NOVELTY - A thermally primed hydrogen-producing fuel cell system is used to supplement a primary power source (130) by heating a hydrogen-producing region (19) of a fuel processing assembly (10) to a threshold temperature; maintaining the hydrogen-producing region at or above the threshold temperature; delivering water and a carbon-containing feedstock to the hydrogen-producing region during a transition period; producing hydrogen gas in the hydrogen-producing region; and generating the power output with the fuel cell stack from oxidant and hydrogen gas produced in the hydrogen-producing region.    USE - A thermally primed hydrogen-producing fuel cell system is used to supplement a primary power source.    ADVANTAGE - The use of thermally primed hydrogen-producing fuel cell system to supplement a primary power source satisfies an applied load from an energy-consuming device.    DETAILED DESCRIPTION - Using a thermally primed hydrogen-producing fuel cell system to supplement a primary power source (130) comprises:    (A) heating a hydrogen-producing region (19) of a fuel processing assembly (10) to a threshold temperature at which the hydrogen-producing region produces a mixed gas stream containing hydrogen gas as a majority component from a feed stream containing water and a carbon-containing feedstock;    (B) maintaining the hydrogen-producing region at or above the threshold temperature during periods in which the hydrogen-producing region is not producing hydrogen gas and in which there is no demand for a power output from the fuel cell system to satisfy an applied load;    (C) delivering water and a carbon-containing feedstock to the hydrogen-producing region during a transition period in which there is a demand for a power output from the fuel cell system to satisfy an applied load;    (D) producing hydrogen gas in the hydrogen-producing region; and    (E) generating the power output with the fuel cell stack from oxidant and hydrogen gas produced in the hydrogen-producing region.    An INDEPENDENT CLAIM is included for a method of supplying backup power to an energy-consuming device from a hydrogen-producing fuel cell system, comprising:    (A) heating the hydrogen-producing region to within the hydrogen-producing temperature range during periods in which the primary power source satisfies the applied load;    (B) maintaining the hydrogen-producing region within the hydrogen-producing temperature range during periods in which the primary power source satisfies the applied load;    (C) delivering the feed stream to the hydrogen-producing region in response to the primary power source becoming unavailable to satisfy the applied load;    (D) producing, by the hydrogen-producing region, the reformate stream from the feed stream;    (E) generating, by the fuel cell stack, a fuel cell power output from the hydrogen gas of the reformate stream and an oxidant; and    (F) powering the energy-consuming device with the fuel cell power output.    DESCRIPTION OF DRAWING(S) - The drawing shows a fragmentary schematic view of a thermally primed hydrogen-producing fuel processing assembly.    Fuel processing assembly (10)    Fuel processor (12)    Hydrogen-producing region (19)    Containment structure (70)    Power source (130)