Portable air conditioner running costs in the UK: worked examples

Running-cost worry stops a lot of people buying air conditioning they would genuinely benefit from — and encourages others to buy an "air cooler" that costs little to run because it does almost nothing. The honest answer is that a portable AC is a significant but knowable expense, and you can work out yours on the back of an envelope before spending anything. This guide shows the sums with real numbers you can swap for your own.

Electricity is billed per kilowatt-hour, and a kilowatt-hour is just power multiplied by time. So your cost is: input power (kW) × hours running × your unit rate (pence per kWh). That is the whole model.

The input power you need is the electrical consumption figure on the rating plate and energy label — typically somewhere around 0.8–1.3 kW for common portable units. Do not use the BTU or kW cooling capacity figure in this sum: that describes heat moved, not electricity consumed, and is roughly two-and-a-half to three times larger than the input figure. Confusing the two is how frightening, wrong estimates get made.

Take a unit with a 1.0 kW electrical input, run from 6pm to midnight — 6 hours — to cool a living room and then a bedroom for sleep.

• Energy per evening: 1.0 kW × 6 h = 6.0 kWh.
• Cost per evening at the assumed 27p/kWh: 6.0 × £0.27 = £1.62.
• A hot week (7 evenings): 7 × £1.62 = £11.34.
• A hot month (30 evenings): 30 × £1.62 = £48.60.

If your rate is 30p, the evening becomes £1.80; at 22p it is £1.32. The structure of the sum never changes — only your two inputs do.

Same 1.0 kW unit, but running from 10am to 10pm — 12 hours — because someone is home all day through a heatwave:

• Energy per day: 1.0 kW × 12 h = 12 kWh.
• Cost per day at 27p/kWh: 12 × £0.27 = £3.24.
• A 10-day heatwave: 10 × £3.24 = £32.40.
• A full hot month, 30 days: 30 × £3.24 = £97.20.

That headline month figure is worth staring at before a heatwave rather than after a bill — but it is also an overestimate for most real use, for the reason below.

The sums above assume the compressor draws full power every minute it is switched on. In practice, once the room reaches the set temperature a thermostat-controlled unit cycles the compressor off and on, or an inverter model throttles down. If the compressor is actually working 60% of the time — an illustrative duty cycle, not a promise — the 6-hour evening costs 0.6 × £1.62 ≈ 97p, and the 12-hour day about £1.94.

How close you get to that depends on the fight the unit is in: a well-sized unit in a shaded, sealed, well-vented room reaches setpoint and cycles; an undersized unit in a sun-baked room with a leaky window seal runs flat out all day and pays the full worst-case figure. Running cost and cooling performance are the same problem.

• The rating plate (usually on the back or side) states rated power input in watts — 1,000 W is 1.0 kW in the sums above.
• The energy label lists consumption and an efficiency ratio; between two units of the same cooling capacity, the higher-efficiency one buys the same cool for less electricity.
• A plug-in energy monitor, bought for a few pounds, sits between the unit and the socket and tells you the actual kWh used per day — the definitive answer for your room, your weather and your settings.
• Your electricity unit rate is on your bill or your supplier's app. Use the actual pence-per-kWh figure, not the monthly total.

• Size correctly. An undersized unit runs continuously at full draw and still fails; the sizing guide covers picking the right band.
• Vent and seal properly. A leaky window seal pulls warm air straight back in, lengthening compressor time — sealing is the cheapest efficiency upgrade there is.
• Shade the room. Blocking afternoon sun through glass removes a large heat load the unit would otherwise pay to pump back out.
• Set 24–26 °C, not 18 °C. Every degree lower extends compressor time; "cool enough to sleep" is much cheaper than "cold".
• Cool the room you are in and close the door. One cooled bedroom overnight costs a fraction of chasing a whole open-plan flat.
• Pre-cool ahead of the peak. Running harder in the cooler morning and coasting through late afternoon can beat starting the fight at 4pm.