Abstract
During a photoelectrochemical reaction only a portion of the light energy absorbed by the semiconductor (
or 
single crystal) is utilized in the electrode reaction. The unused portion of energy is expended through various mechanisms as heat. Therefore by monitoring temperature changes within the photoanode as a function of electrode potential and light intensity, information concerning the efficiency of the process can be obtained. Experimental results are presented and interpreted using a model for the energy balance within the system. This permits the determination of the quantum and energy efficiencies simultaneously without the need to calibrate the light source.