Flats heat up faster than houses and hold the warmth longer, especially top-floor and south-facing ones with big windows and little shade. When a heatwave hits, the instinct is to reach for whatever cools you down, but the gap between the cheapest and dearest option is large. A small fan costs a few pounds across the whole summer. A portable air conditioner left running can add more to your bill in a fortnight than the fan does in three months. This guide puts real numbers on each method using the current price cap, so you can pick the level of cooling that suits your flat and your wallet.
The short answer. For most UK flats, keeping cool over a summer costs very little if you stick to fans. A pedestal or tower fan run for eight hours a day costs roughly 10p to 13p a day, or about £10 to £12 across a three-month summer. A portable air conditioner is the expensive option at around 25p to 31p an hour, which is £2 to £2.50 a day if run for eight hours, and can reach £150 or more over a hot summer. Figures use the Ofgem cap electricity rate of 26.11p per kWh for July to September 2026.
What drives the cost of cooling a flat
Every cooling appliance you plug in runs on electricity, and electricity is the dearest fuel on your bill. At the current cap, each unit (kWh) costs 26.11p, so the running cost of any cooler is simply its power draw in kilowatts multiplied by the hours you run it, multiplied by that rate. A 50W fan draws 0.05kW. Run it for ten hours and that is 0.5kWh, or about 13p.
Three things decide your total summer cost:
- The appliance. A fan moves air using a tiny motor. An air conditioner runs a compressor that physically removes heat, which is why it draws twenty to thirty times more power.
- How long you run it. A fan you switch on for an hour to fall asleep costs almost nothing. A unit left on all day and night is where bills climb.
- Your flat. A well shaded ground-floor flat may need a fan only on the hottest evenings. A top-floor flat under an uninsulated roof can stay warm past midnight, so you run things longer.
If you want to understand the unit rate itself, see what a kWh is and how the energy price cap works.
Cooling methods compared, cheapest to dearest
The table below prices each common cooling method at a realistic daily run time. Fans are costed at eight hours, since people often leave them on overnight. The portable air conditioner is the outlier.
| Method | Typical power | Cost per hour | 8 hours a day | Over a summer (90 days) |
|---|---|---|---|---|
| USB desk fan | 5W | 0.1p | 1p | about £1 |
| Ceiling fan | 25W | 0.7p | 5p | about £5 |
| Pedestal or tower fan | 50W | 1.3p | 10p | about £9 |
| Evaporative cooler | 80W | 2.1p | 17p | about £15 |
| Portable air conditioner | 1,200W | 31p | £2.51 | about £225 |
At 26.11p per kWh, Ofgem cap July to September 2026. Power figures are typical mid-range models; check your own appliance label or use a plug-in monitor.
The pattern is clear. Anything that just moves air is cheap to the point of not mattering. The moment you ask a machine to remove heat, the cost jumps by a factor of twenty or more. Few people run air conditioning a full eight hours every day for ninety days, so the real-world summer figure is usually lower, but it shows the ceiling. For a fuller breakdown of the two extremes, compare a portable air conditioner against a fan.
Fans: the cheap workhorse
A fan does not lower the air temperature. It moves air across your skin so sweat evaporates faster, which makes you feel cooler. That is enough most of the time, and it is the reason fans are the sensible default for a flat.
The numbers are forgiving. A 50W pedestal fan run all night, say ten hours, costs about 13p. Leave it running every single night for the three warmest months and you are looking at roughly £12. A ceiling fan is cheaper still and clears floor space, though installing one in a rented flat is usually off the table. A small USB desk fan pointed at you while you work costs pennies across the whole summer.
To squeeze more from a fan, place it so it pulls cooler air in from a shaded window in the evening, or set it across a bowl of ice for a short top-up. Our guide to fan running costs has the full per-model maths, and sleeping in a heatwave without air con covers the night-time tricks.
Portable air conditioners: real cooling, real cost
A portable air conditioner genuinely lowers the room temperature, which a fan cannot. The trade-off is the running cost and some practical hassle. Most domestic units draw between 900W and 1,500W, so at the cap rate you are paying roughly 25p to 39p for every hour the compressor runs.
Two points push the cost up further in a flat:
- The exhaust hose. Single-hose units vent hot air out of a window, but they also pull warm air in from the rest of the flat to replace it, so they work harder. You also lose cool air through the gap around the hose unless you fit a window seal.
- Run time. Because a flat can hold heat well, the unit may cycle on and off for hours rather than cooling the room once and stopping.
Used sensibly, for an hour or two before bed in the hottest week of the year, a portable unit might add £3 to £5 to a week's bill. Run day and night through a long hot spell and it becomes the single biggest item on your summer electricity. The detail sits in our air conditioning running cost guide.
Cheaper ways to keep a flat cool
The cheapest cooling is the heat you stop getting in. None of this costs much, and most of it costs nothing.
- Close curtains and blinds on the sunny side during the day. Stopping direct sun through glass is the single biggest free win, especially for a south or west-facing flat. Pale or reflective blinds work best.
- Keep windows shut while it is hotter outside than in, then open them wide once the evening air drops below the indoor temperature. Cross-ventilation, with windows open on two sides, clears warm air fastest.
- Switch off heat sources. An old desktop, a games console left on, lamps and the oven all dump heat into the room. Cooking on the hob or in an air fryer rather than a full oven helps on hot days.
- Mind the fridge and freezer. They work harder in the heat and add warmth to a small kitchen, so keep them away from windows and check the seals. See fridge and freezer in hot weather.
For the full set of no-cost and low-cost tactics, keeping cool without air con is the place to start, and energy saving in a flat covers the wider picture for flat-dwellers.
The bottom line
For the overwhelming majority of UK flats, keeping cool in summer is cheap. A fan or two, run sensibly, costs you somewhere between a few pence and a few pounds across the entire season. The free measures, shading windows by day and ventilating at night, cost nothing and do most of the work.
A portable air conditioner is the only option that meaningfully changes your bill, and even then only if you run it hard and often. If your flat is genuinely unbearable in a heatwave and you decide a unit is worth it, use it in short bursts, fit a window seal, and treat it as the exception rather than the default. Reach for the fan first, the closed curtain second, and the compressor last.