In this paper we present two methods for determining the conversion efficiency of TPV devices. In the first, the conversion efficiency is calculated from measurements of the external quantum efficiency and reflection as a function of wavelength, and from the I‐V characteristics under high‐level illumination. This is an indirect method based on separate differential measurements. In the second method, a novel heat transfer technique is utilized to combine both the voltaic diode and spectral control efficiency into a single measurement. ©1996 American Institute of Physics.