Abstract: The present invention provides an uninterruptible power supply (UPS) having a chassis formed using mating panels of a sturdy, lightweight, substantially rigid material, such as ABS plastic, that are fixedly attached together to form a compact, user-friendly chassis using simple connection techniques that entail a minimum of tools and a minimal number of fastening mechanisms. In one general aspect, a UPS in accordance with the invention comprises an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, an inverter operatively coupled to the DC voltage source to receive DC power and to provide AC power, a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply, and a chassis for housing at least the DC voltage source, inverter, and transfer switch.