Eliminating the ac-dc converter (such as a computer's power supply), in a dc system when using a fuel cell based uninterruptible power supply (UPS), serves several primary functions. Firstly, it eliminates the need for a dc-ac inverter, and secondly, it eliminates a usually highly inefficient component-the power supply. Multiple conversions result in multiple inefficiencies. By replacing the computer's ac power supply with a high efficiency dc power supply capable of operating directly from a fuel cell - and thereby eliminating the inverter - the overall efficiency of the UPS can be increased by 50% or more. This is essential considering that the primary function of a fuel cell based UPS is long-term operation of the system, and poor efficiency equates to higher fuel consumption. Furthermore, inefficient systems have greater power demands, and therefore a larger fuel cell stack is needed to power them. At the present cost of fuel cell systems, this is a considerable problem. The easiest way to accomplish a direct dc UPS is to replace the computer's ac-dc power supply with a dc-dc power supply. (c) 2005 Published by Elsevier B.V.