Our data

Every year, Energy Saving Trust’s savings data and statistics are updated. We publish information on how much money and carbon emissions can be saved in the home across a range of measures, from insulating your home to buying the most efficient TVs.

Our savings calculations are a good start for householders to get an idea of what savings could be possible to achieve. However, the data is based on averages and should therefore be treated as a guide. So, if you are looking to install something more specialist or expensive, you should look at getting more in-depth advice that is specific to your home.

Fuel and electricity prices and carbon factors change from year to year, and sometimes quite substantially. Naturally, this affects how much money and carbon emissions you can expect to save from installing energy efficiency measures, and how cost-effective certain measures will be.

Housing is becoming more efficient year-on-year. We also factor this into our savings, as the more measures you install; the less is saved per measure on average.

The biggest changes often come from renewables, where incentives can change and the up-front cost changes annually. 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.

First we look at the state of the housing stock – with data sources such as the English Housing Survey and statistics available from the Department of Business, Energy and Industrial Strategy (BEIS). We can then work out how much insulation is installed and assess the average efficiency of boilers across the UK. We use U-values from the Standard Assessment Procedure, or 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 also update the carbon factors we use each year with the latest published data from BEIS. From there, it’s about cross-checking and validating our existing models, and 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 make assumptions related to the average mileage for each region.

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, or SAP, which is the methodology used to assess the energy efficiency of homes. Recent changes have included incorporating regional differences in climate, say between Scotland and the South West, when it comes to heating requirements. We also look to model new technologies when they come along, such as solar PV diverters and battery stores.

Field trials and other in-situ monitoring are really valuable to improve the accuracy of our data, giving us more data to check our assumptions against. We are always looking for the best available data out there, to best represent the current situation for householders.

We use Ofgem’s price cap for our gas and electricity prices. This is because most households in the UK are currently paying standard variable tariffs (default tariffs) that are capped by Ofgem. For other fuels, we use UK Government’s predicted energy prices for the coming year, together with recent actual price differences between fuels, 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.