Additional volumetric sources to the thermal energy equation are present because not all chemical energy released in the electrochemical reactions can be converted to electrical work due to irreversibilities of the processes. The total source that goes to the thermal energy equation (that is, enthalpy) is:

(20–82)

where is the net enthalpy change due to the electrochemical reactions, is the product of the transfer current and the overpotential in the anode or the cathode TPB, is the ohmic resistivity of the conducting media, and is the enthalpy change due to condensation/vaporization of water.