How Effective Are EPC Ratings at Determining The Energy Efficiency of Homes?

What is an EPC?

An Energy Performance Certificate (EPC) is a property's energy efficiency rating from A to G, with A being the most efficient for the energy used (more energy is useful as opposed to waste energy). EPCs are valid for ten years from assessment and are required whenever a building is rented, sold, or newly constructed. Although some old buildings are exempt from the requirement for an EPC, these include listed buildings and residential buildings used less than four months a year.

How Are EPCs Used to Measure Energy Efficiency In Buildings?

EPCs are calculated by accredited assessors. The assessor considers the type of property, its age, size, and location and assigns a set of precalculated values for each element. This calculation methodology is based on the Standard Assessment Procedure (SAP) for new dwellings and Reduced Data Standard Assessment Procedure SAP data (RdSAP) for existing dwellings, the government-approved method for assessing a property's energy performance.

Are EPCs Accurate at Predicting the Energy-Efficiency of Modern Homes?

During the assessment, there is no actual testing of the fabric efficiency of the materials used, which would require destructive testing; there is no way to quantify the exact W/mK of heat escaping through walls, floors, and roofs without this.

During an EPC assessment, the energy consumption of elements like boilers is not calculated; to calculate a boiler's actual efficiency, the Gross calorific value of the fuel must be worked out by dividing the total energy output of a boiler by the total energy input given to the boiler, multiplied by 100.

As EPCs for existing buildings are calculated using a set of standardised figures (RdSAP), this is a method of measuring the performance of existing dwellings; these are conducted from intrusive tests to measure the thermal resistance conducted in very specific conductions in labs. They use a set of values for each element; for example, a 1930s solid brick wall of a particular thickness will have a U-value of 2.1W/m²K.

In a peer-reviewed article titled 'An analysis of errors in the Energy Performance certificate database,' the researchers found that between 36 and 62% of Energy Performance certificates possess errors.

What Has Been Done to Improve The Efficiency of EPCs?

From April 2024, RdSAP 10 was introduced to upgrade the former way of assessment. Some of the changes are:

• No more assumed values for glazing areas based on the age of the property - assessors will be required to measure all windows.
• Additional options for roof rooms—Assessors will be required to take additional measurements to capture different construction types of common walls and gable walls.
• New age band – Introduction of new age band for properties built from 2023 onwards to show improvement in u-values.
• RdSAP 10 incorporates updated energy factors, so new assessments are based on the latest data.

Whilst EPCs are a great general assessment of the thermal efficiency and performance of dwellings based on building materials, because they are calculated using a set of standardised values, they may only sometimes indicate the actual thermal performance of the home. Even with the latest changes, some aspects will still not be accounted for.