According to one aspect, an uninterruptable power supply (UPS) is provided. The UPS includes a first input constructed to receive input power from a first power source, a second input constructed to receive input power from a second power source, an output constructed to provide output alternating current (AC) power derived from at least one of the first power source and the second power source, a bypass switch having an on state and an off state coupled between the first input and the output, an inverter coupled between the second input and the output and constructed to generate the output AC power. The UPS being constructed to quickly transition from a first power delivery mode that provides output power derived from the first power source to a second power delivery mode that provides output power derived from the second power source.