Energy saving assumptions

Lighting assumptions

The saving for the average CFL is derived from Carbon Emissions Reduction Target (CERT) savings and an average electricity cost of 12.12p/kWh. The higher saving assumes replacing a traditional 100W tungsten filament (GLS) lamp with a 20W Compact Fluorescent Lamp (CFL) and 770 hours of use per year (based on Lighting Association research for the average incandescent lamp fitting in a typical household).

Replacing lamps of lower wattage or usage will give lower savings, though reductions in lighting electricity consumption 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 Recommended model of similar size and an electricity cost of 12.12p/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 lighting, 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 four times (12.12p/kWh) more expensive than natural gas (3.06p/kWh) and emits more than twice as much CO₂ for each unit of energy used.

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 account for the potential increase in space heating that may be required when more energy efficient products are installed. This is the reason for the slight reduction in savings compared to previous figures.

Insulation Assumptions

The costs and paybacks shown are approximate, are provided for illustrative purposes only and based on a gas heated semi-detached house with 3 bedrooms. Installed Costs and paybacks assume that installation is undertaken by a professional installer and are un-subsidised. However, please note that grants and offers are available from local authorities and energy suppliers which can reduce the cost and payback times substantially. Savings assume a gas price of 3.06p/kWh. Some of the savings made may be taken in increased comfort.

¹Assumes insulating to a U-value of 0.45 W/m²K.
²Assumes insulating to a U-value of 0.35 W/m2²K. Installed costs are the full cost of installation £4,500 for a semi-detached house. If work is already being done on the walls of the house, the marginal cost is around £1,900 with a payback of around 5 years.
³Floor Insulation represents the cost of the insulation only.

Heating 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. Heating control costs assume the additional cost for installation when an installer is at the premises already working on the heating system. Savings for condensing boilers assume that their installation is mandatory due to building regulations and hence there is no additional cost when compared with a non condensing equivalent. Savings assume a gas price of 3.06p/kWh.

Condensing boiler costs/payback - see page 38 of the Domestic energy primer - an introduction to energy efficiency in existing homes.