EPC calculations - criteria

Does anybody know the criteria for how EPCs are measured.
I was wondering if the criteria had changed as new technologies for energy efficiency have become available.
There have been a couple of things lately that have made me wonder - I know at least one other member wondering about EPC calculations that don’t add up !! @Ann_Kolodziejski
Is there a calculator that the public can access or do we know of any changes.

Thanks in advance.

EPC’s are really guesswork but officially they follow the rdSAP formula, which I believe is in the public domain.

SAP is used by architects to pass regulations and give themselves a pat on the back. However SAP doesn’t include any cross reference to the actual building designed and built.

The “rd” refers to reduced data” and these are partially specified in the regulations and partially guesswork by the assessor. All guesses should be declared, such as “Solid floor. Uninsulated (assumed)”. The biggest guess is that the building met Building Regs at the time of construction.

I know of a couple of changes to SAP and hence rdSAP over the years. They are linked to updates to building regulations.

A suspicious number of rented properties meet the minimum EPC rating by one point. Either landlords are very good at hitting the mark or assessments are not entirely independent.

EPCs are the start of a narrative – how does the story continue? - UKGBC - UK Green Building Council?

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I’m glad you posted this !
I am currently writing to Carbon Coop to raise this.
The EPC system seems to have irregularities, which will put many at a disadvantage through inaccurate ratings if they are getting it wrong.
What is the criteria for measurement?
Who is authorized to carry out an EPC on your home.
How is it regulated?
One thing is clear, any miscalculations will impact on the statistics about carbon reduction in homes.
Misleading the public and setting us up to fail in meeting net zero.

The link Tim shared for us strongly recognizes these flaws against our progress rooted in inaccurate EPCs.

Here are two examples EPC rating experiences from within our community here on the forum.

How do they add up for you ?

Cases in point 1 - my home originally an E rated EPC, is now suddenly a D after a new assessment with the Green Grant Assessor. from EON - the only measures we have taken in the two years we have been here has been to renew the loft insulation.

Currently based on our income and benefits, we seem to qualify for the Green Grant and chose the heat pump - yet this may end up oversized if it’s fitted before our other fabric measures (triple glazing and insulation in the lean to roof.) are taken into consideration.

Additionally if we have our fabric measures completed before the heat pump assessment, it will no doubt boost our EPC rating up to a C - this will disqualify us from the green grant for the heat pump.
This scenario will leave us unable to have a heat pump - we won’t have the funds - we won’t be able to move away from fossil fuel - we’ll be prevented from doing our bit for the planet.

Sizing and fitting a heat pump before we have our fabric installations, could leave us with a pump 2kw oversized - an inefficient and costly heat pump !! Which ever way we look at this we see potential to put us at financial disadvantage. We will certainly struggle to absorb more financial hits after the impacts Covid has already had on us.

**Case in Point 2

UPDATE: Had an EPC done on my house because the previous one was over 10 years old. Okay, so my house had been rated at ‘F’ in 2009 and in 2021 after having floor, loft and cavity wall insulation, new doors and windows, airtightness work and MVHR unit installed, low energy appliances and lighting, my house is now rated at ‘F’!!! Yep, those measures that I have undertaken seem not to have made a jot of difference to the EPC rating.

Highly irregular are my thoughts.

Does anyone one else have experience of EPC ratings which don’t add up ?

This is a typical quandary and not just for those seeking grants. In reality there is no ideal time to fit a heat pump. I am currently running my boiler at 50°C while I undertake improvements, with the aim of getting the most out of the boiler before fitting a heat pump when they become mass market commodities.

This requires justification from the EPC surveyor. Their name and phone number appear on the certificate. Without details it is impossible to say but I would guess that the house must be around C. I once had a survey which resulted in suggested improvements of PV and solar thermal. I have both already and pointed them out to the surveyor.

This is my new EPC - completed by the surveyor who Eon sent to us.
It makes interesting viewing as it shows what the improvements will gain us if we have more insulation.
We’re still waiting for a heat pump installer to contact us


This is our original EPC


The EPC certificate doesn’t include the practicalities of completing a task, just a very vague price range.

Not many people, particularly in cities, could fit both solar thermal (ST) and PV as they wouldn’t have sufficient roof space for 2 useable systems. Many new developments have one solar panel to pay lip service to PV. I don’t count that as practical but is does tick an EPC box.
Then you need to ask whether ST and wet system heat pump are compatible, particularly with PV. On warm sunny days ST will give plenty of hot water but on those same days the COP will be good and PV would power the system. Conversely, on cold winter days there would be negligible ST, negligible PV and much lower COP. In effect the heat pump would not be needed when most efficient.

You echo my thoughts there Tim.
Although on balance, if I can safe in sunnier times for winter that might mean we break even financially while moving away from fossil
This would be acceptable to us

Maybe end summer holidays and go away when the house is most expensive to run?

That definitely sounds like a plan :raised_hands:t3:

Here is an interesting EPC story- In 2004 the rating on a cottage was D and as the house was put on the market in 2021 it has a new rating of F. Given the upgrades to house and heating how is this possible?

It’s crazy. There doesn’t seem to be any rhyme or reason to it… It’s no wonder that people think the green grant and retrofit is such a risky road where you don’t know who to trust.
You literally have to be a scientist to understand it !! People have never felt more monopolised.and stressed by government decisions and the ignorance that has brought us to this situation, which is the tip of the iceberg when you weigh up the bigger picture.
…Just read back over this and realised I just had a right old moan there !! :sweat_smile:

It is interesting that a system based on assumptions that are so easily proven to be wrong is so difficult to correct .- in fact there is little obligation to correct or acknowledge mistakes!

You said it all there Lloyd…
I think I say the same in many more words :sweat_smile:

The EPC rating is based on rdSAP (rd = reduced data = guesswork). Every release of SAP moves the goal posts as the criteria become more stringent. With so much time passing even hypothetical prefect tests on the same unaltered property would change. The latest version of SAP was released a few months ago.

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So this basically means that as materials and technology improve to be more carbon efficient etc our homes EPCs can up as well as down . And I know you don’t have the answer to this question, but I’m wondering why they don’t assess double glazing on the green grant scheme to replace if failing.
Just one example of the many I guess… :roll_eyes:

As the criteria become stricter houses will get worse results over time, never better.

No government pays householders grants for maintenance tasks, so failed window units or flattened loft insulation are not covered.

The government is again (still) assessing the EPC system. The aim is to make it reflect the real life energy usage of a building! Deja vu or what?

Has anything changed since this thread was last used? I doubt it.

What would members of this forum like to see changed?

The main factor against meaningful change, as far as I can tell, is that the EPC is supposed to be cheap. A proper analysis on a property by property basis will take hours, including the off site element. Time is money, as they say, and if the price rockets there will be an outcry.

I came across this thought provoking table on another site.

My guess is that people living in EPC G rated homes don’t heat the whole property, which skews the result at the bottom on the table but the rest is interesting too.

See also

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