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Home (United Kingdom) > Info > Our calculations

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.

Our calculations are different for Northern Ireland. Go to our Northern Ireland site to find out details.

General assumptions

Appliances

Heating

Insulation

Lighting

Generating your own energy

General assumptions

All our savings figures are based on a typical three-bedroom house heated by gas, with an average boiler efficiency of 78%. Actual savings for a particular household will vary depending on the size and construction of the house, and the way the occupants use the heating system and appliances.

Our 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:

Fuel Gas Oil  LPG  Coal  Electricity (Economy 7) Electricity (standard rate)
Average price (pence/kWh) 4.49 5.87  8.17  3.69  8.54 14.39
Carbon dioxide factor (kgCO2/kWh) 0.183 0.246 0.214  0.296  0.521  0.521

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

Comfort factor

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 figures for heating and insulation savings assume that some of the benefit has been taken as an improvement in comfort.  We apply a standard ‘comfort factor’ to give a financial savings figure that is less than the theoretical maximum.

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.

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 – 14.39 pence per kWh rather than 4.49p. It also emits nearly three times as much carbon dioxide: 0.521kg per kWh rather than 0.183kg. 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. Visit Defra’s website to find out more about these factors .

Appliances

Our calculations are based on replacing an average appliance purchased new in 1999 with an Energy Saving Trust Recommended model of similar size, and include an allowance for the heat replacement effect.

Appliance savings

Appliance EU energy rating Annual saving (up to) Annual carbon dioxide saving (up to)
Fridge freezer A+ or A++ £40 135kg
Upright or chest freezer A+ or A++ £26 90kg
Refrigerator A+ or A++ £17 55kg
Dishwasher A £7 25kg

Heating

The savings for the condensing boiler upgrade are based on changing to an A-rated (88% efficient) condensing boiler and a full set of heating controls.

Old boiler rating Annual saving Annual carbon dioxide saving
G ( < 70%) £300 1,200kg
F (70–74%) £200 810kg
E (74–78%) £150 610kg
D (78–82%) £105 420kg

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.

Insulation

Our calculations are based on:

  • installation by a professional installer
  • all the grants and offers available from the government, local authorities and energy suppliers
  • the lower rate of VAT you get on materials, equipment, and labour, when you have energy-saving work done to your house. ( Details are at the HMRC website. )
  • comfort factor

The savings are based on the figures used for the government’s Carbon Emissions Reduction Target (CERT) .

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.)

Measure Loft insulation (0 to 270mm) Loft insulation (100 to 270mm)
Approximate saving per year up to £175 £25
Installation cost £100 to £350 £100 to £350
Time taken to pay for itself Up to two years Four years or more
DIY cost £50 to £350 £50 to £350
Time taken to pay for itself  Up to two years Two years or more
Carbon dioxide saving per year Around 720kg  Around 110kg

Other insulation savings

Measure Cavity wall insulation Internal wall insulation
to a U-value of 0.45 W/m²K
External wall insulation
to a U-value of 0.35 W/m2²K
Double glazing
EST-recommended
Annual saving Up to £135 Around £445 Around £475 Around £165
Installation cost £100 to £350 £5,500 to £8,500 £9,500 to £13,000 £3,300 to £6,500 (costs vary hugely)
Time taken to pay for itself One to three years      
Annual carbon dioxide saving Around 550kg Around 1.8 tonnes Around 1.9 tonnes Around 680kg

Lighting

These savings are based on replacing old fashioned GLS light bulbs with efficient compact fluorescent bulbs and replacing halogen spotlights with evern more efficient LEDs.  The cost of replacement bulbs has been allowed for, and a factor has been applied to account for the heat replacement effect.

The savings are based on figures used for the government’s Carbon Emissions Reduction Target (CERT) .

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

Lighting savings

Depending on how long your lights are in use every day, just one compact fluorescent lightbulb could save you on average around £3 a year, and as much as £8 for brighter bulbs or those used for more hours a day. And because it will last around 10 times longer than a standard bulb, on average it will save you around £55 before it needs replacing.
Replacing an average halogen spotlight with an equivalent LED bulb wills ave you around £4 per year.
Fit all the lights in your house with energy-saving bulbs and you could save around £55 a year – £870 over the lifetime of all of the bulbs.

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:

Fuel Gas Oil  LPG  Coal  Electricity (Economy 7) Electricity (standard rate)
Average price (pence/kWh) 4.49 5.87  8.17  3.69  8.60 14.39
Carbon dioxide factor (kgCO2/kWh) 0.183 0.246 0.214  0.296  0.521  0.521

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 three bedroom semi-detached house with basic insulation measures installed – loft and cavity wall insulation where appropriate, primary pipework insulation and TRVs. Existing heating systems are assumed to have stock average efficiencies – 78% for gas and LPG, 82% for oil, 100% for electricity and 60% for solid fuel.

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, but do not include Renewable Heat Premium Payments or other grants.

Wood-fuelled boilers

Pellet price of 4.00 p/kWh based on bulk delivery.

Assumes average wood fuelled boiler efficiency of 80% (from CERT)

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.

Ground source heat pumps

Assumes a system efficiency of 250% for a typical installation and 300% for a good installation, based on the results of the Energy Saving Trust field trials.  System efficiency is the total useful heat provided in a year, divided by the electricity required to run the system, including fans, pumps and any direct electric heating.

Our savings figures assume that the heat pump system (including any direct electric heating input) provides all the space and water heating for the property.

Air source heat pumps

Assumes a system efficiency of 220% for a typical installation and 300% for a good installation, based on the results of the Energy Saving Trust field trials.  System efficiency is the total useful heat provided in a year, divided by the electricity required to run the system, including fans, pumps and any direct electric heating.

Our savings figures assume that the heat pump system (including any direct electric heating input) provides all the space and water heating for the property.

Solar electricity

Savings are based on a well oriented unshaded system in an average UK position, generating 846 kWh per kWp per year, based on DTI filed trials.  Assumes that 27% of the electricity generated is used directly in the home, 73% is exported to the grid, but only 50% export is paid for (export is assumed to be deemed, not metered).

Solar hot water

Savings assume a 50:50 split of evacuated tube/flat plate systems, with a collector area of 3.5m2, and typical hot water consumption for a property with average occupancy.

Wind turbines

Assumes 19% load factor taken from the Energy Saving Trust field trials in locations where wind speeds were 5m/s or higher.

Hydroelectricity

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