The mathematical model of the solid oxide fuel cell (SOFC) is presented. The new approach for modeling the voltage of SOFC is proposed. Electrochemical, thermal, electrical, and flow parameters are collected in the 0D mathematical model. The aim was to combine all cell working conditions in as a low number of factors as possible and to have the factors relatively easy to determine. A validation process for various experimental data was made and adequate results are shown. The presented model was validated for various fuel mixtures in relatively wide ranges of parameters as well as for various cell design parameters (e.g. electrolyte thickness, anode porosity, etc.). A distinction is made between the “design-point” and “off-design operation”.