Solar Thermal Panels

The current methodology for EPC assessments of existing dwellings in England and Wales is RdSAP 10. The Government plans to introduce reformed EPCs based on the Home Energy Model (HEM) in the second half of 2027.

Changes to EPC metrics will also affect how property energy performance is reported and how future Minimum Energy Efficiency Standards (MEES) are assessed. For privately rented homes in England and Wales, the Government has set 1 October 2030 as the compliance date for the new MEES standard.

These changes mean that the measures needed to improve a property's current EPC rating may not necessarily be the same as those required to meet future standards. The guidance on this page relates primarily to the current RdSAP methodology.

On this page we will learn:

✔ What solar thermal panels do and how they differ from solar photovoltaic (PV) panels.
✔ Whether solar thermal could be suitable for your property, including the importance of roof orientation, shading, a suitable hot-water cylinder and compatibility with your existing heating system.

✔ How a solar thermal system works, including how heat collected from the roof is transferred to the domestic hot-water cylinder.
✔ How solar thermal is assessed under RdSAP 10, including the information that can be recorded about the collectors, solar pump and hot-water store.
✔ What happens when details of an existing solar thermal installation are unknown, and how RdSAP's prescribed default values are used.
✔ How much solar thermal could improve an EPC rating, including a worked example showing the effect of different levels of information being entered into the assessment.
✔ Why documentary evidence can be important when an existing solar thermal installation is included in an EPC assessment.
✔ When RdSAP 10 may recommend solar water heating as an improvement measure.
✔ Whether solar thermal represents good value as an EPC improvement, including how the potential EPC benefit compares with the cost and practical requirements of installation.

Solar Thermal panels use energy from the sun to heat water for use in the home. They are usually installed on a roof and connected to a hot-water cylinder, reducing the amount of energy required from a boiler or immersion heater.

Unlike solar photovoltaic (PV) panels, which generate electricity, solar thermal systems are designed specifically to provide domestic hot water.

Solar thermal can reduce the energy required to heat water and may improve a property's Energy Performance Certificate (EPC) rating. However, the benefits depend on the existing hot-water system, the property's characteristics and the way the installation is represented in the EPC assessment.

Is solar thermal a worthwhile way to improve your EPC rating? That depends on the property. A system might increase the EPC score, but the cost of installation, the need for a suitable hot-water cylinder and the availability of alternative technologies all need to be considered.

This guide explains how solar thermal works, how it differs from solar PV, how the technology is treated in an RdSAP 10 assessment and how much difference it might make to an EPC rating.

It also includes a worked example based on my own property to illustrate why the potential improvement in an EPC score needs to be considered alongside the cost and practical implications of installation.

Solar thermal and solar photovoltaic (PV) panels both capture energy from the sun, but they use it in different ways.

Solar thermal panels heat water. A liquid circulates through collectors exposed to sunlight and transfers absorbed heat to water that's stored in a suitable hot-water cylinder. This reduces the amount of energy that must be supplied by the property's other water-heating equipment.

Solar PV panels generate electricity. This electricity can power household appliances and other electrical equipment. Depending on the installation, surplus electricity can also be stored in a home battery, exported to the electricity grid or diverted to heat water in a cylinder.

Solar PV therefore offers greater flexibility in how the generated energy is used. Solar thermal, by contrast, is specifically intended to provide hot water.

For a property without a suitable hot-water cylinder, installing solar thermal may involve substantial additional work. A household with a combi boiler, for example, will generally need a different arrangement to store and heat water if a conventional solar thermal system is installed.

If the principal objective is to improve an EPC rating, it is worth comparing the potential rating improvement from solar thermal with other available measures, particularly solar PV.

Neither technology should be selected solely on the basis of a predicted EPC score. The installation cost, the property's characteristics and the likely energy savings also matter.

See also our other pages: Solar PV and Solar PV Diverters.

The Energy Saving Trust puts the typical installation cost of solar water heating at around £5,300, although the actual cost varies with the system and installation requirements.

This roof provides a useful illustration of the difference between the two technologies. The central collector is a solar thermal panel, while the two groups of panels on either side are solar photovoltaic (PV) panels. The remaining roof openings are roof windows.

Solar thermal installations typically use one or two collectors, whereas a PV installation may cover a substantially larger area of the roof.

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.

A solar thermal system uses a separate heat-transfer circuit to collect heat from the sun and transfer it to the property's hot-water cylinder.

The solar collectors on the roof contain pipes through which a heat-transfer fluid circulates. This fluid is typically a mixture of water and glycol, which allows the system to operate in cold weather without the fluid freezing.

When sunlight heats the collectors, the heat-transfer fluid becomes hotter. A solar pump circulates the fluid through the collectors and back to the hot-water cylinder, where the heat is transferred to the stored domestic hot water through a heat exchanger called a heating coil.

The cooled fluid then returns to the collectors and the cycle repeats.

A controller monitors the temperatures of the collectors and the cylinder and operates the pump when there is useful heat available. The system also incorporates components such as an expansion vessel and safety equipment to accommodate changes in fluid volume and pressure as the system heats and cools.

The hot water cylinder also houses a second heating coil driven by the main boiler, and an electric immersion heater is common as a backup. Both of these have been omitted from the diagram above for simplicity.

Before considering Solar Thermal as an improvement measure, there are several practical questions to address.

Do you have a suitable roof?

Solar collectors need an appropriate location with good exposure to sunlight. Roof orientation, pitch and shading can affect the amount of useful heat collected.

Do you have space for a hot-water cylinder?

A suitable cylinder is an important part of a conventional solar thermal system. If the property currently has a combi boiler and no cylinder, installing solar thermal may require significant changes to the existing hot-water arrangements.

Is your existing heating system compatible?

The compatibility of your boiler and hot-water system needs to be established before installation. An installer should assess the equipment and advise on the necessary changes.

Would another technology be more appropriate?

Solar PV generates electricity that can be used for multiple purposes. Depending on the property, it may offer a more flexible way of using available roof space.

If your primary objective is to improve an EPC rating, consider the likely rating improvement alongside the installation cost, the practical requirements and the expected energy savings.

The Energy Saving Trust also recommends obtaining multiple quotations and using appropriately certified installers.

If you already have a Solar Thermal installation in your property, information about it is recorded during an EPC assessment.

The level of information recorded depends on the amount of evidence available of the installation, as follows:

• Details Unknown - Defaults are used by the methodology for all options
• Basic Details Known - Basic details can be evidenced. Default values are used for the remaining option.
• Solar Collector Details Known - If basic details of the system are already available, then additional details of the solar collector can also be entered, if documentary evidence is available
• Solar Store Details Known - If basic details of the system are already available, then additional details of the solar store (water cylinder) can also be entered if documentary evidence is available

Basic Details include:

• Collector elevation (pitch): Horizontal, 30°, 45°, 60° or Vertical
• Collector Orientation: 8 points direction – eg: South West
• Overshading: None or very little/Modest/Significant/Heavy
• Solar Pump: Unknown/PV Powered/Electrically powered
• Type of showers in the property: Non-electric only/Electric Only/Both/None
• Collector Loop Efficiency (if known)
• Incidence Angle Modifier (if known)

Solar Collector Details Include:

• Collector Type: Flat plate/Evacuated Tube/Unglazed)
• Aperture Area of Panel [m2]: 
• Collector zero loss efficiency: 
• Collector linear heat loss coefficient (a1):
• Collector 2^nd order heat loss coefficient (a2):

Solar Store Details Include:

• Is Solar Store Combined: Yes/No
• Total Hot Water Store Volume (litres):
• Dedicated solar volume (litres):

What happens when installation details are unknown?

RdSAP uses prescribed default values when the information required to describe an existing solar thermal installation is unavailable. These assumptions allow the assessment to proceed without the assessor having to establish every technical characteristic of the system.

These defaults are listed in Tables 29 (and 28) of the RdSAP 10 Specification document:

You can see for example that the overshading default is 'modest'.

To ensure you get the most accurate rating for your property, you should ensure you have all documentary evidence available about the installation of your Solar Thermal system so that your energy assessor can record that information in the assessment.

To illustrate the potential effect of solar thermal on an EPC rating, I modelled a system on my own property.

My house is a 1950s mid-terraced property with a gas condensing combi boiler. It currently has an EPC rating of 68D and does not have a hot-water cylinder.

We cover this property in our Case Study 2 page.

To model a plausible solar thermal installation, I first changed the heating and hot-water configuration in the assessment software. The following results therefore relate to this revised configuration, rather than my property's existing combi-boiler arrangement. The baseline represents the revised system without solar thermal, allowing the effect of adding the technology to be compared on a consistent basis.

I recorded the presence of an Alpha NXS 30 system boiler and a large 200 litre hot water cylinder with 50mm factory fitted foam insulation and a cylinder stat. The boiler controls are Programmer, Room thermostat and TRVs.

The following table shows the modelled results when we add Solar Thermal:

The modelling indicates an increase from 67D to 68D when we have a Solar Thermal system with unknown details. This is an improvement of one integer SAP point (or 0.66 SAP points). 

When we model known details from a specific installation I have encountered before, the rating improves to 70C. This is an increase from the baseline of 2.89 SAP points.

This is a useful illustration of how solar thermal can affect an EPC rating, but it also highlights a couple of important considerations:

• Would such an installation represent good value for money if you were to install one today?
• If you already have such an installation, it may be important to provide documentary evidence to your energy assessor to get the most accurate EPC rating.

In the case of my own property, it would need a suitable hot-water arrangement.

I currently have a combi-boiler so I would probably need a new boiler and space would need to be found for the additional hot water cylinder.

For me, the installation would therefore involve much more than simply fitting panels to the roof.

The Energy Saving Trust currently gives a typical installation cost for Solar Thermal of around £5,300. This is not a quotation for any individual property however, and property owners would need to get a quote from a suitably experienced heating engineer.

Given the likely expense and the relatively modest improvement in my modelled EPC score, I would be far more likely to install solar PV instead.

The result will differ between properties. If you are considering solar thermal primarily to improve your EPC rating, contact a Domestic Energy Assessor to model the likely effect before you commit to an installation.

RdSAP 10 allows a number of details about an existing solar thermal system to be entered into the assessment, including the collector type, aperture area, collector efficiency and heat-loss coefficients, as well as information about the hot-water store.

If these details cannot be established, the methodology uses prescribed default values instead. Where documentary evidence is available, it can therefore allow the installation to be represented more accurately in the EPC assessment.

This could include information such as the manufacturer's documentation for the solar collectors and hot-water cylinder, or other documentation that confirms the technical characteristics of the installation.

As the worked example on this page demonstrates, the information available about a solar thermal installation can affect the calculated EPC score. If you have a solar thermal system, it is therefore worth making any relevant documentation available to your energy assessor when the EPC is carried out.

Under RdSAP 10, the improvement measure is called Solar Water Heating.

The RdSAP 10 specification identifies this recommendation for houses and bungalows where the main dwelling does not have a thatched roof.

The measure is considered where solar water heating is not already present.

The modelled improvement uses the solar water-heating parameters specified by the methodology. It also accounts for the size of the hot-water cylinder, increasing a normal or unknown cylinder size to medium where applicable.

The rating shown after the recommended improvement is a modelled outcome based on the assumptions used in the assessment. It should not be treated as a guarantee of the score that a particular installation will achieve in practice.

The recommendation will only appear if it improves the EPC rating by at least 1 SAP point. 

(The solar panel is modelled with parameters as per Table 29 : Heating and hot water parameters in the RdSAP10 specification document).

• Solar thermal panels use energy from the sun to heat domestic hot water.
• A suitable hot-water cylinder and compatible heating system are important considerations when planning an installation.
• Solar thermal can improve an EPC rating, but the size of the improvement varies between properties and depends on the assessment inputs.
• RdSAP 10 uses prescribed default values when details of an existing solar thermal installation are unknown.
• Documentary evidence can allow an existing installation to be represented more accurately in the EPC assessment.
• Solar PV offers greater flexibility because the electricity generated can be used for a range of purposes around the home.
• The Solar Water Heating EPC recommendation in RdSAP 10 is based on defined model assumptions, so the rating shown after the recommendation should be treated as an indication rather than a  guarantee of the score a particular installation will achieve.
• If you are considering Solar Thermal primarily to improve an EPC rating, seek modelling advice from a Domestic Energy Assessor before committing to the work.
• Also, consider any potential rating improvement alongside likely installation costs and practical requirements.

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