$\varepsilon_{\mathrm{thermo}}=\frac{\Delta \mathrm{G}_{\mathrm{rxn}}}{\Delta \mathrm{H}_{\mathrm{rxn}}}=\frac{\text { useful } \mathrm{E}}{\text { total } \mathrm{E}}$.
Related Theory
Fuel cells are more efficient than thermal plants. The efficiency of fuel cells to produce electricity is $70 \%$. Hydrogen-oxygen cell, methyl-oxygen-alcohol cell and propaneoxygen cell are some of the examples of fuel cells.