Chapter 12: EPCs - Energy Performance Certificates That Don't Work

Energy Performance Certificates are supposed to inform users of the energy costs and environmental impact of buildings. There are separate certificates for residential homes and commercial premises. EPCs were introduced in the year 2007 source and are required whenever a building is offered for rent or sale.

For a variety of reasons, the EPC system as it exists at the moment is dysfunctional.

Quality of Assessment

The current EPC can be undertaken by assessors with minimal training - typically just a few days of classroom instruction and some supervised assessments. The going rate for an EPC assessment is about £60-80, reflecting the low barriers to entry and the commoditized nature of the service. For most vendors and landlords, it is a tickbox exercise to satisfy legal requirements rather than a genuine attempt to understand or improve the property's energy performance.

The assessment process itself is heavily reliant on standardized assumptions rather than detailed analysis. Assessors typically spend 20-30 minutes in a property, taking basic measurements and checking for obvious features like double glazing, cavity wall insulation, and boiler types. Many factors that significantly affect energy performance - such as air tightness, thermal bridging, or the quality of installation - are either estimated from standard assumptions or completely ignored. It is a fact almost universally accepted that EPCs overestimate the heat loss of walls in most older houses, as the methodology assumes worst-case scenarios for thermal performance rather than accounting for the actual thermal mass and construction techniques used in traditional buildings. The result is that two identical properties can receive different EPC ratings depending on when they were assessed, who conducted the assessment, or even which version of the assessment software was used.

This variability undermines the credibility of EPCs as reliable indicators of energy performance. Personal experience bears this out - I've seen significant inconsistencies between EPCs on the same house over time, with insulation levels mysteriously falling between assessments despite no physical changes being made to the property. Even more concerning are the differences between houses clearly constructed at the same time on the same estate, where identical properties can receive different EPC ratings based purely on which assessor conducted the evaluation. Homeowners and buyers cannot trust that an EPC rating accurately reflects the actual energy efficiency of a property, making it difficult to make informed decisions about energy improvements or property purchases.

Outdated Emissions Methodology

The current EPCs in use employ a methodology that was agreed in 2012. Since then, Britain's electricity system has undergone a dramatic transformation. Coal generation has been phased out entirely, while wind and solar now provide over 40% of our electricity. However, in the methodology used by current Energy Performance Certificates, the share of coal in electricity is still assumed to be around 30% and the emissions from electricity are calculated as 450-500 g/CO2/kWh, which implies the emissions from using a heat pump are almost as high (at around 130-150 g/CO2/kWh) as a gas boiler (around 210 g/CO2/kWh).

Given the actual carbon intensity of electricity in 2024 is more like 115 g/CO2/kWh, the emissions of a heat pump operating at 350% efficiency are now around 33 g/CO2/kWh, making them approximately 84% lower than a gas boiler.

Missing Outputs

The outputs shown on an Energy Performance Certificate focus around the annual costs and energy usage. Although the calculation framework considers each of the following, they are not presented to the end user:

Variations in energy prices

The cost to run a house in an energy performance certificate uses standardized energy prices set by the EPC methodology, which are updated periodically but not automatically with current market prices. Over the last 5-10 years there have been dramatic swings in energy costs, with electricity prices rising from around 14p/kWh in 2012 to 25p/kWh in 2024, and the 2022 energy crisis causing unprecedented volatility. During the energy crisis, wholesale gas prices peaked at over £400/MWh, causing household energy bills to double within months. However, EPC calculations continue to use the standardized prices from the methodology, meaning the running costs shown on certificates become increasingly disconnected from actual energy bills over time. This creates a misleading picture for homeowners and potential buyers, as the EPC running costs don't reflect the real financial impact of the property's energy performance.

Heat Loss

The heat loss of a building is the amount of heat that it looses at a given temperature. It can be expressed either as a number of Watts per degree that the outside air temperature falls below 16°C or so, e.g. 100W/°C, or alternatively a design heat loss e.g. 5KW, which is the amount of heat needed to keep the house inside at a comfortable 21C when the outside temperature is at the minimum that can be reasonably expected, depending on where in the UK you are this is between -2C and 2C or so.

Knowing the heat loss of buildings had some value, but ultimately wasn't that important in an age when most buildings were heated with boilers. Most boilers are dramatically oversized for the heat loss of the building, which allows them to rapidly heat them on demand, which in turn allows the central heating to be turned on separately in the morning and evening on a day when all occupants go out during the daytime. To give you a feeling of typical oversizing, a typical gas combi-boiler might have an output of 25KW or so, even though the design heat loss of a typical house might be 5-10KW.

That said, the power rating of combi boilers is usually set to provide hot water at nearly full flow rate; for instance, a flow of 10l/min heated up from 10°C to 55°C, i.e. ΔT of 45°C, requires about 33kW. Consequently, the majority of home users are now accustomed to appliances with high peak power output which a heat pump is unlikely to match.

However, for heat pumps, and in particular those thinking of installing a heat pump, the design heat loss is incredibly important. It's the single figure that indicates how large the heat pump you might want to install in the house is. It's a pre-requisite for rough installation cost estimate.