Solar panels. Array of solar panels (photovoltaic cells) on the roof of a building. 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. As solar 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. Solar energy is relatively cheap, as once the panels are installed, they continue to produce electricity indefinitely. Photographed in Germany.