Solar cells. View of the array of the solar cells (photovoltaic cells) of the Rappenecker Hof Inn, near Freiburg, Germany. The cells contain a semiconducting material which converts sunlight directly into electricity. Solar power is a renewable, inexhaustible source of energy and does not contribute to global warming. This was originally a farmhouse which dates back to the 17th century. The power demands of the inn are largely met by the solar cells seen in the image. Excess energy is stored in batteries and used during the night or during unfavourable weather conditions. A diesel generator is also on hand should the batteries totally discharge.