The geometry of on-membrane thermoelectric power supplies has been optimized to obtain the highest useful electric power for a given heating power. Assuming the use of materials with moderate thermoelectric performances, an analytical one-dimensional model has been developed to find the optimum thermoelectric film thickness and leg length together with the best heat source configuration. The optimal geometry of these thermoelectric devices is discussed according to the thermal properties and emissivity of the materials as well as based on their working environment, i.e. air or vacuum, and on their operational mode. These investigations have been extended to more realistic micro-device geometries and to more efficient thermoelectric materials, using a numerical model. The performance of on-membrane thermopile is predicted for various materials and geometry combinations.