In the H2-mediated case, we have H2 reducing NAD, as shown in the Z-scheme in panel A of Figure 2, which results in the loss of some efficiency due to the potential drop. Each fuel has an electron demand of nef and a combustion energy per molecule of fuel of Efuel, and assuming that we otherwise have 100% conversion efficiency at the enzymes, our thermionic efficiency ηSTF from power received from the sun Psolar and the current in the biological cell are