Our data

One of our ambitions is to support UK-wide access to comprehensive, independent, and tailored energy advice for households and small to medium enterprises.

To support this, we regularly produce and update representative costs and savings data and statistics. We publish information on how much money and carbon emissions can be saved in the home. This covers a range of measures including insulating your home, installing solar panels, and buying the most efficient appliances.

Our calculations give an initial idea of potential savings. They are not intended to replace costs and savings based on surveys or specific data about a particular home.

We want our data to represent real-life situations as accurately as possible. We gather data from diverse sources, including national surveys and government reports, and use various statistical methods to draw meaningful conclusions.

We look at housing stock data from sources including statistics from the Department for Energy Security and Net Zero (DESNZ). We calculate how much insulation is installed and assess the average efficiency of boilers across the UK. We use U-values from the Standard Assessment Procedure (SAP), and government ‘in-use factors’ to account for the difference between modelled savings and in-situ savings. We also account for the ‘heat replacement effect’.

We update the carbon factors we use each year with the latest published data from DESNZ. We also cross-check and validate our existing models, making changes where necessary.

When it comes to our water savings, we assume all bill savings from water meters based on a weighted average of water and sewerage charges for all companies in England and Wales. For transport related savings, we use average petrol and diesel prices from the previous year for the UK and Scotland and use assumptions related to the average mileage.

We review and update our typical installation costs every year. We do this by using installation costs data relating to the last 12 months depending on availability. The data comes from several sources such as national energy efficiency schemes, installers, retailers, and professional industry organisations. These sources are analysed and adjusted according to inflation, location, and date.

Costs used on our website try to illustrate a typical cost only. Costs vary widely and may depend on a property’s size and construction, as well as geographical factors such as having less choice of installers, or higher living costs in cities like London. Our costs don’t typically represent any suitability assessment or remedial work such as surveys, structural alterations or redecoration.

Costs for different house types are calculated using fixed dimensions from the Building Research Establishment (BRE). Where no house type is mentioned we use the average or median cost to represent the most common cost experienced by householders.

If you calculate payback periods using our costs, please be aware that our costs do not include any replacement parts or any maintenance. Using a simple payback methodology (installation cost divided by annual savings) does not consider discount rates, future energy prices, inflation or the lifetime of the installation; therefore, this should be used with caution.

Our numbers provide indicative costs for installing various measures to give householders an idea what to expect. They are in no way intended to replace quotes based on an on-site survey with specific data and actual costs to householders may vary significantly. We always recommend you get at least three quotes.

As time goes on, more criteria can be measured and factored in to help improve the quality of our data. For example, our heating savings are modelled alongside improvements to the Standard Assessment Procedure, which is the methodology used to assess the energy efficiency of homes. We also apply statistical methods to help inform how much confidence we have in our assumptions.

Field trials and other in-situ monitoring help us to validate our calculations. We are always looking for high quality data to best represent the current situation for householders.

We use Ofgem’s price cap for gas and electricity unit rates. This is because most households in England, Wales and Scotland are currently paying prices set by this cap.

For other fuels, we use a combination of the UK Government’s predicted energy prices for the coming year, current prices from suppliers, and historic prices to produce our estimate of likely future fuel costs in the near term.

While we cannot predict future costs with certainty, this gives the best indication of the likely financial impact of any changes householders might make to their energy use. In our calculations we assume the following average fuel prices and carbon factors for GB and NI households.

We review all our savings and costs assumptions and methodology at least annually. For example, as a trend, the UK’s housing stock average becomes more efficient year-on-year, as new-built homes are better insulated, and some existing homes are improved. We also factor this into our savings, acknowledging that fewer savings are achieved per individual measure as more measures are installed.

Renewables can also frequently change, where incentives can change and the up-front costs react quickly to market demand. In the case of solar PV in particular, the system costs fell quite dramatically over a number of years, but have recently started to rise again. We review some assumptions with a higher frequency than others.

Gas and electricity prices have changed quickly in the last few years, and carbon factors change from year to year, sometimes quite substantially. These factors influence how much money and carbon emissions you can expect to save from installing energy efficiency measures or low-carbon technologies, and how cost-effective certain measures will be. The frequency of our updates considers all the above, and we release an update when these changes are likely to influence the decision a householder may make about their home.  

If you want to use our statistics, please clearly source:

• Energy Saving Trust
• the date the information was last updated (this is displayed on the relevant webpage)
• the energy prices the data is based on, where applicable

We do not recommend using our statistics for any financial modelling or analysis. We have different statistics and methodologies for this purpose and can provide a consultancy service for businesses or anyone who needs more support.