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Our calculations

To work out the figures shown throughout the site for how much you could save when you make energy-saving measures, we’ve made some assumptions.

Changes to calculations

A number of the Energy Saving Trust’s energy efficiency savings were based upon figures approved by DECC under CERT (the Carbon Emission Reduction Target). With the end of CERT and the introduction of Green Deal and ECO, the Energy Saving Trust now model many savings using SAP, the method that underpins the Green Deal Assessment procedure.

General assumptions

Our figures are based on current fuel prices. In general we use these average fuel costs and carbon dioxide factors.





Wood pellet

Average price (pence/kWh)





Carbon dioxide factor (kgCO2/kWh)







 Coal / solid fuel

 Electricity (off peak economy 7)

Electricity (standard rate)

Average price (pence/kWh)




Carbon dioxide factor (kgCO2/kWh)




Our gas and electricity prices are sourced from DECC’s domestic energy price statistics from December 2013 and are exclusive of fixed charges for these fuels. All other fuel prices are sourced from Sutherland tables averaged over 2013 to account for fluctuations in fuel process

In our savings figures we allow for a comfort factor and the heat replacement effect wherever appropriate.

Adjustment factors

If you make improvements to your insulation or heating system,you could end up with a warmer house rather than lower bills.  In practice, many people end up with a bit of both. 

Our savings figures for heating and insulation include green deal “in use factors”. These account for differences between modelled savings and observed in-situ savings. Where these in use factors are based on empirical evidence from field trials and NEED ( the national energy efficiency data-framework) our savings are provided as a single figure that includes the in use factor. Savings with in use factors based only on expert recommendation, are presented as a range between the modelled saving and the modelled saving plus in use factor.

The adjustment factors do not account for comfort taking where some households may choose to heat their homes to a higher temperature after installing energy efficiency measures. If your house was warm enough already, you could save more money when you insulate by adjusting the heating controls to make sure you don’t overheat the house.

 Heating and Insulation

Our savings figures are based on homes heated by gas, unless stated otherwise, with an average boiler efficiency of 81 per cent. The standard dimensions and heat loss parameters for our standard dwellings are as follows:


 Semi-detached house

 Detached house

 Detached bungalow

 Mid-terrace house

 Mid-floor flat

Number of bedrooms






Floor area (m2)






Window area  (m2)






Door area (m2)






Exposed wall U-value (W / m2 K)






Roof U-value (W / m2 K)






In our standard dwellings we assume that 80 per cent of the home’s windows are double glazed the remainder single glazed. Our model assumes that each home’s living area is heated to 21 degrees Celsius and that the remaining areas are heated to 18 degrees Celsius.

Boiler replacement savings:

The figures below show the estimated annual gas bill savings in 5 typical house types for replacing an old boiler with a new condensing boiler. The savings will vary depending on how efficient your previous boiler was.

Old boiler rating

Semi detached house

Detached house

Detached bungalow

Mid terrace house

Mid floor flat

G ( < 70%)






F (70–74%)






E (74–78%)






D (78- 82%)






The A to G rating system has now been withdrawn, but is included here as it is still widely quoted. The efficiencies shown are SEDBUK 2009 efficiencies, the current standard for boiler comparison.

Loft insulation savings

The figures show the savings when you insulate an uninsulated loft, and when you top up 100mm of insulation to 270mm. The recommended depth for mineral wool insulation is 270mm but other materials need different depths. These savings assume the following heat loss values (U-values) for roofs with the following levels of loft insulation: no insulation – 2.3 W/m2 K; 100mm of insulation - 0.4 W/m2 K and 270mm of insulation 0.19 2.3 W/m2K.

Loft insulation (0 to 270mm)

Detached house

Semi detached house

Mid terrace house

Detached bungalow

Potential saving per year





Installation cost





Carbon dioxide saving per year

1050 kg

620 kg

580 kg

830 kg


Loft insulation top up (100 to 270mm)

Detached house

Semi detached house

Mid terrace house

Detached bungalow

Potential saving per year





Installation cost





Carbon dioxide saving per year

110 kg

65 kg

60 kg

95 kg

Heat replacement effect

Electrical appliances and lighting in the home give out heat when they are switched on or on standby. In heated living spaces, some of the heat they give out contributes to the warmth of the building.

If your buy more efficient appliances, or use the appliances less often, then the amount of waste heat is reduced.  The temperature inside the building can only be maintained by adding heat from another source. This is known as the Heat Replacement Effect.

Most of the heat given off by electrical equipment does not contribute to your space heating.  Also, using heat from lighting and appliances is an expensive way to heat a space.  Electricity is typically more than three times as expensive as natural gas – 13.52 pence per kWh rather than 4.21p. It also emits nearly three times as much carbon dioxide: 0.480kg per kWh rather than 0.184kg. So replacing old inefficient appliances and lighting with new more efficient versions will save you a lot more in electricity than it cost you for heating.

Our calculations take the heat replacement effect into account.  Our lighting, appliances and standby savings include reductions to account for the potential increase in space heating that may be needed when more energy-efficient products are used.

We use the heat replacement effect factors revised by Defra in March 2010. 


Our lighting savings assume the following lifetimes and annual usage for lightbulbs:

 Bulb type

 Lamp Lifetime

 Lamp usage per day


 1,000 hours

 1.2 hours


 10,000 hours

 1.2 hours


 25,000 hours

 2.16 hours

For lifetime savings the cost of replacement bulbs has been allowed for, and a factor has been applied to account for the heat replacement effect.

The higher figures reflect the saving from replacing brighter or higher-use bulbs with energy-saving light bulbs. Replacing lamps of lower wattage or usage will give lower savings, though you will still use 75-80 per cent less electricity.


All bill savings from water meters assume a tariff of £2.89 per cubic meter based on a weighted average of water and sewerage charges for all companies in England and Wales.

Our general advice:

  • for an eco showerhead, assumes a 13-litre-per-minute showerhead is replaced with a 7.7-litre-per-minute showerhead and each person showers 4.9 times a week
  • for spending one minute less in the shower, shows the maximum saving calculated for a 16-litre-per-minute shower
  • for replacing one bath with a shower, shows saving weighted by all shower types and assumes a bath is 80 litres
  • for washing up, assumes filling a bowl four times a day instead of running a hot tap for five minutes at five litres a minute four times a day.

Generating your own energy 

Savings for all microgeneration systems assume the same costs and carbon dioxide factors that we use for our energy-efficiency savings.  Figures are based on current fuel prices and do not include any allowance for future price increases. We use these average fuel costs and carbon dioxide factors: Savings for electricity generation systems are based on the typical system size currently being installed in domestic properties. 

Renewable heating savings are based on the heating requirements for a typical four bedroom detached house with basic insulation measures installed – loft and cavity wall insulation where appropriate, primary pipework insulation and TRVs.

System costs are based on typical prices paid through recent grant programmes, or on information from industry if more up to date. Prices include installation.

Air source and ground source heat pumps

Our savings assume a level four seasonal performance factor of 2.82 for ground source heat pumps (GSHPs) and 2.45 for air source heat pumps (ASHP), as found in phase 2 of the Energy Saving Trust heat pump field trial. That means for every unit of electricity input into the system, 2.82 and 2.45 units of useful heat are generated by the respective heat pumps. You can read more about the trials in our report The Heat Is On. The heat pump system is assumed to replace a boiler with average space heating efficiency of 84 per cent for oil heating (an average non-condensing oil boiler) and 75 per cent for gas (an average non-condensing gas boiler). Electric storage heaters are assumed to be 100 per cent efficient.

For most homes, RHI payments will be deemed, based on the estimated total heat demand of the property, and the seasonal performance factor of the heat pump and distribution system based on its rating in the MCS heat emitter guide. Our example RHI income calculations assume a system with an SPF of 3.4 for GSHPs and 2.7 for ASHP (a system rated 3* in the heat emitter guide), which provides 100 per cent of space heating and 100 per cent of water heating.

These savings and RHI payments are presented as a range for each property type. The upper end of the range of savings and RHI payments are for homes with solid walls, and assume that the property has 270mm of loft insulation. The lower end of the range is for homes with insulated cavity walls and 270mm of loft insulation.


No savings have been given for hydroelectricity turbines as their performance is highly dependent on the water source and size of the system.

Solar electricity

Savings are based on a well oriented unshaded 4kWp system in Westminster, generating 3,730 kWh per of energy per year. They assume that 25 per cent of the electricity generated is used directly in the home, 75 per cent is exported to the grid, but only 50 per cent export is paid for (export is assumed to be deemed, not metered).

Solar hot water

Annual energy savings are based upon the median saving achieved in the Energy Saving Trust solar hot water field trial. RHI payments assume a system is installed with a zero loss efficiency, first and second order heat loss coefficient of 0.76, 3.3 W/mK and 0.011 W/m2Krespectively. These coefficients are based on a sales weighted average between evacuated tube and flat plate panels.

Wind turbines

These savings assume a 19 per cent load factor taken from the Energy Saving Trust field trials in locations where wind speeds were 5m/s or higher. They assume that 37 per cent of the electricity generated is used directly in the home, 63 per cent is exported to the grid, but only 50 per cent export is paid for (export is assumed to be deemed, not metered).

Wood-fuelled boilers

Assumes average wood-fuelled boiler efficiency of 75 per cent. You will not always see financial savings when replacing some fuels with wood fuel. This is because wood heating can be more expensive than using gas. However as wood fuel is a renewable fuel you can still greatly reduce the carbon footprint of your house.


Our consumer transport savings are based on a petrol price of 134.14p/litre and a diesel price of 140.39p/litre as of December 2013. Average mileage is assumed to be around 8,400 miles a year, sourced from Dft. Our vehicle emissions factors have been sourced from DEFRA’s GHG Conversion Factors 2013.