School of Built Environment, Engineering and Computing

Leeds Beckett researcher contributes to wide ranging review on government models used to assess the energy efficiency of the nation’s homes

Dr. David Glew is Head of Energy Efficiency and Policy at the Leeds Sustainability Institute (LSI). He was interviewed as part of a Government funded research project focussed on improving the Standard Assessment Procedure (SAP), which is used to generate Energy Performance Certificates for houses. Following the publication of the report into SAP, Dr. Glew provides a summary of the main findings here.

House model

We may all be familiar with the A to G energy ratings on our fridges and TVs, but we may not know our house has one too. It is called an Energy Performance Certificate (EPC) and homes must have one when they are built, sold or rented out. Currently, over 18 million homes have had EPCs in England and Wales. If you want to know your home’s score (or if you want to see the EPC for your neighbour’s house), then you can look it up in the Government’s open EPC database.

When you take a closer look at EPCs more critically, however, blots on this wonderful national dataset start to appear. A database is only as good as the information you put into it and as my LSI colleague, Dr Adam Hardy’s research has shown EPCs are riddled with errors. 

Beyond this, perhaps one of the most frustrating things about EPCs is that they are routinely misunderstood and misused. I will let you into a secret; EPCs were not designed to predict how much energy your house will use; they were supposed to be an “asset rating”. To understand this, we must consider the following:

EPCs need to know if our walls or floors are insulated, how efficient our boiler is, how much insulation is on our lofts etc. It’s very time intensive to measure these accurately so, instead, “best guesses” are taken using defaults provided by the reduced version of SAP (rdSAP). This is less important when undertaking housing stock level assessments as most of the small mistakes will cancel out. However, on the scales of individual homes, an EPC will be wrong more often than not.

Secondly, just as driving our cars erratically vs. smoothly, can impact our miles-per-gallon, using a home differently will affect how much energy it uses. To get around this, EPC’s ignores who lives in the house and assumes “standardised occupants”. This is a fantastic way to ensure houses can be assessed against each other in a fair and consistent way. It also means that, unless you follow exactly the assumed occupancy schedule, the energy use estimated by your EPC will be wrong. 

Both these problems mean that one can’t expect EPCs to correctly estimate energy savings from retrofit measures (e.g. cavity wall insulation) on an individual home. Indeed, EPCs were not designed for this. But guess what? This is exactly what EPCs are being used for. Some changes have been made to make EPCs better able to consider how homes are used, and improvements to the inputs and default values are routinely made. But these problems persist. I, therefore, have sympathy for EPCs; it hardly seems fair to criticise them for getting the wrong answer when in reality, the wrong questions are being asked.

This new extensive report faces these problems head-on, asking fundamental questions about aligning the production of EPCs with their intended purposes. The goal of the report is to make EPCs more useful and accurate, something our own research project is exploring. This report is an impressive attempt to capture multiple stakeholder opinions and condense all the data into a series of deliverable recommendations. EPCs are here to stay, they are fundamental to the Government’s route map to zero carbon and changes may be needed if they are to become fit for purpose in the future.

You may download this report here

Dr David Glew

Reader / School Of Built Environment, Engineering And Computing

Dr Glew is Head of Energy Efficiency at the LSI and manages the LSI interdisciplinary research projects on domestic retrofits. Over the last 5 years he has been responsible for delivering research contracts worth over £3 million, aimed at informing energy policy. He has published in multiple peer review publications, supervises doctoral students in the LSI and has been appointed to government advisory panels.

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