Abstract We derive a power factor and an efficiency factor for comprehensive evaluation of thermoelectric generator (TEG) materials, and approve that only using the power factor (or efficiency factor) is sufficient to determine the output power (or thermoelectric conversion efficiency) potential of a material. We also show that the effect of thermal conductivity is underestimated in the traditionally used figure of merit. In fact, the traditionally used power factor and figure of merit are simplified versions of the new factors for the special “constant surface temperatures” condition (the heat transfer coefficients on the hot and cold sides are infinitely large), which rarely occurs in practical applications. These two new factors can be conveniently used for material evaluation and design optimization, for example, to determine the optimal interface temperature and length ratio of a segmented TEG.