Solar PV Diverters

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. Note that a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed to start in 2026. In addition, government consultations indicate future changes to the format of EPCs and an increase in effective minimum rating levels required for private rental properties. Methods for improving EPC ratings will therefore change in the future.

Solar Diverters enable the use of excess power generation from solar photovoltaic (PV) panels to heat up water stored in a hot water cylinder, rather than be exported back to the national grid.

This should mean your hot water cylinder will need less contribution from your boiler or immersion heater in order to heat up the water within it, thus saving energy and money.

The myenrgi eddi is a well known example of a solar diverter:

The presence of a solar diverter could not be recorded in EPC assessments of existing dwellings prior to 15th June 2025.

Under the new RDSAP 10 methodology, solar diverters in existing dwellings can now be recorded, however there are limitations.

The RDSAP 10 Specification document directs us to section G4 of the underlying SAP 10.2 document to see how solar PV diverters are handled in assessment calculations. There, it states:

“For optimum performance, hot water cylinders may be larger than typical practice. To maximise the diversion of electricity, a dwelling with a PV diverter may store water at a higher temperature than typical practice, also increasing cylinder heat losses – this is reflected in a correction factor of 0.9.

For a PV diverter to be included in the EPC rating calculation:

1. A solar photovoltaic system must be present and connected to the dwelling’s electricity supply.
2. There must be a hot water cylinder with a volume greater than the calculated average daily hot water use
3. There must be no solar water heating system present [ed: i.e. Solar Thermal]
4. There must be no battery storage system present"

So for example if you also have a Solar Thermal system, or a Home Battery or a hot water cylinder that is deemed too small, then the solar diverter will not contribute to the assessment calculation.

During the assessment of an existing dwelling where a PV diverter is present, the energy assessor will take photographs of the PV diverter and related documentation the homeowner has, then select the option to indicate the presence of the diverter - no details of the diverter are otherwise required in the software. The assessor however needs to comply with part of Convention 9.05 that reads:

"For PV diverters, written evidence of manufacturer and model of diverter system installed is required. If this cannot be obtained, then the diverter cannot be entered [ed: in the assessment]"

I notice that the words 'photographic or' are missing before the word 'written' in that paragraph. Those words are present in the corresponding paragraph about the recording of home batteries.

This leads me to wonder whether a photograph of a PV diverter with the make and exact model evident on the device or on an attached product label, would be sufficient in itself in the case of an EPC audit, or not.

Either way, if you have a PV diverter and you want it recorded in the EPC assessment, locate any documentation you have and present that to the energy assessor when they arrive.

For domestic EPCs, the headline EIR rating is calculated from the financial cost of running that property over a year (lighting, heating, ventilation, hot water). This is related to the floor area of the property through a couple of equations to end up with the rating.

The EPC methodology for existing dwellings was updated to RDSAP 10 on the 15th June 2025. The fixed fuel costs used within the methodology to calculate the EPC rating were kept the same as used previously under RDSAP 9.94, rather than being updated to those used since 2022 in the newer underlying methodology, SAP 10.2, for new-builds.

An advantage of that decision is that it keeps new EPC ratings for existing dwellings more consistent with previously issued EPCs (RDSAP ones).

Here are the costs used for the most common electricity tariffs:

For reference, the cost used for mains gas is 3.48 p/kWh.

The value of solar PV generated electricity consumed within the home is considered by the methodology to have the same value as single rate electricity drawn from the grid, or a weighted average of the low and high rates for dual rate electricity.

For existing dwellings, the financial value given to excess solar electricity exported to the grid is still equal to the cost of single rate electricity drawn from the grid.

There is therefore no/little percieved value differential between exporting excess solar to the grid and drawing it back again later, versus using that solar power in the home straight away.

These things all contributed to the observations about Home Batteries on our Solar PV and Home Batteries page, and will also contribute towards how Solar PV diverters are valued in EPC assessments of existing dwellings.

The situation is different in SAP (for new builds) where there has been a differential since 2022 between the value of exported excess solar generation, versus the cost of single rate electricity drawn from the grid.

An energy assessor has to manually select 'Addenda 15' if solar PV is about to be recommended on an EPC that they are lodging.

This results in the following text being added to the final EPC:

“When considering the PV installation consider installing PV battery and a PV
diverter for water heating”

A recommendation for a PV diverter is available for display on EPC certificates of existing dwellings.

Referred to as Recommendation 'U' in the documentation, it is considered when:

• PV is present, and
• PV battery storage is NOT present, and
• Solar water heating is NOT present

This recommendation is triggered when:

• A hot water tank is present

The recommendation would improve the assessment by:

• Add a PV diverter

Supporting information advises that a recommendation is made on an EPC only if it increases the SAP rating by at least 1 (one) SAP point, or 0.5 SAP points in the case of cylinder insulation, draughtproofing and Low Energy Lighting (LEL).

I'm interested in the effect a Solar PV Diverter would have on the EPC rating of our Case Study 1 property.

This is a 1950s mid-terrace house with a gas combi boiler. This would not be suitable in it's current form for testing with a PV diverter because there is no hot water cylinder.

I have therefore changed the configuration of space and water heating in the assessment of this property to make it suitable for a PV diverter to be added.

For the space and water heating this property now has:

• Ideal Logic Max System2 boiler (BRE ref: 19023) with radiators, thermostat, programmer and TRVs
• No forms of Secondary Heating (as before)
• A 'Large' hot water cylinder (i.e. greater than 170 litres)
• Water heating primarily provided by the Ideal boiler above
• Foam insulation present: 50mm thickness
• Cylinder thermostat present

For the solar we have:

• 4kWp solar capacity - single array
• 45 deg elevation
• South facing
• Overshading: Little or none
• Single rate, Smart, electricity meter
• Solar array connected to the electricity meter

The new baseline of this property with the above heating configuration, but without solar PV and without a diverter = 72C.

The results I get by adding 4kWp solar and then a diverter, whilst having an export capable electricity meter are:

So with just the 4kWp solar array added, I get a 21 SAP point increase from 72C to 93A. This is a level of increase that I would expect, having modelled the presence of solar PV on our case study page for this property.

Note that in this configuration there are no Recommendations present for a PV Diverter or a PV Battery.

But then after additionally including a solar diverter, I get an 8 SAP point reduction from 93A to 85B.

This would be mildly disappointing to a homeowner.

I suspect the rating is lower because the methodology considers it has lost the financial benefit of some excess solar which it previously exported at a rate of 13.19p/kWh. It is also using that electricity now to heat hot water, displacing some use of the gas boiler which would have operated at 3.48p/kWh, therefore resulting in an overall higher financial cost. 

Let's try that again, this time with an electricity meter that is not export capable:

With a non export-capable meter, our rating only rises by 7 SAP points from 72C to 79C when we add the 4kWp solar array. This is much less than the 21 point increase we had above with an export capable meter, because any excess solar generation cannot be exported to the grid, and is simply 'lost', or rather not utilised.

EPC Recommendations are present in this configuration for both a PV Diverter and a PV Battery.

When we add the diverter this time, some of that 'lost' excess generation appears to be used and the rating rises a further 3 SAP points from 79C to 82B.

This sounds understandable, albeit the ratings are low.

In practice, I think electricity meters would almost always be export capable when a solar PV array is present, so generally the observed behavior would be as per the first table here, for this type of boiler, hot water cylinder etc.

Next, to test whether changing the size of the hot water cylinder has the expected effect that is described in the methodology documentation, i.e. that if the cylinder capacity is not 'larger than average' useage, the diverter is not included in the rating calculation.

I change the size of the hot water cylinder and find:

Changing the HWC size to 'Normal' (from 'Large')

The diverter is no longer taken into account, the ratings with the diverter are the same as without the diverter.

Changing the HWC size to 'Medium' (from 'Large')

The diverter still appears to be included in the calculation and the ratings change as per the tables above.

So that behaviour confirms the description in the methodology documentation that a hot water cylinder size that is deemed too small will result in the diverter not being taken into account in the rating calculations. The threshold of what is considered too small will depend on each individual property of course, and will be dependent on things such as assumed occupancy which depends on floor area and other factors.

There is also an option in the assessment software to specify the exact size in litres of the hot water cylinder. By entering figures into this field we find:

Cylinder Size >> Volume Known >> 1 through to 500 litres

Any value entered seems to make no difference to the EPC ratings. The ratings reflect those in the tables above without a diverter present. It seems the diverter is not being taken into account in the calculations, despite very large numbers entered into this field.

I wonder if this latter behaviour here is a fault in the assessment software I use?

The next step I want to try is the same as above but with a main space-heating system that is more expensive to run than mains gas. Choices include Storage Heaters of different types, or even the worst case scenario of electric panel heaters.

I have chosen the worst cost-case of electric panel heaters, but with programmers and room thermostats. In summary:

For the space and water heating this property now has:

• Electric panel heaters, with programmers and room thermostats
• A 'Large' hot water cylinder (i.e. greater than 170 litres)
• Water heating provided by dual immersion heaters
• Foam insulation present: 50mm thickness
• Dual rate, Smart, electricity meter (dual rate due to the dual immersion heaters)

For the solar we have:

• 4kWp solar capacity - single array
• 45 deg elevation
• South facing
• Overshading: Little or none
• Solar array connected to the electricity meter

The new baseline of this property with the above heating configuration, but without solar PV and without a diverter = 49E.

The results I get by adding 4kWp solar and then a diverter, whilst having an export capable electricity meter are:

In this case we have a 21 point increase followed by a 6 point decrease.

The changes in EPC rating are similar to the example above where we had a gas system boiler, although the absolute ratings are lower.

Let's try again with a non export-capable electricity meter:

The pattern with the non export-capable meter is similar again, although adding the diverter gives a larger increase than the example with a gas boiler.

The next step is to check again with using Normal/Medium/Large cylinder sizes and also entering in a direct figure for the cylinder size in litres.

My young daughter has just nabbed my iPad though......

I have gaps in my knowledge that limit a full understanding, but my current thoughts are:

For our Case Study 1 property, I would currently be wary of fitting a PV diverter. The EPC ratings under the current methodology (for existing dwellings) appear not to favour the presence of them, and I'm not sure whether it would actually be financially advantageous alongside a regular solar installation where you are able to export all of your excess generation.

The vast majority of properties in the UK that have solar PV installations will also have export-capable electricity meters

I'm not considering CO2 emissions here at all by the way, we'll come back to that shortly. I'm only considering financial cost at the moment, which is the consideration used in the calculation of EPC ratings for dwellings in the UK.

Clearly, if your DNO imposes a limit on your export, and you are able to generate more than that limit, then in the real world it would be better to use that additional excess-generation to heat hot water rather than letting it go un-used.

The RDSAP 10 methodology however does not have the facility to record a DNO export limit, and appears to consider all excess solar generation as exportable to the grid.

In my own home, I have a gas combi-boiler, and I'm still on a dual rate electricity meter for historic reasons. I don't have a hot water cylinder, or solar PV currently, but if I did have both (and a system boiler rather than a combi-boiler) and my solar capacity was lower than my DNOs export limit, I would probably just export any excess solar generation and use the gas boiler to heat a hot water cylinder rather than use a PV diverter. This would be for cost reasons.

In the real world, at the time of writing in July 2025, Octopus would pay me 15p/kWh for exported solar, and I would heat up the hot water cylinder using mains gas at a fuel cost of 6.2p/kWh at any time of the day (prior to consideration of cylinder losses etc.).

Alternatively, if I had purely electric hot water heating using a hot water cylinder with dual immersion heaters, perhaps due to having electric space heating as well, it may still be advantageous with a regular solar PV installation to export excess solar during the day (Octpus: 15p/kWh) and heat up the water cylinder at night from grid electricty - my current night rate is 12.5p/kWh, and if I had an Electric Vehicle (EV), Octopus' rate would be as low as 7-8p/kWh.

The RDSAP 10 methodology uses fixed fuel costs that have been used for many years, and the values are relatively low, compared to current real-life costs. This is fine in itself, as it is the comparative costs between fuels that is more important, and the fixed costs have helped retain a level of consistency in EPC ratings of existing dwellings over the years.

It does however consider the value of exported excess solar generation at the same rate as imported single rate electricity, which in itself limits any financial advantage in the model from using excess solar generation in the property  rather than exporting it to the grid and drawing it back in again later.

The government have issued a couple of Consultations recently involving the future format of EPCs and the suggestion that the single cost-based metric used currently could be replaced by perhaps three or four separate metrics instead, each measuring different aspects of a dwelling.

This could include such metrics as a Smart Readiness metric, an Emissions metric, the existing Cost metric and a Building Fabric metric. It's possible that an emissions metric or a smart readiness metric could benefit from having a solar PV diverter present. Time will tell, once the future direction becomes clear, and any calculation criteria for future metrics is declared.

Before we finish this page, as I mention on every page, always consult a local Domestic Energy Assessor (DEA) before making any changes to your property with the intention or expectation of improving your EPC rating.

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