The third part of the paper concludes the design tool of a stand-alone power system based on a proton exchange membrane (PEM) fuel cell stack by means of Microsoft Excel. The design of the push-pull converter, single-phase inverter and of the output filter are described in separate sections of the paper. All the data are derived from the specific load conditions for the stationary application. The design tool is used in order to choose an adequate topology for the PEM fuel cell power system.