How much does an air conditioner cost to run?
With 28°C outside and a 21°C target, a 3.5 kW air conditioner draws about 0.31 kW while cooling. Running it for 8 hours a day costs about 66.1p a day, £4.62 a week or £20.11 a month at 26.32p per kWh.
Adjust for your room
A 7°C gap means the unit works at about 35% of its 3.5 kW rating.
While it is running
Worked example
With 28°C outside and a 21°C target, the gap is 7°C. The room needs about 1.225 kW of cooling (7 × 0.175 kW per degree). At that gap the unit’s efficiency is an EER of 3.905, so each 1 kWh of electricity moves 3.905 kWh of heat out of the room. It therefore draws 1.225 ÷ 3.905 ≈ 0.314 kW.
Over 8 hours that is 2.51 kWh. At 26.32p per kWh it costs 66.1p a day and £4.62 for a full week. Run that way through a 8-week hot spell, the bill is about £36.99.
What each degree costs
Every degree you ask the unit to cool further widens the gap and makes it work harder. A 20°C target instead of 21°C raises the cost from 66.1p to 77.2p a day, about 17% more. A 23°C target instead brings it down to 45.2p a day.
As a heater
Most split units also heat. In this estimate, with 5°C outside and 21°C inside, the room needs 2.8 kW of heat. The unit’s COP is about 2.92, so it draws 0.96 kW and costs 25.2p an hour. A fan heater or electric radiator giving the same heat uses 2.8 kW and costs 73.7p an hour. See the fan heater and oil-filled radiator pages for their own figures.
How to cut the cost
Clean the filters, keep doors and windows shut, and close curtains or blinds on sunny windows so the unit has less heat to remove. Your unit’s label shows its seasonal efficiency, SEER for cooling and SCOP for heating, and a higher number means a lower bill. A timer or sleep mode stops it cooling an empty room.
How this estimate works
- The room needs 0.175 kW of cooling or heating for every °C between outside and inside, up to the unit’s 3.5 kW rating.
- Efficiency starts at an EER of 4.5 for cooling and a COP of 4.2 for heating, then falls by 0.085 and 0.08 for every °C of gap, and never below 1.5.
- Power drawn is the load divided by the efficiency, capped at 1.25 kW.
- These are simple estimates for a typical single-room unit, not manufacturer test data. They ignore room size, sunshine, insulation, standby use and defrost cycles.
Questions
Is an air conditioner expensive to run in the UK?
Often less than people expect, because it moves heat instead of making it. With 28°C outside and the room held at 21°C, a 3.5 kW air conditioner draws about 0.31 kW, which costs 8.3p an hour at 26.32p per kWh. The cost climbs quickly as the gap between outside and inside widens.
How much does it cost to run an air conditioner in a heatwave?
With 35°C outside and the room held at 22°C, the gap is 13°C and the unit draws about 0.67 kW. Over 10 hours that costs about £1.76.
How much does it cost to run an air conditioner overnight?
On a warm night of 22°C, cooling the bedroom to 19°C is a 3°C gap. The unit draws about 0.12 kW, so 8 hours costs about 26p.
Is it cheaper to heat a room with an air conditioner?
In heating mode the unit moves heat in from outside, so it can deliver more heat than the electricity it uses. In this estimate, with 5°C outside and 21°C inside, the room needs 2.8 kW of heat and the unit draws about 0.96 kW, which costs 25.2p an hour. A fan heater or electric radiator giving the same heat uses 2.8 kW and costs 73.7p an hour.
How accurate is this estimate?
It is a guide, not a measurement. The calculator uses a simple model of how hard a typical single-room unit has to work, and it ignores room size, sunshine, insulation and standby use. Your unit’s energy label shows its seasonal efficiency, and your own meter readings show the real cost.
Other appliances
Electricity rate: Ofgem energy price cap, 1 October to 31 December 2026, Great Britain average, Direct Debit, default tariff: 26.32p per kWh. Your own tariff and region may differ, so change the rate above to match your bill. Appliance figures are typical estimates; your appliance's energy label or manual is more accurate. Last checked 2026-11-03. Next review: January 2027, when Ofgem announces the next quarter's cap.