Domestic water heating is perhaps the best overall potential application for active solar heating in the UK and accounts for approximately 6% of the total national delivered energy use. Domestic water heating demand continues all the year round and still needs to be satisfied in the summer when there is plenty of solar energy available.

Incoming mains water is usually at a temperature of around 10 degrees Celsius in the UK and has to be heated, under recent recommendations, to a storage temperature of at least 60 degrees Celsius.

For water heating, a typical UK house uses 5kWh per day of useful energy (that is the energy content of the water leaving the taps). This figure can vary considerably from house to house, dependent on household size and water usage.

The actual amount of delivered energy (the energy registered on a gas or electricity meter) can be considerably higher, particularly in summer. Running a boiler with a continuously burning pilot light and uninsulated hot water pipe runs for small quantities of hot water reduces efficiency considerably. Even electric immersion heaters may only manage 50% efficiency in terms of useful energy at the tap, boilers considerably less.

We could try to design a water heating system to give as close as possible to 100% of the daily requirements in winter. It would, however, be at an unacceptable cost relative to the financial returns and would give us too much hot water in the summer. Hence, solar water heating systems for domestic hot water are usually designed to achieve almost all of summer requirements, and will use a backup heat source to provide the short fall over the rest of the year.

A well designed system will typically provide 80% of a family's hot water requirements during the summer months of April to September and make a useful contribution during the rest of the year. In a typical year the system will overall provide 30-70% of a family's annual domestic water heating needs. This percentage is sometimes referred to as the solar fraction.

Agricultural and commercial systems have not received much attention in the UK in terms of solar water heating. Dairies, hotels, sheltered housing, etc., use considerable quantities of hot water, which make them good candidates for solar systems. In broad terms, such systems are only larger versions of domestic systems.