Solar Thermal Panels

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 9.94. This updates to RDSAP 10 on 15th June 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consulations indicate future changes to the format of EPCs and minimum rating levels required for priate rental properties. Methods for improving EPC ratings may therefore change in the future.

Solar Thermal panels absorb energy from the sun to heat up a liquid which is then circulated through your domestic hot water cylinder and heats up the water stored there.

The water stored in the cylinder therefore does not need to be heated up so much by your main heating system or any immersion heater which would otherwise have been used exclusively instead.

This results in a saving of energy and therefore cost to the home owner. With EPC ratings being entirely cost-based at the moment, the EPC rating of the property would normally be higher as a result.

Solar Thermal is a different technology compared to Solar Photovoltaic (PV)  which we cover seperately on our Solar PV & Home Batteries page.

Wheras Solar Thermal helps heat up water stored in a cylinder, Solar PV generates electrical power which can supply loads in the dwelling and any excess can be stored in a home battery or exported to the National Grid.

The roof above is one that I walk past four times a day during school drop-offs and pick-ups.

A solar thermal panel can be seen in the centre of this roof. Normally just one or two such panels would be present in such an installation.

This compares to the two blocks of three panels either side, which are solar photovoltaic panels.

Finally, the two others rectangles in the roof are Velux style windows.

Solar thermal has largely fallen out of favour in recent years to Solar PV which is more flexible in that the generated electricity can be used for a variety of different uses as mentioned above.

In addition, excess electrical power from solar PV can be used with a solar diverter to heat up hot water in your cylinder, rather than using Solar Thermal.

Here, I model the presence of a Solar Thermal system on my own property and model the improvement in EPC rating that I could get. I have a 1950s mid-terraced house, which we cover in our Case Study 2 page.

So, I would get an improvement of 2 SAP points on my EPC rating by having Solar Thermal installed.

It would cost a lot of money, and involve the installation of a hot water cylinder which I don't currently have, thus losing some space in the house, and probably pushing me into having the roof re-felted and re-battoned and the Finlock guttering removed at the same time.

The estimated cost of just the solar thermal recommendation on my EPC is £4,000 to £6,000 which is a significant amount of money. I would probably choose to spend that money on a Solar PV installation instead because I would get a greater SAP points return for the money and I would probably consider the generated electrical power to be more flexible.

As is always the case, contact a local Energy Assessor to advise you on the potential EPC rating of your specific property before undetaking any improvement measures.

A recommendation for Solar Thermal can appear on an EPC if the measure would improve the EPC rating enough to be included. This appears to be the case most of the time.

In the Recommendations list it is referred to as 'Solar Water Heating' and the conditions required are:

• Property type is House or Bungalow (not thatched roof on main dwelling)
• No Solar Thermal system present

The rating indicated after the measure has been installed is based on a Solar Thermal installation with parameters as indicated in table S18 of the RDSAP Appendix S document on the BRE website, but with a medium sized cylinder.

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.