Can reducing the thermostat 1°C really save 10%?

It seems extraordinary that reducing the thermostat setting by just one degree can save more than 10% on your gas bill. A back of the envelope calculation suggests this has to be an exaggeration. However, when you take into account other sources of heat it can be an underestimate. Of course it depends on various factors and for a less well insulated house the relative saving is less, though the kWh and £ savings will be more.

The back of the envelope calculation: less than 8%.

Suppose you start with the setting at 20°C. The average temperature outside during the heating season (in Cambridge) is 7.6°C. So you need heat for an extra 12.4°C on average. Reducing the temperature by 1 degree saves you 1/12.4 = 8%. So not quite 10%. If you add in hot water heating which is not affected by the thermostat at all, the savings ought to be lower still.

However,  some of your heat comes from solar gains, appliances etc.

However, in reality, you get quite a lot of heating from other sources including your use of electrical appliances, from sunshine through windows, and even from your own body heat. This chart shows sources of heat through the year for a house using weather for Cambridge (modelled, using EnergyPlus). The space heating is extra heat, on top of the basic gains. In the summer, you need less heat and you get more solar gains so you do not need any extra heat.

Modelled energy sources for a medium sized semi detached house with filled cavity walls through the year.

Reducing the thermostat setting means less extra heat is needed.

When you reduce the thermostat setting, you need less extra heat from the heating system. This chart shows the annual gas use (including hot water heating) with thermostat settings for 21°C down to 18°C in the living area (other rooms 1°C less) for a semi-detached house using two different wall types: solid walls on the left without insulation and filled cavity walls on the right.

Modelled annual gas use reducing the thermostat level from 21°C  to 18°C for two homes identical except for the type of walls: solid walls versus filled cavity.

Savings are greater for a less well insulated home (but a lower proportion).

Reducing the thermostat level by just one degree from 21°C to 20°C reduces gas demand by 1560 kWh (£160 under the price guarantee) in the solid wall case, and a little less: 1320 kWh (£130) in the cavity walls case. The savings are greater for the solid wall case because it is more heat-leaky though in relative terms the savings are less: only 10% for the solid wall case and 12% for the cavity wall case. This is because the cavity wall home gets a lower proportion of its heat from the heating system.

Cutting use of appliances will increase heating bills - but the heating is more cost-efficient.

When you use electricity in the home, for anything from running the freezer to playing computer games this also supplies heat (except for things like external lighting, which heats up the air outside, and vented tumble dryers because the heat escapes). If you reduce your electricity bill your heating bill will increase a little to make up for the missing heat. However, electricity costs more than gas per kWh so you still make cost savings. Even if you use an electric heat pump instead of a gas boiler, its super efficiency means you will make net savings.

Summary

• Quite a lot of your heat comes from solar gains, use of appliances, and other sources in the house; your heating system supplies the extra you need to keep warm.
• Reducing the thermostat by 1°C can reduce your total heating bill by 10% or even more.
• Savings (measured in £ or kWh) are greater for a more heat-leaky house (even though less in relative terms).
• Reducing use of electrical appliances will mean more extra heat is needed - but there are still net savings in £.
• Even if you have an electric heat pump for space heating, this is more efficient than using your TV.