The demonstration system will be composed of a PV array, a PEM fuel cell system and a PEM electrolyzer, batteries, storage tanks for H2, O2 and H2O, as well as associated converters. This paper presents the obtained optimal sizing curves for the hybrid system (Ppv [kWp], QH2max [kg]) by taking into account the predefined constant load (6.5 kW) functioning 24/24 hours and the system energy balance for a loss-of-load probability LLP = 0. These results give the sizes of each subsystem and will make possible to build the best energy flows control mode.