Solar hot water heaters (also called solar domestic hot water systems) use the sun's heat to provide hot water. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't. Most solar water heaters require a well-insulated storage tank, which have an additional outlet and inlet connected to and from the collector.
In case of cloudy days or times of high demand, solar water heating systems almost always require a backup system, such as a conventional storage water heater, which usually provides backup and may already be part of the solar system package. A backup system could also be part of the solar collector, such as rooftop tanks. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it could potentially be packaged with a demand water heater for backup.