How much does air conditioning cost to run in the UK?

Air conditioning has a reputation in the UK for being expensive to run, mostly earned by portable units and old systems. Modern split and air-to-air units are heat pumps: they move heat out of the room rather than making cold, and for every unit of electricity they use they move around six units of heat. That changes the sums completely.

How the running cost is worked out

The electricity a unit uses is the cooling it delivers divided by its efficiency. Mainstream wall units sold in the UK are rated A++ for cooling, which means a seasonal efficiency (SEER) of at least 6.1. A room unit is typically around 2.5 kW, but it only runs flat out on the hottest hours; averaged over an hour of use it delivers about half that. Multiply the electricity by your unit rate and you have the cost.

Cost by system

Why portable units cost so much more

A portable unit puts the whole machine, including the hot side, inside the room, then pushes the heat out through a hose in a window gap. Warm air leaks back in around the hose, and the unit pulls replacement air in from the rest of the home. It is rated on a single-point efficiency of around 2.6, against 6.1 for a split unit, so it uses more than twice the electricity for the same cooling before those losses. It is a reasonable stop-gap for a rented room or a short heatwave, not a long-term answer.

What moves your bill most

• Hours of use. Cooling a bedroom for the first few hours of the night costs a fraction of running it all day.
• Setpoint. Setting 24°C rather than 20°C cuts the work the unit does sharply, and 24°C to 26°C feels comfortable when the air is dry.
• Shading and closed windows. Every watt of sun you keep out is a watt the unit does not have to remove.
• Your tariff. A time-of-use tariff with cheaper overnight or afternoon rates changes the answer, and so does solar power on your roof, which peaks exactly when cooling is needed.

Will it cost more as summers get hotter?

Yes, because the unit runs for more hours. Cooling degree days measure how far, and how often, the daily mean temperature sits above 22°C. For Islington, the Met Office's local projections go from about 78 cooling degree days a year in the 2001 to 2020 climate to about 131 in a world 2.5°C warmer than pre-industrial, a rise of about 68%. The unit's cost per hour stays the same; the number of hours you need it does not.

Does air conditioning also heat?

Most split units can run in reverse and heat the room, and an air-to-air heat pump is designed to do both jobs all year. Used as the main heating, it can replace a boiler, which is the one case where a grant comes into play.

For cooling alone, the running costs above are what matter; for a system that also heats your home, the checker compares the heating side against the fuel you use today.