The PHOEBUS demonstration plant supplied energy to part of the Central Library in Forschungszentrum Julich, Germany, for 10 years. The technical feasibility of a self-sufficient energy supply system based on photovoltaic, battery and hydrogen storage was demonstrated. The overall efficiency based on the annual energy balance excluding the photovoltaic efficiency varied from 51% to 64%. The battery bank was able to supply energy to the load for three days in the absence of solar radiation. Around 50-52% of the demands were delivered by the battery. In addition, another 20-25% of the demand was supplied by the fuel cell, which indicates that the energy should be stored in a long-term storage system. Thus, the highest level of energetic reliability can be achieved with relatively low battery capacity and hydrogen storage. (C) 2003 Elsevier Ltd. All rights reserved.