Supercapacitors are energy storage devices. They are charged from an external power source, such as a wall socket, and store the energy for later use. As an energy storage system, they are compatible with alternative, or clean, energy sources, such as windmills or solar panels. For example, a supercapacitor may collect and store the energy produced by solar panels during the day and provide back this energy at night. Supercapacitors are also called electrochemical capacitors and ultracapacitors. In certain cases, they may be called double-layer capacitors.