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Solar thermal panels use the sun’s rays to provide hot water to houses – find out if they’re worth the investment
Solar thermal panels differ from traditional photovoltaic (PV) solar panels in that they are used to provide hot water (rather than electricity), which can also be used with radiators to heat internal spaces. They are generally cheaper to buy and install than solar PV panels.
However, the efficiency of thermal panels varies during the year. While they are around 85 per cent efficient during the summer, this falls to around 25 per cent during the winter.
Solar thermal panels are mounted on the roof of a building in the same manner as solar PV panels (although you can also install them as ground-mounted systems). Unlike solar PV panels, solar thermal panels contain liquid – normally a mixture of water, glycol and antifreeze – that is heated by the sun and then stored in a cylinder. This is then used to heat water in the building, which can also flow through radiators to heat internal spaces.
However, it may be necessary to retain a conventional boiler or immersion heater as a backup system. Solar thermal panels can only provide around 70 per cent of your hot water usage across the year. This is because their efficiency drops dramatically in winter. Solar thermal panels are also not compatible with electric showers or cold-fill washing machines or dishwashers.
It may also be a good idea to invest in a thermal storage system alongside the solar thermal panel system itself. Although it is still an emerging technology, it works in the same way as a battery connected to a solar PV system in that it stores heat for use at a later time.
As with solar PV panels, it is a good idea to speak to a range of solar thermal installers and have a Microgeneration Certification Scheme (MCS) certified surveyor inspect the property. The surveyor will measure the orientation and inclination of your roof and check for any problems from potential overhead shading. They may also provide you with an estimate of the solar thermal system’s output based on your geographical location and investigate your hot water consumption needs, which will probably be based on your property’s current Energy Performance Certificate (EPC) rating.
Installing a solar thermal system will require a small amount of electrical wiring as well as the assembly of the system and installation on the roof. You will also need to replace your existing water cylinder, so it may be an ideal time to consider solar thermal panels when your current cylinder needs replacing.
The system will be installed using frames on the roof that are attached with stainless steel brackets. There are several mounting methods, none of which should compromise the integrity of the roof. If an evacuated tube system is installed, it will include a heat transfer unit that is bolted to the frame. The tubes themselves will not be connected to this unit until everything else has been installed because they will start collecting heat immediately, and damage can occur to the heat transfer unit if the fluid is not present in the tubes and they are not at the optimal pressure.
Following the installation of the panels, the new water cylinder will be installed, either in the loft or in the upper floor of the property.
Given the variability of sunlight throughout the year, you may have to consider retaining a conventional boiler as a backup or installing a new one if necessary.
There are two types of solar thermal panels: flat-plate collectors and evacuated tube collectors.
Flat-plate collectors have a flat surface and look like solar PV panels. They consist of a dark absorbent surface, a transparent layer, an insulating backplate and the fluid. The absorbent surface can be made from a variety of materials. Copper is the most expensive, but it’s a durable conductor. Other materials include polymers, aluminium or steel. Polymers are often used in colder climates because silicon is more resistant to freezing than metal.
Evacuated tube collectors, often called vacuum tube collectors, consist of several glass tubes through which the fluid flows. These tend to be more efficient than flat-plate collectors, particularly in colder climates. While flat-plate collectors tend to lose heat, evacuated tube collectors resist heat loss. However, this puts them at risk of overheating in warm weather, which can affect their efficiency. In colder climates, snow falls between the tubes, thereby minimising the impact of snow on efficiency.
It is possible to install both solar thermal panels and solar PV panels, and a number of households do this.
There are also several hybrid solar panels coming to the market. These combine solar PV cells and solar thermal tubes. These panels are great for properties with minimal space on the roof, although they tend to be more expensive, as they are not yet a mainstream solar panel solution in the way that solar PV panels and solar thermal systems are.
Given that solar thermal panels are more efficient than solar PV panels (around 70 per cent efficient compared to 25 per cent), you will not need as many solar thermal panels on your roof as solar PV panels if you choose to install both systems
A three-bedroom house with three to four occupants would require a solar thermal system of 3 or 4m2, costing between £3,000 and £5,000, including installation costs.
As with standard cost of solar panels, the cost of a solar thermal panel system will depend on the size of the system, the manufacturer and model, and the amount of hot water needed.
Additional costs are minimal, mainly consisting of occasional maintenance requirements. A replacement pump, should you need one, will cost around £90, while new antifreeze, introduced during the five-year maintenance check, will cost around £100.
It used to be possible to receive a government grant for the installation of solar thermal panels under the Domestic Renewable Heat Incentive, offering homeowners 10.92p per kilowatt hour (kWh). However, this scheme was discontinued in March 2022, and its replacement, the Boiler Upgrade Scheme, does not offer any financial incentives for homeowners with solar thermal systems.
How much solar thermal panels will reduce your hot water bills depends how much energy you use, the price of gas, and the time of year.
A study by the Department for Energy Security & Net Zero in March 2024 showed that the median annual hot water usage in the UK is 2,281 kWh.
With a solar thermal system producing around 70 per cent of your hot water, that means you could save £87.50 a year.
Based on the July to September 2024 Ofgem gas price cap of 5.48p per kWh.
Installing solar thermal panels can generate significant savings on household energy bills. However, they are typically only used to heat water and internal spaces within the property, so many householders choose to install solar PV panels instead.
You would need to weigh up the initial costs of both forms of solar energy, and what option is practically best suited to your property.
Of course, solar thermal panels aren’t just a financial consideration. Installing such a system is a significant step towards making your home more eco-friendly by reducing your reliance on fossil fuels.
A reliable solar thermal installer should be covered by the MCS. It is worth contacting local solar panel installers rather than national ones, as local and regional installers are more likely to offer a better deal on installation costs.
Some householders might want to install a solar thermal panel system themselves. This will reduce the costs down to around £1,500-£2,500, but the big drawback is that a DIY-installed system will not be covered by the MCS and so will not be eligible for any future financial incentives or grants the government may introduce.
Yes. The liquid flowing through the solar thermal panels heats water, which can then be used to heat radiators.
Solar thermal panels work in winter, but they are much less effective given the variability of solar energy across the year. Their efficiency will reduce to 25 per cent in winter, as opposed to 70 per cent during summer.
Both thermal and PV solar panels are covered by permitted development regulations. Solar thermal panels typically do not require planning permission unless they are installed on listed buildings or in a conservation area.
