How Are EPC Ratings Calculated?

By Ian on Thursday 21st December 2023

Cold home that requires an energy assessor

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In the UK an Energy Performance Certificate (EPC) is needed whenever a home is built, sold or rented. The certificate will give your property a rating, but how does your assessor arrive at this result? What aspects of your home will affect the EPC rating and what can you do about them? As EPC Assessors with years of experience working in properties across the North West, we’ll give you everything that you need to know about how your EPC result is worked out.

Table of Contents

What is an EPC Rating?

We go in to more detail about what an EPC rating is in another article. In short, once your EPC is complete, your property will be placed in a Banding from A through to G which reflects how energy efficient your home is. A banding of A is for the most energy efficient houses and a banding of G, the least. The reason for the banding is that it gives prospective buyers, tenants or even lenders a quick overview of the amount of energy this property uses. If the property has a Band G or F EPC, it cannot be rented.

What Factors Affect the EPC Score?

To get an EPC, you need a qualified Domestic Energy Assessor (this is us!) to carry out an assessment of your home. The purpose of this assessment is to gather information about the house and how well it uses and retains energy.

First up, we need to know about the house itself. What is it made from? How old is it? Is it a terraced, semi or detached home? How big is it? This information can be vastly different from home to home and it’s an important step to get right.

Next we look at how the home is insulated. There are three main areas of insulation in a home. We’re looking for 270mm of loft insulation (current building regs) in the loft without any compression. Wall insulation in the form of either solid or cavity wall insulation. Floor insulation is also an important aspect. Improving any or all three of these areas will improve your final EPC banding.

Once we understand the envelope of your home, we can look at what’s inside and how this contributes to energy usage.

Heating systems are one of the biggest factors when calculating an EPC banding. The difference between an old inefficient boiler and a new efficient one can be as much as two bands! Your controls are also an important factor too. A thermostat, programmer and TRVs will get you top marks for your controls. Your EPC Assessor will also note any fires you have and the fuel they use.

Although it doesn’t cause a huge swing in results, having energy saving light bulbs is an easy way to gain a tick or two. LEDs or CFLs use much less energy than traditional light bulbs and sometimes it’s the difference between a D and a C band.

Finally, we look at any renewable sources of energy you may have. These generally have a decent positive impact on your EPC rating. The most common forms of renewable energy we encounter are:

  • Solar panels (PV)
  • Solar water heating
  • Air source heat pumps (ASHP)

How is the EPC Score Calculated?

Once we’ve collected all the information, we put it all into software called RdSAP (Reduced Data Standard Assessment Procedure). This software takes all the data and gives us back a certificate with an EPC rating. Your assessor is not in control of the score you get (not even for an extra fiver). The software does all the calculations based on what we’ve assessed at your property.

It is important to note here that the assessment does not take in to account your actual energy usage. We don’t look at any bills or read any meters as part of the survey. A standard energy usage is applied to every home assessed that levels the playing field to provide the actual efficiency of the home, rather than the habits of its occupants.

Part of this calculation process looks at how your home could benefit from improvements. Each EPC comes with 3-5 recommendations that will improve not only your EPC score, but its energy efficiency too. These are specific to your property.


Each property for sale or rent in the UK needs an EPC. The EPC will rate your property on a scale of A-G based on how well it uses energy. Your EPC assessor will survey your home and collect important information about your building and its energy usage. We’ll look for details about the building, its insulation and things like boilers, heating controls lightbulbs etc. This information is input into a software program which automatically calculates the result and generates the certificate!

About the Author

About Ian Kay

Ian is a seasoned energy assessor helping his customers, reduce energy usage and save money on their bills. He holds qualifications from both City & Guilds and ABBE for Domestic EPCs and Commercial EPCs (Level 3 NDEA and Level 4 NDEA). With a background in the building trade he can provide his customers with a unique perspective and advice. Ian combines his deep industry knowledge with practical advice to create blog posts that help visitors understand the complexities of energy performance certificates and reducing operating costs. When not immersed in the world of energy assessments, Ian enjoys exploring the great outdoors and spending quality time with his family.

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