power supply, separate from the means for charging, coupled to the input of the inverter circuit to receive DC power and configured to provide DC power to components of the uninterruptible power supply in the bypass mode of operation. a first input to receive input power from a first power source; an output to provide output power; a bypass input to receive bypass power, wherein the bypass input is selectively coupled to the output to provide output power from the bypass power; an input power circuit, configured to receive power through the first input from the input power source, coupled to the first input and having a DC output that provides DC power having a first DC voltage level; a back-up power source coupled to the input power circuit to provide DC power having a second DC voltage level in a back-up mode of operation; an inverter circuit having an input coupled to the DC output of the input power circuit and an output coupled to the output of the uninterruptible power supply and configured to provide the output power derived selectively from the input power in a normal mode of operation and power from the back-up power source in the back-up mode of operation; a bypass switch coupled between the bypass input and the output of the inverter circuit and controlled to operate in a closed position in a bypass mode of operation to provide bypass power to the load in the bypass mode of operation through the bypass switch; means for controlling the inverter circuit in the bypass mode of operation to convert AC power from the bypass power source at the output of the inverter circuit to DC power at the input of the inverter circuit; means for charging the back-up power source in the bypass mode of operation; and a power supply, separate from the means for charging, coupled to the input of the inverter circuit to receive DC power and configured to provide DC power to components of the uninterruptible power supply in the bypass mode of operation.