The energy efficiency of a polycrystalline ZnO photoanode in an electrolyte solution containing redox agents was investigated by measuring the temperature change of the electrode surface. The results of photothermal measurements show that the energy efficiency depends on the kind of redox agents used. Reducing agents with more positive potentials gave larger energy efficiencies, but the potential must be negative enough to decrease ZnO dissolution. This work is pertinent to solar energy conversion.