​Solar thermal panels

As the British public look for alternative ways to reduce their energy consumption and the associated costs, increasing numbers of people are turning to the sun as a free source of energy. Despite the UK’s typically inclement weather, households can still harness the power of solar energy to contribute towards heating their hot water.

Inner workings

Solar thermal heating systems use solar panels to absorb the sun’s heat, which is then transferred to the water for use in homes. Although these systems will continue to provide hot water during the colder months, the output is considerably less than during the warmer summer months.

Although solar thermal panels look similar to solar photovoltaic (PV) panels, they carry out different tasks. PV panels are used to produce electricity, which can be diverted to heat water. However, solar water heating is more efficient.

“Solar thermal technology works in conjunction with conventional water heating systems to reduce the amount of energy needed to bring the water up to temperature,” explains Ghassan Beldawi, technical manager at Woodford Heating. “By using the heat absorbed by the panels to pre-heat the water in the storage cylinder, these systems can help save energy, while reducing running costs and carbon emissions.”

Solar water heating systems can either use flat plate panels or evacuated tubes, which collect solar energy and convert the infrared portion of light into heat. The collectors are filled with a water and glycol solution. This fluid is pumped round a circuit, which passes through the hot water cylinder.

In direct or open-loop systems, the water heated by the solar panels goes directly into the domestic hot water cylinder. However, these are rarely used in the UK due to the risk of freezing or overheating. Therefore, most solar water heating systems in the UK use the indirect method where the heat absorbed from the sun is transferred to the water in the hot water cylinder via a copper coil.

Although these systems work all year, water will still need to be heated further with a boiler or immersion heater, particularly during the colder months. According to the Energy Saving Trust (EST), during the summer these systems should provide around 90% of a household’s hot water requirements, dropping to around 25% in the winter.


As with any piece of equipment, solar thermal heating systems will require regular maintenance, such as checking the pressure gauge to ensure there are no leaks.

Most installation companies offer an annual service check, but it’s advised that systems are thoroughly checked every five years. This must include the draining and flushing of the system and replacement of the fluid because the anti-freeze protection offered by the glycol will reduce over time, leading to inefficient operating.

There are other components within a solar thermal system, such as the pump and valves, which must be well maintained to ensure equipment is in good working order. Regular servicing will not only minimise running costs and optimise efficiency, but maximise the life span of the system too.

Untapped resource

Although conventional boilers and hot water cylinder systems are often compatible with solar water heating, the significant potential of hot water storage has been overlooked in the Heat and Buildings Strategy.

Energy storage technologies can play an integral role in the zero carbon strategy, yet hot water cylinders have not been recognised as a resource to help the green infrastructure in the UK.

Isaac Occhipinti, director of external affairs for the Hot Water Association, said: “If the government is serious about decarbonisation then we need to encourage homeowners, at the very minimum, to keep their hot water cylinder in order to future proof their heating system and maximise the UK’s energy storage potential.

“In addition to meeting multi-outlet demand, storage systems are essential partners to any renewable energy input as these sources need to be harvested and stored. Hot water storage is the only practical solution to turning the energy into something useful and banking it for when it needs to be used.”

Despite the potential positive impact hot water storage technology can have in helping to reach government targets, the problem lies in encouraging property developers to create the space for them.

Mike Foster, chief executive officer of the Energy and Utilities Alliance, said: “As we move to an increasingly more thermally efficient new build, the space heating load reduces from its current levels, but our hot water demand remains constant. Hot water is often overlooked in the debate around net zero. Hot water cylinders can utilise renewable solar, renewable wind and yes, work with clean gas too. That versatility needs to be recognised.”

Good to know

  • A boiler or immersion heater is needed to supply hot water through the winter and to supplement supply on cloudy days.
  • Installing an average solar water heating system costs £3,000 to £5,000.
  • Solar water heating systems are not always compatible with combi boilers.
  • Gravity-fed heating systems will require an extra valve or pump.
  • A solar-compatible water cylinder, which is large enough to hold around two days of hot water, will be required.

Find out more

For more information, visit: energysavingtrust.org.uk

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