Solar water heating systems use free energy from the sun to heat domestic hot water.
A conventional boiler or immersion heater can be used to make the water hotter, or to provide hot water when solar energy is unavailable. Solar water heating panels should not be confused with photovoltaic panels which produce electricity from sunlight.
A solar water heating (SWH) system consists of three major components:
- one or more solar collectors (tubes or panels, usually mounted on a roof),
- a pump connected to a controller, and
- a hot water cylinder with two heating coils (twin coil cylinder).
The collector uses the solar energy from sunlight to heat a thermal fluid (water with antifreeze). If the fluid in the collector is hotter than the water in the cylinder then the controller activates the pump, to pump the fluid through the coil in the cylinder, thus heating water in the cylinder for showers and hot water taps. The second heating coil in the cylinder is connected to a back-up boiler (e.g. gas boiler) to provide hot water when there isn’t enough sunshine.
Source - Which? Energy (link)
Regulations in the UK strongly discourage DIY solar water heating. It's a system that can utilise pressurised steam and is therefore potentially hazardous; only professionals are certified to install these. DIY also prohibits eligibility for incentive programmes such as the Domestic Renewable Heat Incentive scheme (RHI).
For detailed information and expert advice, consult the CAT information service page found here.