Businesses specialising in solar heating systems design, supply and install equipment that uses energy from sunlight to provide heat, most commonly for domestic hot water. Solar thermal systems typically use roof-mounted collectors to warm a heat-transfer fluid, which passes heat to water stored in a cylinder. They may work alongside a boiler, heat pump or immersion heater, which can provide additional heat when solar input is limited. These systems are different from solar photovoltaic panels, which generate electricity. Services can include assessing a property's suitability, specifying collectors, pipework, controls and hot-water storage, and integrating the system with existing heating equipment. Installers may also replace or upgrade components, test operation and explain system controls. Maintenance can involve checking pumps, sensors, fluid levels and protective fluids, as well as investigating leaks or poor performance.