Solar water heating or Solar Hot Water Heater is water heated by the use of solar energy. Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are more specific examples.

In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

Residential Solar Hot Water Heater can be subdivided into two kinds of systems: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50°C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.

Active Solar Hot Water Heater employ a pump to circulate the water or heat transfer fluid and a controller to turn the pump on and off depending on the temperature of the tank and collectors. Active systems are usually significantly more efficient that passive systems but are more complex, more expensive, more difficult to install and rely on electricity to run the pump and controller. During active heating, solar energy is stored, collected, and distributed in buildings, providing hot water or space heating. When sunlight falls on a building's collector, it is transformed into heat and conveyed into a carrier fluid. It is then pumped into a conversion, later into a storage, and finally into the distribution system.

Newer electronic controllers permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.

The most commonly used solar collector is the insulated glazed flat panel. Less expensive panels, like polypropylene panels (for swimming pools) or higher-performing ones like evacuated tube collectors, are sometimes used.