According to one aspect, embodiments of the invention provide a UPS comprising a line input, an interface configured to be coupled to a DC power source, an output, a PFC converter, a positive DC bus coupled to the PFC converter and configured to provide a positive DC output, a negative DC bus coupled to the PFC converter and configured to provide a negative DC output, a DC-DC converter coupled to the interface, and a controller configured to operate, in an online mode of operation, the PFC converter to provide regulated DC power, derived from input AC power, to the positive DC bus and the negative DC bus, and operate, in a backup mode of operation, the DC-DC converter to convert backup DC power to unregulated power, and the PFC converter to provide regulated DC power, derived from the unregulated power, to the positive DC bus and the negative DC bus.