In my August post, I commented on the importance of gaining a ‘feel’ for one’s individual carbon footprint. This post is a case in point.

Recently, we had one of the coldest August Bank Holidays recorded in the UK. A friend commented that she had put on the heating in her home. Separately, during a call about sustainability practices at a local school, someone raised the suggestion that pupils should always be encouraged to turn the lights off when leaving schoolrooms.

Which is more significant in terms of carbon: leaving the lights on, or turning up the thermostat? Here’s a simple comparison.

Consider a modern kitchen lit by eight LED bulbs in the ceiling, each one with a power output of 5W. If the lights in this kitchen are left on for 24 hours, the electricity usage is about 1 kWh. The carbon intensity of electricity in Britain is now around 0.2 tCO2/MWh, or equivalently 200 gCO2/kWh, so the carbon emissions would be about 200 gCO2.

Now imagine turning up the thermostat by, say, 3 degrees Celsius. A typical home in the UK (three-bed semi-detached, built in the first half of the twentieth century) might require about 7 kWh of gas per day per degree Celsius of extra temperature. To a rough approximation, the required heat energy is proportional to the temperature differential between inside and outside, so turning the thermostat up 3 degrees might require around 21 kWh of extra gas per day. The carbon intensity of natural gas, by coincidence, is about the same as the current electricity coefficient, i.e. about 200 gCO2/kWh. So the carbon emissions would be about 4200 gCO2, over twenty times the figure for leaving the lights on.

Of course, I am comparing here the impact of switching on lights in one room, the kitchen, with turning up the heating for the whole house. But then we normally switch on lights one room at a time, but switch the (central) heating on for the whole property.

It was not always thus. Back in 1990, say, around three-quarters of our electricity was generated from coal-fired power stations; and incandescent light bulbs were normal. Combining these two effects, it is quite possible that the carbon footprint of lighting a kitchen would have been around ten to twenty times what it is now. Back then, the two actions in this comparison might have been similar in their carbon impact – and it is likely that switching off the lights in the house as a whole would have been more significant than moving the thermostat by 3 degrees, even allowing for a less efficient gas boiler back then.

But times change, and we need to keep up-to-date. What we decide to do with the updated information is another matter – but perhaps we could enable and educate school pupils to turn down the heating first, before turning off the lights, whenever they leave a classroom.