What are solar thermal panels?

Solar thermal panels differ from solar photovoltaic (PV) panels in that they are used to provide hot water, which can also be used with radiators to heat internal spaces. They are generally cheaper to buy and install, and have a shorter payback time.

However, the efficiency of thermal panels varies during the year; while they are around 70 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. However, 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. 

Homeowners may be more attracted to solar thermal panels than solar PV panels because thermal panels are more efficient. This is because heat waves carry more energy than sunlight. Additionally, no energy is lost during the generation of heat energy, where there is when converting the DC electricity generated by PV panels to AC electricity that is used in homes. 

Furthermore, solar thermal panels tend to be cheaper than solar PV panels, so the payback time is shorter.  

Solar thermal panels operate well in colder climates and overcast weather and are resistant to strong winds. They also tend to include an energy storage system. Although they come with a five to 10 year warranty, they normally last longer than the normal 25 years for solar PV panels. 

They also require little maintenance, but, as with solar PV panels, they work best on a south-facing roof in locations with little or no overhead shading. You can also install them as ground-mounted systems, provided they are not subjected to any overhead shading. However, they are not compatible with electric showers or cold-fill washing machines or dishwashers. 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. 

It may 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.

It is also worth bearing in mind that, as with solar PV panels, the amount of sunlight varies throughout the year, so this means that solar thermal panels will not cover all your hot water needs. It may be necessary to retain a conventional boiler or immersion heater as a backup system because although solar thermal panels can provide up to 90 per cent of your hot water during the summer, this is likely to fall to just 25 per cent during the winter.

Although solar thermal panels are low maintenance, your installer should provide guidance regarding any maintenance tasks you may need to carry out. This includes checking the pressure gauge in case there are leaks and monitoring the control panel for any warnings if you are not receiving hot water or the pipework is cold. Some installers conduct an annual service check, which takes about 10-20 minutes. Additionally, it is a good idea to have the system checked thoroughly every five years. This typically entails draining and flushing the system and replacing the fluid, given that the protection delivered by the antifreeze will reduce over time. The efficiency of the system may decline if this five-year check isn’t carried out. 

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. 

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. 

The main types of solar thermal panels

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. 

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 solar PV 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.

How can I reduce the cost of solar thermal panels?

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. 

Will solar thermal panels reduce my bills?

UK government research has found that the installation of a solar thermal system will reduce household water heating bills by around 50 per cent.

In January 2023, given the gas price of 10.3p per kWh, householders installing a solar thermal panel system could expect to save £75 per year, based on the supply of 730 kWh of hot water from the system. 

For a while, householders could expect to save between £145 and £275 per year under the Energy Price Guarantee scheme, but this was discontinued in June 2023. 

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. 

Despite this, the payback time for solar thermal panels is much shorter than that for solar PV panels.

This means it really depends on your motivation for installing a solar thermal panel system. If you want a system with a short payback time, installing solar panels may be worthwhile. But if you are focusing purely on energy bill savings throughout the year, it probably isn’t given that the savings will decrease considerably during the winter. 

A reliable solar thermal installer should be covered by the MCS. It is worth contacting local 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.