Portable Air Conditioner Running Costs UK: What You'll Really Pay Per Hour

Every heatwave follows the same script: portable air conditioners sell out by Thursday, and four weeks later the electricity bill lands. Having installed and serviced cooling kit for years, I can tell you the running cost is rarely the horror story people fear, but it is never the 'pennies per hour' the marketing implies either.

This guide gives you the actual arithmetic: what a typical unit draws, what that costs at the current unit rate, and worked examples for an evening, a week and a full heatwave, with every figure checkable yourself using a £12 plug-in meter.

Most portable air conditioners sold in the UK draw between 0.8kW and 1.5kW of electrical input. Under the July to September 2026 price cap, electricity averages about 26p per kWh on a standard variable tariff, give or take a penny depending on your region. The maths is simple: a compact 0.9kW unit costs about 23p per hour flat out, a mid-size 1.2kW unit about 31p, and a large 1.5kW machine about 39p.

Those are worst-case figures. Once the room reaches the set temperature, the compressor cycles on and off, and real-world consumption drops to roughly 70 to 85 per cent of the plated figure. On a time-of-use tariff with a cheap overnight window, running one through the small hours can cost half the daytime rate. For context, a decent 9,000 BTU unit costs £250 to £450 to buy, so a full summer of evening use typically costs less than a fifth of the machine itself.

The number on the box is almost always cooling output, quoted in BTU: 7,000, 9,000 or 12,000 for most UK models. What you pay for is electrical input, which is a different and smaller number, found on the rating plate on the back of the machine. The ratio between the two is the energy efficiency ratio, or EER, and for portables it usually lands between 2.4 and 2.8.

Worked through: a 9,000 BTU unit produces about 2.6kW of cooling. Divide by an EER of 2.6 and the electrical input is roughly 1.0kW, so about 26p per hour at cap rates. A 7,000 BTU compact works out near 0.8 to 0.9kW input. If you are choosing between sizes, our guide on how to size an air conditioner covers the kW-per-room-size method; an undersized unit that runs flat out all evening costs more than a correctly sized one that cycles.

Three honest scenarios at 26p per kWh, using an 80 per cent duty cycle after the first hour of pull-down; these match what I have measured on real installations.

• •0.9kW compact (7,000 BTU), one evening: 5 hours from 6pm to 11pm. First hour at full draw (23p), then four hours cycling (about 19p each). Total: roughly £1.00 per evening.
• •0.9kW compact, one week of evenings: 7 evenings at £1.00 gives about £7 per week, or £28 to £30 across a warm month.
• •1.2kW mid-size (9,000 BTU), one evening: 5 hours costs about £1.35 with cycling. A week of evenings: around £9.50.
• •1.2kW unit in a proper heatwave: 10 hours a day for 7 days at 85 per cent duty comes to about £18.50 for the week. Uncomfortable on the bill, but £2.65 a day for a cool bedroom during a 33°C spell is a defensible spend.
• •For comparison, a 50W pedestal fan costs about 1.3p per hour: a full 8-hour night is 10p, and a week of nights is around 73p.

Two identical units in two different rooms can cost wildly different amounts to run. These are the factors that matter, in the order they usually bite.

• •The window kit. A single-hose portable pumps air out, pulling warm outside air back in through every gap. An unsealed sash window can double the running time. Fit the fabric seal kit properly; our draught-proofing guide applies in summer as much as winter.
• •Room size and sun. A 9,000 BTU unit in a 14m² north-facing bedroom cycles happily. The same unit in a 25m² south-facing loft room runs flat out and still loses, costing 30 per cent more for a worse result.
• •Thermostat setting. Each degree lower adds roughly 5 to 8 per cent to consumption. Set 24°C, not 19°C; you want relief, not a meat locker.
• •Dirty filters. A clogged intake filter forces longer run times. Rinse it fortnightly in season; how to clean an air conditioner covers the five-minute job.
• •Timing. Cooling the bedroom from 9pm is far cheaper than fighting the 3pm peak heat, and cheaper again on an off-peak overnight tariff.

A fan does not lower the room temperature at all; it cools people by moving air across skin. That distinction decides which nights each tool earns its keep.

• •Pedestal or desk fan (45 to 70W): 1 to 2p per hour. Below about 28°C room temperature, this is all most people need.
• •Ceiling fan (about 30 to 60W): similar cost with better coverage; make sure it is spinning the right way, per our guide on ceiling fan direction in summer.
• •Portable air conditioner (0.8 to 1.5kW): 23 to 39p per hour, but it genuinely drops the room 5 to 8°C. Above 30°C, it is the only thing on this list that works.
• •Window unit (0.5 to 1.0kW): more efficient than a portable because the hot side sits outside, though fitting one to UK windows takes some carpentry. See our window air conditioner picks if your window suits.
• •Evaporative 'air coolers' (60 to 80W): cheap to run but nearly useless in humid UK heat, and they add moisture to the room. Do not confuse them with air conditioning, whatever the listing claims.

Do not take the box's word for it, or mine. A plug-in energy monitor costs £10 to £15 from Screwfix or Argos, sits between the plug and the socket, and logs cumulative kWh. Run the unit for a typical evening, read the total, multiply by your unit rate from your bill. That is your real cost, duty cycle included.

Treat certain claims as red flags. 'Costs pennies to run' with no wattage stated usually means an evaporative cooler dressed up as air conditioning. A 12,000 BTU claim on a machine priced under £200 is almost certainly inflated. If the seller cannot tell you the input wattage, walk away.

All costs use the Ofgem price cap electricity unit rate for July to September 2026, taken as 26p per kWh; regional rates vary by roughly a penny either way, and fixed or time-of-use tariffs differ, so substitute the rate from your own bill. Duty cycles of 70 to 85 per cent reflect meter readings on sealed, correctly sized installations; a leaky window kit or an oversized room pushes real consumption towards 100 per cent. Purchase prices were checked in August 2026 against Argos, Currys and Screwfix. When the cap changes in October, scale every figure in proportion: the arithmetic stays the same, only the pence per kWh moves.