Laboratory tests show that the proton exchange membrane (PEM) fuel cell can be a fast-response power source able to produce full power in less than 200 μs. For every standard cubic foot (28.3 liter) of hydrogen, approximately 45 Whrs of electrical energy can be produced from a PEM fuel cell. A 100 kW PEM fuel cell would weigh approximately 300 lbs (136 kg) at the current level of material technology. S i n c e t h e f u e l c e l l c a n o p e r a t e f o r many years continuously, the duration of power sustainment depends upon the quantity of stored reactants. Using hydrogen and oxygen, a PEM fuel cell results in one of the highest energy density power devices for the UPS application. Hydrogen storage methods discussed include gas, l i q u i d, metal hydride, and carbon a d s o r p t i o n.