Air conditioning running costs in the UK: the maths, with current prices

Quick answer: Cost is electrical input (kW) x hours x unit rate. At Ofgem's 26.32p per kWh, a 3.5 kW split costs about 10p to 20p an hour to cool a room and a portable about 30p to 35p. A split running 350 full-output hours a year costs roughly £35 to £53.

Prices checked 10 October 2026. The sum is simple. The part that trips people up is converting "3.5 kW of cooling" into the electricity used, so we do it step by step.

The sum: kW x hours x unit rate

Air conditioners are sold by the heat they move (output), but you pay for the electricity they draw (input). Efficiency links the two.

Electrical input (kW) = cooling output (kW) ÷ efficiency Cost = electrical input (kW) × hours × unit rate (£ per kWh)

Worked example for a 3.5 kW cooling output at an efficiency of 6.1, at 26.32p per kWh:

1. Input = 3.5 ÷ 6.1 = 0.57 kW
2. Cost per hour = 0.57 kW × 26.32p = about 15p
3. Over 350 hours = about £53

Use the running cost estimator to run your own figures. It takes the same three inputs.

Which unit rate to use

Ofgem's price cap for 1 October to 31 December 2026 sets average rates for people paying by Direct Debit in England, Scotland and Wales:

Ofgem says there is no VAT on electricity from 1 October 2026 to 31 March 2027, and that it sets the price cap level every three months (Ofgem's price cap explainer), so these rates cover 1 October to 31 December 2026 only. Your own rate depends on your region, payment method and tariff, and a fixed deal can sit above or below the cap. Check your bill. Which? uses the same 26.32p figure for its running-cost estimates. Standing charges do not change when you add an air conditioner, so we leave them out of every example below.

What the efficiency numbers mean

Three figures appear on datasheets, and mixing them up is the commonest mistake.

UK law (retained Regulation 626/2011) sets the label classes for air conditioners up to 12 kW. A+++ starts at SEER 8.50, A++ at 6.10, A+ at 5.60, A at 5.10 and B at 4.60. For heating, A+++ starts at SCOP 5.10, A++ at 4.60, A+ at 4.00, A at 3.40 and B at 3.10. The label assumes 350 hours of cooling and 1,400 hours of heating in an average climate year, which is how it produces its "kWh per year" figure.

Efficiency is not constant, which is why seasonal figures are higher than full-load ones. The certified Daikin RXM35A/FTXM35A has an EER of 4.63 at full 3.5 kW output. At lower outputs the certified figures are higher: EER 7.23 at 2.58 kW, 11.51 at 1.66 kW and 14.30 at 1.25 kW. (These are the certificate's part-load points. The record does not give the outdoor temperature for each, so we do not split the gain between lower load and other test conditions.) Its SEER is 9.25 and its SCOP is 5.20. Daikin UK's size guide gives about 0.1 kW per square metre as a rule of thumb for choosing the capacity, and the size calculator helps with a first estimate.

Cooling cost: split systems and portables

All figures below are for a 3.5 kW cooling output at 26.32p per kWh. Efficiency inputs come from the legal class boundaries and the certified Daikin record, which we picked as a named, checkable example. The class-boundary rows show what the lowest unit in each class would cost.

The last two rows use the EER bands for portable (single and double duct) units in Annex II. Compare the "full load" rows with each other and the "seasonal" rows with each other. A seasonal figure is flattering because it includes gentle part-load running, so on a very hot day, when the unit works hard, expect closer to the full-load rows. The like-for-like gap is about 20p against 30p to 35p an hour, which is one and a half to nearly two times as much for the class A portable. One more caveat: Annex VII tests splits and double-duct portables with 27 °C air indoors, but single-duct portables with 35 °C indoors, a tougher test. The 35 °C condition applies only to single-duct units, so the like-for-like gap may be smaller than the table suggests for a single-duct portable, and it is not a rule for every portable.

Which? sells the same message in a different form. It says most portable units need hose venting, that a window left partly open lowers efficiency, and that portable units are heavy and awkward. Its own running-cost column for tested models sits behind its paywall, so we do not quote it.

From hours to a yearly bill

"Hours" here means hours at full cooling output. A unit rarely works flat out for a whole clock hour, so use the label's 350 hours as a standard year, 100 as a light user and 700 as a heavy one.

Against the £1,723 a year that Ofgem quotes for the 1 October to 31 December 2026 price cap, 350 hours on an efficient split adds about 2 to 3%. Homebuilding quotes a Daikin representative at about £900 a year for a five-room multi-split used for both heating and cooling six hours a day, seven days a week, and about £600 at four hours a day in a well-insulated house. Homebuilding does not state the unit rate behind them, and they cover a whole season of heating as well, so treat them as the scale of a whole-house system rather than a summer cooling bill.

Heating cost: heat pump against gas

A reversible split is an air-to-air heat pump, and Which? says it is far more efficient than direct electric heating. Heating cost uses SCOP, and the price comparison with gas is where the electric rate matters. Gas is cheaper per kWh, but a boiler delivers less than 1 kWh of heat per kWh of gas, and a heat pump delivers several.

For 4 kW of heat output at 26.32p per kWh electricity and 7.97p per kWh gas:

The 90% figure is the Energy Saving Trust's for an A-rated boiler. The 80% figure is our own assumption for an older boiler, not a sourced figure. Swap in your own boiler's seasonal efficiency. Put the break-even in a sentence: at those rates a heat pump matches gas on fuel cost at a SCOP of about 3.0 for a 90% boiler and about 2.6 for an 80% one. The certified Daikin record also lists a COP of 3.15 at -10 °C outdoor temperature, so even in a hard cold snap that unit stays above the 90% boiler break-even. This ignores standing charges, the cost of fitting the unit, and the fact that SCOP is a laboratory seasonal figure and not a guarantee for your house.

Which? also warns that if you have never had air conditioning, using a heat pump for cooling will add considerably to your annual energy use, and recommends insulation and shading first. Both cost nothing to run once they are in place.

How to cut the cost

• Size it right. Daikin warns that wrong sizing risks a more expensive unit, higher running costs and reduced comfort. Daikin adds about 20% for the South East of England against Scotland.
• Clean the filters. Daikin and Samsung UK both recommend cleaning about every two weeks. Toshiba's maintenance guidance lists keeping energy efficiency up and operating costs down among the reasons for regular servicing, and says airflow should not fall below 80% of the specified value.
• Pay for efficiency where you will use it. On the figures above, going from SEER 6.1 to 9.25 saves about £18 a year at 350 cooling hours. If the unit also heats for months, higher SCOP saves more.
• Use the schedule. Which? says air-to-air units are programmable, and many offer smart control.
• Close the room up. Which? notes that a window left partly open makes a portable unit less energy-efficient. Keep windows closed while cooling.
• Cool the building as well as the air. Shading and insulation first, as Which? advises.

Our servicing guide covers what a yearly service costs, and our guide to choosing a home system covers efficiency classes in more detail. To compare installers who show prices and efficiency ratings, start at our Brighton and Hove installer directory.