Auxiliary fuel cell system that provides backup power to a primary power source, and energy-consuming systems that are otherwise associated with at least one primary power source and at least one auxiliary fuel cell system. The auxiliary fuel cell system includes at least one fuel cell stack that is adapted to produce an electric current from a fuel and an oxidant, such as from hydrogen and oxygen gases. The auxiliary fuel cell system further includes a source of fuel, such as a source of hydrogen gas, and this source may include a fuel processor that is adapted to produce the hydrogen gas or other fuel. The energy-consuming system is adapted to selectively draw an electric current from a primary power source or an auxiliary fuel cell system. A source selection system selectively establishes electrical communications between an energy-consuming system and the primary power source or the auxiliary fuel cell system.