Power loss mechanisms in illuminated and nonilluminated photovoltaic cells are investigated by an ac calorimetric method. In this electrothermal (ET) method, periodic temperature changes, caused by periodic electrical excitation, are sensed as a function of external parameters, e.g., applied voltage. ET signals on crystalline silicon and thin-film CuInSe2/CdS solar cells are measured as a function of applied voltage. The results are compared to those obtained from an energy balance model. Inherent to the model is the occurrence of injected carrier cooling. The ability of ET measurements to separate different power loss mechanisms is discussed and compared to that of conventional photothermal ones (e.g., photoacoustic).