The aim of the present paper is to develop and apply a software tool for designing hybrid renewable energy systems. The hybrid system consists of a wind generator and photovoltaic modules which are the renewable technologies for energy production. The programme has I,een applied for simulating a hybrid system with the above mentioned technologies in order cover the electricity and water needs of the Merssini village on Donoussa island in the Aegean Sea of Greece. The Merssini village is occupied by 20 year-round residents while the population is doubled during the summer period. The village is non-electrified and faces a problematic scarcity of fresh water. In the analysis that follows, the considered technical data as well as the results of programme runs for winter and summer seasons are presented. The electricity consumption consists of both the household and desalination plant consumption. The system is supplemented with batteries and a micro hydraulic plant fbr energy storage. The simulation programme was used to optimise the design of the system as well as to manage the energy supply and energy storage, The results prove that this simulation programme constitutes a valuable tool for the determination not only of the optimum combination of technologies, but;also the optimum energy management of complex hybrid systems.