Solar Water Heating - The Basics:

Solar water heating (SWH) units are a mature renewable energy technology that have been accepted and widely used in most countries for many years.

Solar water heating units are designed to deliver the optimum amount of hot water for most of the year. However, in winter there sometimes may not be enough solar heat gain to deliver sufficient hot water. In this case, a gas or electric element is normally used to help heat the water.

Residential solar thermal installations fall into two groups: direct, where the solar collector and tank are one unit and the hot water goes directly into the tank. For indirect, the tank is separate, usually inside the roof and the collector on top of the roof. Both typically include an auxiliary energy source (e.g an electric heating element) that is activated when the water in the tank falls below a minimum temperature setting ( 55°C). Hot water is therefore always available.

"In order to heat water using solar energy, a solar thermal collector, often mounted on a roof or a wall facing the sun, heats up cold water that is driven by natural convection (Thermo siphon passive system)."

The collector usually consists of a simple glass-topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black, or a set of glass tubes surrounded by an evacuated (near-vacuum) glass cylinder. The tiny tubes inside carry water or another fluid (like antifreeze) into the box to be heated. The black painted absorber plate helps make things hotter.

The collector sends the hot water into a well-insulated storage tank. Heat is stored in a hot water storage tank. The volume of this tank needs to be larger with solar water heating units to allow for bad weather, and because the optimum final temperature for the solar collector is lower than a typical immersion or combustion heater.

No electricity is involved in this process. However, SWHs do include an electrical element in case there is a long patch of cloudy weather and there isn't enough sun shine to heat the water to the required temperature. So you always have hot water.