NOVELTY - Operating a fuel cell system comprises beginning production of the system electrical output from the fuel cell system responsive to a system start condition; initiating production of electrical output from a second fuel cell stack of the fuel cell stacks responsive to a second stack start condition that is different from the first stack start condition; and ceasing the production of electrical output from at least one of the first and second fuel cell stacks of the fuel cell stacks responsive to a stack stop condition.    USE - The method and the fuel cell system are useful for generating electricity (all claimed).    ADVANTAGE - The fuel cell systems may operate efficiently over a given range of electrical outputs. Moreover, the fuel cell system may efficiently provide electrical power.    DETAILED DESCRIPTION - Operating a fuel cell system comprising an oxidant source adapted to provide an oxidant stream, a fuel source adapted to provide a fuel stream, an energy storage device adapted to store electrical energy and provide an energy storage device electrical output, and fuel cell stacks, comprises beginning production of the system electrical output from the fuel cell system responsive to a system start condition; where the process for beginning production of the electrical output comprises supplying the electrical output from the energy storage device; initiating production of electrical output from a first fuel cell stack of the fuel cell stacks responsive to a first stack start condition; initiating production of electrical output from a second fuel cell stack of the fuel cell stacks responsive to a second stack start condition that is different from the first stack start condition; and ceasing the production of electrical output from at least one of the first and second fuel cell stacks of the fuel cell stacks responsive to a stack stop condition; where the fuel cell stacks are adapted to receive the fuel stream and the oxidant stream and to produce a stack electrical output; and the fuel cell system is adapted to produce a system electrical output to supply to an energy consuming device when a primary power system is unable to supply the energy consuming device. INDEPENDENT CLAIMS are:    (1) a fuel cell system adapted to produce a system electrical output to supply an energy consuming device, comprising an oxidant source adapted to supply an oxidant stream; a fuel source adapted to supply a fuel stream; fuel cell stacks, where each of the fuel cell stacks is adapted to receive the fuel stream and the oxidant stream and to produce a stack electrical output; an energy storage device comprising a state of charge and is adapted to produce an energy storage device electrical output; an energy consuming device adapted to receive the electrical output from the fuel cell system; and a controller adapted to control the operation of the fuel cell system; where the controller is adapted to begin the production of the system electrical output responsive to a system start condition, initiate production of electrical output from a first fuel cell stack of the fuel cell stacks responsive to a first stack start condition, initiate production of electrical output from a second fuel cell stack of the fuel cell stacks responsive to a second stack start condition, and cease production of electrical output from at least one of the first and the second fuel cell stack of the fuel cell stacks responsive to a stack stop condition; and    (2) a fuel cell system bank comprising fuel cell systems, which comprises a first fuel cell system and a second fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow chart of the process for operating a fuel cell system.