Solar hot water systems capture thermal energy from the sun and use it to heat water for your home. Domestic hot water systems can be a cost-effective way to generate hot water for your home. Solar hot water systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a tankless or demand-type water heater for backup.