The thermal efficiency,η of a fuel conversion device is the amount of useful energy produced relative to the change in enthalpy, ΔH between the product and feed streams η=
useful energy
ΔH
In an ideal case of an electrochemical convertor, such as a fuel cell, the change in Gibb’s free energy, ΔG of the reaction is available as useful electric energy at that temperature of the conversion. Hence ηideal=