A simplified model has been formulated in order to easily and adequately determines the efficiency of a solid oxide fuel cell (SOFC) stack system, independently on the fuel choice. Simple analytical formulas for the calculation of electromotive force (emf) generated in a SOFC system as well as for the estimation of its efficiency are proposed. It was found that both emf and efficiency are functions of the amount of carbon atoms of the fuel (oxygenated or not) as well as of the operational temperature. The steam to fuel molar ratio, defined as reforming factor, m, for each fuel and temperature has been adjusted to be low enough so as to ensure optimal conditions in the SOFC operation but high enough to avoid carbon formation within the cell. It has been also demonstrated that the efficiency values predicted by the model are of low relative difference compared with the numerical ones and, therefore, the approach presented here can be considered as sufficient enough for practical use.