Solar panels. Array of photovoltaic cells (solar panels) on a roof. The cells contain a semicon- ducting material which converts sunlight directly into electricity. Solar power is a renewable source of energy and does not contribute to global warming. As photovoltaic cells do not work in low light, residences which use solar power must have a reserve power supply, such as batteries which store excess power during the day. Photographed at the Schwarzwald-Einoedhof Rappenecker-Hof" near Freiburg, Germany. The houses in this research project have all their power needs met by a combination of solar power and energy-saving devices."