The influence of emitter size on the fuel to electrical conversion efficiency in thermo-photovoltaic generators is investigated. Based on simple and reasonable physical assumptions such as a black body emitter and thermal equilibrium between emitter and exhaust gases we evaluate the electrical efficiency as a function of the emitter area for thermo-photovoltaic generators equipped with Ge, GaSb and Si photovoltaic cells. It is found that for a given thermal energy introduced there are optimal values of the emitter surface which maximize the generator efficiency. The corresponding values of energy per unit area of the emitter are higher than in the realized prototypes. This suggests that it is possible to obtain more efficient thermo-photoconverters with reduced emitter and photovoltaic cell area. (C) 2013 Elsevier B.V. All rights reserved.