The theory of thermal matching of a thermoelectric generator with the environment has been applied in this work to a wearable thermoelectric generator. This enabled evaluation of its top performance characteristics in typical environmental conditions. To correctly perform the modeling, the relevant properties of the human body as a heat generator for a small-size thermoelectric generator have been studied and presented in the paper as well. The results have been practically validated in different wearable thermoelectric generators. In particular, a power over 1 mW per square centimeter of the skin has been practically demonstrated on a walking person at ambient temperature of –2 °C. The comparison with wearable photovoltaic cells shows that in typical situations thermoelectric generators provide at least ten times more power.