NOVELTY - The method comprises: characterizing a mixture of fuels provided into a fuel cell system by obtaining a measure of a quantity of carbon in a sample of the fuel mixture; obtaining a water flow rate in the fuel cell system; and calculating a steam:carbon (S:C) ratio based on the characterization of the fuel mixture and the water flow rate. The step of characterizing the fuel comprises determining a composition of the fuel based on one of location or time of year. The method further comprises providing the fuel and an oxidizing gas to the cell system based on the calculated S:C ratio.    USE - The method is useful for operating a fuel cell system such as solid-oxide fuel cell system (all claimed).    ADVANTAGE - The method provides simplified parameters to characterize the fuel and the oxidizing gas of the fuel cell system to control the fuel cell system thus ensuring efficient operation of the fuel cell system under various atmospheric conditions and various fuel sources.    DETAILED DESCRIPTION - The method comprises: characterizing a mixture of fuels provided into a fuel cell system by obtaining a measure of a quantity of carbon in a sample of the fuel mixture; obtaining a water flow rate in the fuel cell system; and calculating a steam:carbon (S:C) ratio based on the characterization of the fuel mixture and the water flow rate. The step of characterizing the fuel comprises determining a composition of the fuel based on one of location or time of year. The method further comprises providing the fuel and an oxidizing gas to the fuel cell system based on the calculated S:C ratio; and generating electricity using the fuel cell system. The step of obtaining the measure of the quantity of carbon in the sample of the fuel comprises determining moles of carbon per gram or milliliter or mole of fuel, obtaining the fuel characteristics from a fuel supply source, and comprises measuring or testing the fuel. The S:C ratio is calculated by an equation of water flow rate/(fuel flow ratex mol carbon/mol fuel). A computer or a logic device performs the calculating step.