You are here

Our calculations

Within our advice we often give you an indication of the amount you could save, on fuel bills and CO2 by making energy efficiency improvements. These saving figures are either calculated using models or are based upon evidence from field trials of products. The amount of energy and money that can be saved be that by installing energy efficiency measures such as insulation, or by reducing energy consumption by changing behaviour, will differ from household to household.

For measures like insulation and new heating systems the amount you could save will vary depending on the way you heat your home (ie the type of heating system you have, the temperature you set your home to and when you have you heating on) and the way your home is built (ie its size, shape and how well insulated it is to begin with). For our savings, we present figures as simply as possible that we expect to best represent typical homes commonly found across the UK.

Fuel prices and carbon intensity

Fuel prices are constantly changing. Therefore we regularly review the savings figures we provide to match the most up to date prices. Our fuel prices and carbon factors were last reviewed in February 2015. In all our calculations we assume the following average fuel prices and carbon factors for GB households:

England, Scotland and Wales

Fuel prices

Gas

Oil

 LPG

Wood pellet

Average price (pence/kWh)

4.29

5.36

8.32

4.77

Standing charge (£/year) £87.92 - - -

Carbon dioxide factor (kgCO2/kWh)

0.185

0.246

0.214 

0.000

 

Fuel prices

 Coal / solid fuel

 Electricity (off peak economy 7)

Electricity (standard rate)

Average price (pence/kWh)

4.00

7.22

14.05

Standing charge (£/year) - £80.43 £69.97

Carbon dioxide factor (kgCO2/kWh)

0.296

0.490

0.490

 

Northern Ireland

Fuel prices

Gas

Oil

 LPG

Wood pellet

Average price (pence/kWh)

4.91

6.83

8.59

4.77

Standing charge (£/year) - - - -

Carbon dioxide factor (kgCO2/kWh)

0.185

0.246

0.214 

0.000

 

Fuel prices

 Coal / solid fuel

 Electricity (off peak economy 7)

Electricity (standard rate)

Average price (pence/kWh)

3.89

8.75

17.41

Standing charge (£/year) - £35.83 -

Carbon dioxide factor (kgCO2/kWh)

0.296

0.490

0.490

 

 

Energy modelling: SAP

SAP (Standard Assessment Procedure) is the UK Government’s recommended method for estimating the energy performance of residential dwellings. The SAP methodology is used to produce Energy Performance Certificates and is used in a number of other government programmes, such as the Green Deal, Energy Company Obligation and the Renewable Heat Incentive to estimate the amount of energy typically used in a home. It is also the same methodology that we use to calculate the majority of our savings figures for upgrading insulation, draft proofing, glazing and heating systems (including renewable space heating). In the section below we outline the details about the homes we model in SAP.

 

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

 3

 4

 2

 3

 2

Floor area (m2)

 89

 149

 67

 79

 60

Window area  (m2)

 17

 24

 9

 13

 7

Door area (m2)

 3.7

 3.7

 3.7

 3.7

 1.85

Exposed wall U-value (W / m2 K)

 1.42

 1.37

 1.37

 1.59

 1.42

Roof U-value (W / m2 K)

 0.34

 0.34

 0.32

 0.42

 0

Percentage double glazed 94% 89% 94% 87% 81%

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. The model assumes that the home situated in the middle of Great Britain and is heated during 68 weekend days with 16 hours of heating and 170 weekdays with 9 hours of heating (2 hours in the morning and 7 hours in the evening) every year.

Adjustment factors

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.

U-values

For the following insulation measures we assume the following changes in U-value for the dwelling:

Measure Before (Wm-2K-1) After  (Wm-2K-1)
Loft insulation (0mm - 270mm) 2.30 0.16
Top-up loft insulation (120mm - 270mm) 0.35 0.16
Cavity wall insulation 1.60 0.50
Internal solid wall insulation 2.10* 0.30
External solid wall insulation 2.10* 0.30

*A recent study found that the U-value of solid walls is typically better than this (around 1.3 Wm-2K-1) however adjustment factors used in our saving account for this difference in performance.

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 costs 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. 

Lighting

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

Bulb type

Lamp lifetime

Lamp usage per day

Incandescent

1,000 hours

2.5 hours

CFL

10,000 hours

2.5 hours

LED

25,000 hours

2.16 hours

The incandescent and CFL usage hours are based on average hours for a main lounge light fitting. The usage hours for LED bulbs are based on the average hours for a kitchen light fitting, since these are most likely to be halogen spots suitable for replacement.

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.

Water

All bill savings from water meters assume a tariff of £3.01 per cubic meter based on a weighted average of water and sewerage charges for all companies in England and Wales.

Our general savings:

  • 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 5 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 an 8 litre bowl four times a day instead of running a hot tap for five minutes at six litres a minute for 10 minutes.

Generating your own energy 

Renewable Space Heating: Heat pumps and biomass boilers

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 with heating controls including TRVs.

We assume that wood-fuelled boilers have an efficiency of 80 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. Our prices for wood pellets assume that the fuel is bought in bulk as opposed to bagged.

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 83 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.

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 0.01 W/m2K2 respectively. These coefficients are based on a sales weighted average between evacuated tube and flat plate panels.

Hydroelectricity

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 3,820 kWh of energy per year. Scottish savings are based on a 4kWp system in Stirling, generating 3,180 kWh of energy per year. For both, it is assumed 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).

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 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).

Travel

Our consumer transport savings are based on a petrol price of 130.43p/litre and a diesel price of 137.08p/litre as of December 2014. Scottish savings are based on a petrol price of 128.52p/litre and diesel price of 132.95p/litre. Average mileage is assumed to be around 8,108 miles a year for the UK, and 8,700 miles a year for Scotland, sourced from Dft. Our vehicle emissions factors have been sourced from DEFRA’s GHG Conversion Factors 2014.