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Homebuyers need accurate evidence of their property’s actual energy use

Credit: Adobe

Sarah Kostense-Winterton, Executive Director

Sarah Kostense-Winterton, Executive Director | Mineral Wool Insulation Manufacturers Association

6 min read Partner content

Energy Performance Certificates in their current form are not fit for purpose. They need a revamp to capture the real measured performance of buildings.

Our home is the biggest purchase of our life and we spend many years, searching, planning and saving to buy a home (if we are so lucky). However, one area that is often overlooked in the house buying process is whether the energy use of the property reflects the actual energy bills we will receive each month when we move into our dream home. This is even more crucial given the cost of living crisis.

In a nutshell, the answer is no. EPCs (or Energy Performance certificates) – the method used to calculate energy use – are based on theoretical energy estimates which do not measure the real energy efficiency performance of a home. Often, they can be grossly inaccurate and do not reflect the impact of poorly or inappropriately installed products. This can mean billpayers are paying considerably more than they should for their energy, or investments in energy efficiency measures are not being recovered at the rate assumed.

Much like cars, differences between the theoretical and real energy performance of a building can vary radically. For example, in new homes, energy use can be up to 3-4 times that stated on an EPC. While older homes that have undergone retrofits often still show energy usage of 30% higher than their EPCs.

Furthermore, as the UK aims to transition its buildings to low carbon heating, the requirements for well-insulated homes only grows – as the cost-effectiveness of low carbon heat systems is very closely linked to the efficiency of the home itself. As we shift to electrified heat, the need for a much more accurate picture of electricity demand is essential to de-risking the transition to Net Zero and ensuring grid and district network reliability.

The Problem

There is already widespread agreement that the EPC regime needs radical reform and the Committee on Climate Change has made a series of recommendations to this effect, as has the House of Commons Environmental Audit Committee and several other expert bodies such as NESTA.

The Government’s EPC Action Plan of September 2020 acknowledged the need to build consumer confidence by making EPCs ‘accurate, reliable, and trusted’ and work is underway in both DESNZ and DLUHC to update the EPC regime alongside tightening Minimum Energy Efficiency Standards for rental properties and consulting on the Future Homes Standard 2025.

However, the process used to estimate a home’s energy use, its carbon emissions, and energy efficiency – known as SAP (Standard Assessment Procedure) – uses assumptions for the spec of the building and its appliances, size, occupancy, and behaviour. It assumes that there is perfect installation of all products used in the home and doesn’t measure anything in situ. The reality of a poor installation of a product leads to a significant performance gap between the theoretical and actual contribution of measures such as loft insulation and cavity wall insulation as examples. Actual energy usage (and therefore carbon emissions) of a home can be significantly higher than that estimated by its EPC.

BEIS’s ‘Building for 2050’ study found that the space heating requirements of new-build properties were as much as 2-4 times higher than that stated on their Energy Performance Certificate in terms of kWh. Performance gaps of 30% on homes retrofitted with energy efficiency measures are also regularly observed. Part of the reason that the performance gap for new homes can be so high is that the greater the ambition of reducing energy usage, the higher the potential gap between EPC rating and the actual energy usage of a property if the measures have been poorly installed.   

The Solution

Unlike desktop studies, ‘real’ or ‘in-use’ performance reports the energy usage of buildings under standardized conditions based on observed measurements in a specific property over time. It harnesses the power of sensor and smart meter data to provide a granular picture not just of the energy used in the building overall but of the specific contributions that efficiency interventionslike installed insulation have made to that performance. This has the benefit not only of accuracy but also of allowing homeowners to isolate the contribution (or lack of contribution) that specific measures in which they have invested are having on their actual energy bills and carbon emissions.

Real Performance technologies are now able to measure fabric efficiency ‘under average occupancy and average weather’ despite the homes being occupied and users likely not behaving ‘averagely’. Of course, different real performance measurement technologies have different approaches in their implementation, but in general, the process begins with sensors deployed in the property alongside real-time weather and smart meter data.

Now we can shift away from guessing to measuring the actual performance of fabric-based interventions in reducing energy use in homes.

Next steps

Everyone wants their home to be warm, comfortable and affordable to heat. To secure this outcome Government must act now to transition the EPC from a crude estimate of home efficiency performance to instead rely on tools able to verify that homeowners have received the efficient home they were promised – whether a new or retrofitted home. Two steps are needed:

  1. By 2025, Energy Performance Certificates (EPCs) in the UK and the subsidy schemes based on them, should transition from being theoretical estimates of energy use, to being based on the measured performance of buildings – using tools and software verified by the UK Government.
  2. Government must set out a timetable for this transition as part of the EPC Action Plan along with transitioning the ECO4 and Great British Insulation schemes and the Future Homes Standard. An ambitious level for accuracy must be set, followed by DESNZ’s Technical Advisory Panel accrediting the measurement technology suppliers that meet this high standard.

Sarah Kostense-Winterton, MIMA’s Executive Director, says: “Historically, retrofit has focused on the ‘process’ of installing ‘insulation measures’ but not the desired outcome – real reductions in energy demand for heating. This has left the risk of a poor installation with the homeowner.

Transitioning to the use of real performance tools to measure actual energy use will give certainty to homeowners and to local and national government that the energy efficiency measures in which they invest do in fact drive real-world savings in energy use and (therefore) energy bills.

A clear timetable for the deployment of real performance measurement would help the whole housing sector considerably and put the onus on delivering quality installations."

For further information please contact Sarah Kostense-Winterton ( 

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