The fluctuating offer of renewable energies and their, in most cases, not synchronous use make it necessary to develop processes of energy storage both for short-term, relatively small amounts of energy with great power fluctuations and for long-term (seasonal) large energy volumes. The growing share of solar electric energy makes the storage problem an ever increasing priority since grid storage is only possible to a limited extent. A hydrogen storage system based on water electrolysis, storage of the H-2/O-2 product gases, and reconversion into electric energy via a fuel cell is well suited for the long-term storage of electricity. As part of the North Rhine-Westphalian Working Group on Solar Energy, the Research Centre Julich operates the PHOEBUS demonstration plant for the further intensive development of these technologies comprising all important components for an autonomous all-year energy supply from solar energy.