When the house gets uncomfortably hot, the choice usually comes down to a fan or a portable air conditioner. They look like two answers to the same problem, but they are really different machines doing different jobs at wildly different running costs. Knowing which one you actually need, and on which days, saves both money and disappointment.
The short answer. A fan moves air, which cools your skin for about a penny an hour. A portable air conditioner removes heat from the room and pumps it outside, which genuinely lowers the temperature but draws a kilowatt or more, costing roughly 26p to 39p an hour, thirty to fifty times as much. For most warm British days a fan is enough; a portable air conditioner earns its higher cost only on the small number of days when the air itself is simply too hot.
They do different jobs
The key thing to understand is that a fan does not cool the air at all. It moves it, and that moving air cools you by speeding up the evaporation of sweat from your skin, so you feel cooler while the room stays the same temperature. A portable air conditioner does something fundamentally harder: it uses a compressor and refrigerant to pull heat out of the room's air and pump it outside through a hose, actually dropping the temperature. One makes you feel cooler; the other makes the room cooler. That distinction explains both the comfort difference and the cost difference between them.
The running-cost gap
Because the two do such different work, they sit at opposite ends of the cost scale. The table uses the standard price-cap rate of about 26p per kWh, current in mid 2026, and the gap is stark.
| Device | Typical power | Cost per hour | Cost per 8-hour day |
|---|---|---|---|
| Pedestal or tower fan | 50 W | ~1.3p | ~10p |
| Evaporative air cooler | 60 to 100 W | ~2p to 2.6p | ~13p to 21p |
| Portable air conditioner | 1,000 to 1,500 W | ~26p to 39p | ~£2.09 to £3.13 |
At an example 26p per kWh. A portable air conditioner can cost in a single day what a fan costs in a month of nightly use. Full figures and how to cut them are in the air conditioning running cost guide.
Where an evaporative cooler sits
Between the fan and the air conditioner sits a third option worth knowing about: the evaporative cooler, sometimes sold as an air cooler. It blows air over a wet pad, and as the water evaporates it takes a little heat with it, so the air coming out is slightly cooler than the air going in, unlike a plain fan. It uses only a little more power than a fan, so it is cheap to run. The catch is that it adds moisture to the room and works best in dry heat; in the muggy, humid warmth of a typical British hot spell its effect is modest, because the air is already damp and evaporation slows. It is a cheap half-step up from a fan, not a substitute for real air conditioning.
The portable air conditioner's catches
A portable unit does genuinely cool a room, but it comes with strings beyond the running cost. It must vent its heat outside through a hose in a window, and the window gap has to be sealed, or hot air pours back in and it never wins. Cheaper single-hose models have a built-in inefficiency, drawing replacement air in from the rest of the house as they blow air out, so part of their effort is wasted. They are also bulky, noisy, and only really cool the one room they stand in. None of this makes them useless, but it does mean a portable air conditioner is a considered purchase for genuine need, not a casual grab, and the running cost guide covers getting the most from one.
How to decide
Work from the bottom up. For the large majority of warm days, the free measures plus a fan are enough: shade the windows against the sun, flush cool air through at night, and let a cheap breeze do the rest, all set out in the keeping cool without air conditioning guide. Reach for the fan first because it costs almost nothing. Consider an evaporative cooler if you want a touch more for not much more power, accepting it does little in humid heat. Buy a portable air conditioner only if you genuinely face a run of days when the air itself is too hot to sleep or work in, you cannot get relief any other way, and you accept the running cost and the venting faff. For most British summers that is a handful of days a year, which is worth weighing against the price of a machine that sits in a cupboard the rest of the time.
The bottom line
A fan and a portable air conditioner are not two versions of the same thing. The fan moves air to cool your skin for about a penny an hour and handles most warm days; the air conditioner removes heat to cool the room for pounds a day and is for the rare days nothing else can manage. An evaporative cooler is a cheap middle option that struggles in humid heat. Start with shading and a fan, step up only when you must, and you keep a British summer comfortable without letting the cooling become the thing that runs up the bill.