Solar Thermal Technology

Hot water from the sun

A solar thermal system uses solar absorber panels, commonly installed on the roof of a property, to absorb radiant heat from the sun, using it to heat the hot water of the property.

The panels have a network of pipes inside them containing water and antifreeze. This liquid is heated by the sun and then pumped from the solar panel to a heat exchanger coiled inside the hot water cylinder of the property. The warm mixture of water and antifreeze never comes into contact with the water in the tank, but it does heat it up providing hot water for showers, baths and washing up – just as with a normal boiler.


What does it look like?

The panels tend to be 2-4m2 in size and come in two basic types:

  • the first looks like a simple ‘flat plate’ black panel and has a similar appearance to a sky-light when installed;
  • the other is an array of glass tubes (‘evacuated tube’ collectors). This system can be more efficient in its ability to provide hot water in the winter, but tends to cost more.

How much hot water does the average system provide?

For UK residential properties, solar hot water systems are designed to work in conjunction with another water heating system, such as a gas, oil or perhaps even a biomass (wood) boiler.

The average home system might provide almost all a household’s hot water requirements during the summer months, even on cloudy days. Through the rest of the year, it will help preheat the water stored in your tank, which will then be brought up to the required temperature by the boiler or whatever secondary heating source is being used. Overall, most solar thermal systems should provide around 50-60% of an average household’s average hot water consumption.

What does it cost?

A typical solar water heating system costs about £3,000-£5,000 to retrofit into an existing home. For self-installed systems, costs can fall to around £1,500-£2,000, though you may become ineligible for financial incentives (see below). Larger systems cost slightly less per unit of output.

How long does it last?

The average solar hot water system is likely to last over 20 years and, like solar PV, requires very little maintenance.

Where should it go?

The solar panels need to face south-east to south-west (though within about 15° of south is best) with an optimum elevation of 40° – although between 15-60° is acceptable.

It is still possible for the panels to work if they are in a bit of shade, however, output will be greatly reduced. You should ensure, as much as possible, that the panels will not be overshadowed by trees or other buildings.

When considering installing solar water heating to a property it is necessary to consider its integration with the existing heating system – an installer will be able to advise on this. For example, combi boilers are not directly compatible because they do not use hot water storage tanks, therefore, some form of retrofitting will be necessary.

Financial incentives

Solar thermal systems are supported under the Government’s Renewable Heat Incentive (RHI).  The RHI pays a beneficial rate of 9.4 p/kWh for each unit of ‘green heat’ generated by non-domestic solar thermal, and is similar to the Feed-In Tariffs already in place for renewable electricity generation. Payments have been running since late 2011 (see separate Non Domestic RHI fact sheet).

The Renewable Heat Incentive for domestic properties was launched in April 2014. Again, the tariffs are paid per-kWh and are currently set 19.2p/kWh for solar thermal hot water.  For more information check out our downloadable Domestic RHI fact sheet.