A method of energy analysis applicable to a steady‐state electrochemical reactor has been developed, based on a rigorous thermodynamic treatment of entropy production by heat, mass, and charge transfer developed by Fo̸rland and Ratkje. For one‐dimensional reactors equations are derived that enable calculation of the temperature distribution in the electrolyte, the steady‐state heat loads at each electrode, and the total required electrical energy input to operate the reactor at a given current density. The method requires no knowledge of single iron thermodynamic properties or electrochemical potentials. Detailed calculations are presented for a case study of a high temperature water vapor electrolysis cell employing solid zirconia electrolyte.