Summary

If no energy saving actions have been taken in the past, the owners of a typical house of the 1950s might aim for a 40% or 50% reduction in energy costs, with no loss in comfort.

Savings of 10% in cost might be achieved by reducing electricity usage, for example by:

• not using electricity for heating when other options are available,
• using low-energy light bulbs extensively, and switching off lights when they are not needed,
• improving the efficiency of the fridge or freezer, and
• switching off devices at the wall when they are not being used.

Another 10% might be saved from the energy for heating by:

• setting the heating thermostat slightly lower,
• reducing the temperature of the hot-water cylinder, and
• using less hot water e.g. by using a plug in a basin and having showers not baths.

Inexpensive changes could save another 20% in energy costs, such as:

• draught proofing,
• adding more insulation to the separate hot-water cylinder, if fitted, and insulating the pipes that heat it from the boiler,
• adding more insulation to the ceiling or roof, and
• insulating the walls by cavity insulation if appropriate.

More expensive changes of:

• replacing an old boiler with an efficient condensing boiler, and
• double glazing or adding secondary glazing,

should save at least another 10% in energy costs.

Estimated Savings from Different Actions

The cost savings that can be achieved depend on:

• the size and structure of the house,
• the behaviour of the occupants,
• the appliances available,
• the fuels used, and
• the prices charged by the energy supplier.

Each saving is estimated as a percentage of the total energy cost for a dwelling if no energy saving actions had previously been taken. The percentage is probably reasonably valid for houses of different sizes. However it may be difficult to estimate what the energy bill would be if no energy saving measures have been taken.

The saving is also estimated as the cash saved each year. The cash saved will, of course, depend on the house and the cost of energy. These savings have been estimated with energy prices significantly lower than those applying in August 2008, and so should be fairly conservative.

The model used to calculate these savings is described later in this page.

* The SAP calculation is very pessimistic about the benefits gained from low energy lights. It assumes that even if all the fixed lights are of low energy, there is an equal lighting load from plugged in lights which are all incandescent. This may be correct if the occupants have no interest in saving energy, however the figure above assumes that the occupants are actively using low-energy lighting to effect savings.

** The savings predicted by the SAP model for draught exclusion will be too low in many real life cases.

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Payback on Energy-Saving Measures

Saving Electricity

Electricity is the most expensive kind of energy and results in the most greenhouse gases. Replacing it by other forms of heating can reduce costs by 60–70%, and greenhouse gases by 50%. Reducing usage is an even more direct saving.

It may be helpful in seeking savings on electrical energy to fit an electricity monitor as discussed in our energy monitoring page.

Electric room heating

While electric radiators and fan heaters are a very convenient form of heating, it is expensive to use them when other forms of heating are available. Typically over half the spend on energy for a house is for space heating and 10% of the heating is electric. Electrical costs about 3 times as much as gas for the same energy. So the electrical heating may cost 30% of the heating bill. If the proportion of electrical heating was reduced to 5%, the total energy bill for heating the house should reduce by about 10%.

Immersion heaters

A typical house might use 4000 kWh of energy to supply its hot water. About 2500 kWh represents the energy in the hot water used, 600 kWh as energy lost in heating the water and 1000 kWh lost from the hot-water pipes and the hot-water cylinder. If an immersion heater is used in summer, it might consume 2000 kWh. If a gas or oil central heating boiler is used instead, this will use more energy but as the fuel is a fraction of the price for the same energy the annual saving is likely to be around £150 to £250.

Cooking

If the use of an electric cooker is approximately that of two 2 kW electric cooker elements being used for an hour each day, this will use about 1,400 kWh per year. This might cost £150 a year. A gas cooker would provide the same heat for about a third of the price. Microwave ovens, however, are relatively efficient as the energy is concentrated on heating the food.

Fridges and freezers

A fridge or freezer can consume 1 or 2 kWh per day. An old unit may use 30% to 100% more than a modern low energy one. The saving might be between £12 and £36 per year.

The performance of a fridge or freezer is also greatly affected by its surroundings. A build up of ice in the freezer makes cooling take more energy, particularly if the ice begins to stop the door closing properly. Cooling is also less efficient if the radiator of the unit cannot get rid of the heat it is extracting easily as the air round it is too hot, for example if the radiator is in an enclosed space or in a small room where the heat cannot escape.

Lighting

The Government's Standard Assessment Procedure for energy rating of dwellings (SAP) in the UK estimates the average energy consumption for lighting is taken as 9.3 kWh/sq. metre annually if no low-energy lighting is used. In a typical house this might equate to 1000 kWh/year at a cost of about £100.

Low-energy light bulbs take less than a third of the energy of incandescent bulbs for the same light output. It is unlikely that most households will manage to reduce their electricity cost by the full amount of £66 per year if only low-energy bulbs are used, however savings of £33 to £50 per year should be possible. Low-energy lighting is covered in more detail on a separate page.

Significant savings can also be made by switching off lights when they are not used.

Devices on standby

The European Union is considering a new standby regulation which sets a maximum allowed power consumption for standby of either 1 or 2 watts in 2010. From 2013, the admissible power consumption level will be lowered to 0.5 or 1 watt, close to what can be achieved by the best available technology. In the meantime many devices take much more energy that this when not being used. If a unit taking 1 watt is left on standby for a year this would use 8.7 kWh at a cost of about £1. Some televisions take 7 watts when "switched off" by the remote control, though some modern flat-screen ones are far better. A computer can take a similar amount when in "sleep" mode. The power taken by other equipment when not in use may be less well publicised. Some examples are:

• printers, some of which take nearly as much power when switched off at the printer as they do when switched on,
• power bricks plugged into the mains and feeding portable devices such as radios,
• microwave ovens,
• bread makers,
• beds and chairs with electrical adjustments,
• low-voltage lamps with mains connections, where the switch on the lamp only breaks the low voltage connection.

It is not surprising that many homes will have a standby load of more than 40 watts. A small proportion of this standby load is useful, for example when a video recorder maintains a guide of forthcoming programmes. However it should be possible to save £20 a year by switching other devices off at the mains when not required. The issue is discussed in more detail on a separate page.

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Other Savings

Lower temperatures

Surprising amounts of energy can be saved by turning down thermostats. The model shows that 5% of the total energy bill might be saved by running the house at just 1 degree cooler that it is at present. A large proportion of the heat in a hot-water tank is also typically lost rather than being used as hot water. If the water tank temperature can be reduced so that no cold water is needed when the hot tap is run, this heat loss is minimised. The only reason for a higher temperature is if the hot-water tank cannot hold enough water for the immediate demands on it.

Use less hot water

Hot water can be saved if showers are used instead of baths – though energy is unlikely to be saved if the shower is electric and the bath water would be heated by other means. Heat can also be lost by dripping taps, and if the hot tap is left to run for example when washing hands.

Draught proofing

Draughts are not only uncomfortable but cold air needs to be heated, and heating this air can be a very considerable part of the overall heating bill. Draught exclusion should be fairly straightforward.

More insulation

Adding insulation to ceilings, and filling an unfilled cavity wall are both very effective in saving energy costs.

The SAP model

The calculations used to determine likely savings are based on the "Government's Standard Assessment Procedure for Energy Rating of Dwellings" (SAP 2005 version 9.81). This is concerned with the design of buildings and uses estimates for the heating, lighting and hot water required by the occupants. It does not account for the energy used by appliances such as cookers or TV in the dwelling. The standard SAP calculation yields the following estimates:

• The energy used per year.
• The cost of this energy. This is based on energy prices which are no longer valid, but the comparative benefits of different changes to save energy should still be valid.
• A SAP rating, derived from this cost of energy divided by the floor area of the property. Ratings are typically between 1 and 100 where 100 represents a house with no overall energy usage. A rating of 100 can only be achieved if energy from renewables is generated locally.
• An energy band between A and G, derived from the SAP rating. A is the best rating with a SAP rating of 91 or greater.
• The emissions rate which is an estimate of the kg of CO₂ generated per square metre per year. Buildings Regulations specify the emissions rate to be achieved based on the emissions rate for a building of the same dimensions with standard assumptions for the design.

The following table uses the standard SAP calculation to relate the energy performance of the modelled building with that required for a new building under current building regulations.

Modifications to the SAP model

The standard model was adjusted to allow estimates of the effect of electrical appliances and life style changes which are not included in the standard calculation. Some of these adjustments can be made within the model:

• reducing the room temperature sought,
• changing the assumption on the use of electric heating, and
• reducing the assumption for the requirement for hot water.

Estimated energy requirements of appliances were added. These requirements vary considerably between different households. The assumptions used were as follows:

No allowance was made for the contribution of appliances to heating the dwelling in winter, as the appliance consumption is so variable. If included it might reduce the overall cost of running the appliance by about 15%.

The energy costs used for calculating the financial benefit of savings were increased from those assumed in the standard SAP calculation, but not to the level actually being charged in August 2008. The relative figures are as follows:

Thus the financial savings estimated are probably lower than can be achieved. The percentage savings are likely to be approximately correct.

The House Modelled

The house modelled is a 2-storey property, with a pitched roof. It is freestanding with a footprint of 8 by 8 metres. The room height is 2.4 metres. It has cavity walls with a 50 mm cavity, There is 50 mm of insulation below the loft. Heating is by gas, using a boiler with an efficiency averaging 73%. The controls include a timer/programmer, thermostats in the main room and on the hot-water tank, and some radiator thermostatic controls. The hot-water pipes are not lagged. The hot-water tank has 50 mm of loose fitted lagging.

Other assumptions arise from the SAP model for reference houses, e.g. there are 30 sq. metres of windows – 25% of the total floor area.

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