Energy saving assumptions

Lighting assumptions

The saving for the average energy saving lightbulb (CFL) is derived from Carbon Emissions Reduction Target (CERT) savings and an average electricity price of 12.50p/kWh. The higher figures reflect the saving from exchanging brighter or higher use bulbs, for example by exchanging a 60W incandescent with a 15W CFL (and 1,370 hours of use per year) or replacing a 100W with a 20W CFL (and 562 hours of use per year).

Replacing lamps of lower wattages or usage will give lower savings, though reductions in lighting electricity consumption per bulb replaced of the order of 75% - 80% should still be expected.

Appliance assumptions

Savings assume replacing an average appliance purchased new in 1998 with an Energy Saving Trust Recommended model of similar size and an electricity cost of 12.50p/kWh.

Heat Replacement Effect

Electrical appliances and lighting in the home give out heat when they are switched on or are on standby. In the case of incandescent (old-fashioned) light bulbs, about 95% of the electricity is wasted as heat with only 5% being used to provide light.

Most lights and appliances are situated in heated living spaces, where some of the energy they consume contributes to the warmth of the building. If the waste heat they emit is reduced through design improvements, the temperature inside the building can only be maintained by adding heat from another source such as a heating system. This effect is known as the Heat Replacement Effect.

Using your lighting and appliances to heat your home, for instance when you may have gas central heating, is not recommended. Electricity is typically three to four times more expensive than natural gas (12.50p/kWh versus 3.67p/kWh) and emits more than twice as much CO₂ for each unit of energy used (0.539kgCO2/kWh versus 0.185kgCO2/kWh). Therefore in most cases a saving will still be made by replacing old inefficient appliances and lighting with new more efficient versions, because the saving on electricity bills will be higher than the increase in heating bills.

Where appropriate, adjustments to account for the Heat Replacement Effect have been made in our calculations. For instance, our lighting, appliances and standby savings incorporate reductions to the savings to account for the potential increase in space heating that may be required when more energy efficient products are installed. These heat replacement effect factors have recently been revised by Defra (March 2010), which has led to a small change in many of our lighting and appliance savings this year.

Insulation Assumptions

The costs and paybacks shown are approximate, are provided for illustrative purposes only and are based on a gas heated semi-detached house with 3 bedrooms. The savings are the same as those used for CERT and we assume a gas price of 3.67p/kWh.

The savings also include a reduction factor for an effect known as 'comfort taking'. This is where a householder experiences a lower saving than could actually be achieved because they opt for a more comfortable living environment - this could be by leaving the heating on for longer than needed. The savings shown here are therefore an average which includes this reduction. You could save more than the figure shown here by making sure your heating controls are set correctly for the new needs of your insulated home.

Installed costs and paybacks assume that installation is undertaken by a professional installer and both loft and cavity wall insulation costs include a subsidy which can be obtained under CERT. The total, unsubsidised cost of installing either loft or cavity wall insulation is typically around £500.

A variety of grants and offers are available from the government, local authorities and energy suppliers.

¹Assumes insulating to a U-value of 0.45 W/m²K.
²Assumes insulating to a U-value of 0.35 W/m2²K.
³Costs for double glazing are highly variable and dependent on the specific work needing done in each home. Costs given here are for illustration only.
⁴Loft DIY insulation offers change frequently. Look for a good deal. The less you pay for the insulation the more quickly it will pay for itself.
⁵Floor Insulation DIY cost represents the cost of the insulation only.

Heating assumptions

The savings for the condensing boiler upgrade are for changing from an old G rated boiler to an A rated condensing boiler and a full set of heating controls. Savings shown are approximate and are provided for illustrative purposes only, and are based on a gas heated semi-detached house with 3 bedrooms. Savings assume a gas price of 3.67p/kWh.

Fuel price assumptions