Uninterruptible power supplies (UPSs) are generally discussed herein with particular discussions extended to fuel-cell-based UPSs used in conjunction with DC power supplies for improved operating efficiencies. With a wide voltage DC power supply, a DC-AC inverter may be omitted from the UPS and power from a back up power source, such as a battery or a fuel cell, may be applied directly to the DC power supply without performing two power conversions. The end result is a more efficient system capable of longer operating time.