What Does It Cost to Run an Air Conditioner?
A 9,000 BTU air conditioner (about 2.6 kW of cooling) with a coefficient of performance of 3.5 draws about 740 watts. Run for 6 hours a day over 30 hot days, with the compressor on 70% of the time, it uses 93.2 kWh, which costs about £24.54 at the Ofgem price cap rate of 26.32p per kWh.
Try your own numbers
Last checked 10/11/2026. Sources: Ofgem: Energy price cap unit rates and standing charges; U.S. Department of Energy: Estimating appliance and home electronic energy use; Virginia Cooperative Extension: Estimating Appliance and Home Electronic Energy Use (typical wattages, 2020).
How these figures are checked. Sourced figures link to the publisher’s own page and carry a last-checked date; our own rules of thumb are labelled as such. Run by JB, an independent publisher based in Australia. Spotted a mistake? Email hello@calcfig.com.
How the cost of air conditioning is worked out
An air conditioner is rated by the cooling it delivers (BTU per hour, or kilowatts) and how efficiently it turns electricity into cooling. The electricity it draws is the cooling divided by the efficiency. 9,000 BTU per hour is about 2.6 kW of cooling; with a coefficient of performance (COP) of 3.5, the unit draws about 2,600 ÷ 3.5 = 740 W. The COP of 3.5 is an assumption for the example, not a measured figure for any model: your unit’s label or specification sheet gives its real power draw, and portable units are often much less efficient than fixed split systems.
740 W for 6 hours is 4.44 kWh at full power. A compressor does not run flat out all day, so the example assumes it is on 70% of the time: 3.108 kWh a day. Over 30 days that is 93.2 kWh, and at 26.32p per kWh the spell costs £24.54, or about 82p a day. The 26.32p is the Ofgem price cap unit rate for October to December 2026; the standing charge is not included because you pay it whether or not you use the air conditioner.
UK summers are short, so the days of use matter more than the size of the unit. Change the hours and days to match yours.
Common questions
How many watts does a portable air conditioner use?
It depends on its size and efficiency. A 9,000 BTU unit at a COP of 3.5 draws about 740 W while the compressor runs; portable units are often less efficient, so check yours.
Does the compressor run all the time?
No. It cycles on and off to hold the temperature. The calculator lets you set the share of time at full power; 70% is an example, not a rule.
Does the standing charge matter?
No. You pay the standing charge every day whether or not you run the air conditioner, so only the unit rate is used here.