Every heatwave, the cheap end of the cooling market fills up with evaporative coolers, often sold as air coolers or swamp coolers, promising air-conditioning comfort for a fraction of the running cost. They sip electricity and the headline figures look brilliant next to a portable air conditioner. The catch is the one thing the box never mentions: they only work well in dry air, and the UK in summer is rarely dry. So the real question is not whether they are cheap, but whether they cool a damp British room enough to be worth buying.
The short answer. Partly. An evaporative cooler costs very little to run, roughly 2p to 4p an hour, but it only cools effectively when the air is dry. In a typical humid UK summer, with relative humidity often 60 to 80 per cent, the temperature drop is small and the cooler also adds moisture to the room. They are a decent budget fan-plus on a dry, hot day, and a disappointment on a muggy one.
How an evaporative cooler actually works
An evaporative cooler is a fan that blows room air through a wet pad or filter. As the water evaporates, it pulls heat out of the air, so the air leaving the unit is cooler and more humid than the air going in. It is the same effect you feel when you step out of a swimming pool on a breezy day and suddenly feel cold.
This is completely different from a refrigeration-based air conditioner, which uses a compressor and refrigerant to move heat out of the room, usually through a hose to a window. That is why a true air conditioner can chill a sealed room on any day, while an evaporative cooler is at the mercy of the weather. The drier the incoming air, the more water it can absorb, and the bigger the temperature drop. The more humid the air already is, the less it can absorb, and the weaker the cooling.
There is a hard physical ceiling here called the wet-bulb temperature. An evaporative cooler can never cool the air below it, and in practice gets nowhere near. In dry desert climates that ceiling sits well below room temperature, which is why these machines are popular in parts of the United States, Spain and the Middle East. In a humid maritime climate the ceiling is much closer to the actual temperature, so there is little room left to cool.
Why UK humidity is the problem
The UK is a group of islands surrounded by sea, so the air arriving over the country is usually carrying a lot of moisture. Even on hot summer afternoons, relative humidity commonly sits between 50 and 80 per cent, and recent muggy heatwaves have pushed it higher still. That is the worst possible starting point for evaporative cooling.
On a dry day at, say, 30 per cent humidity, a good cooler might drop the air leaving it by 8 to 12 degrees Celsius. On a muggy UK day at 70 per cent, you might get 2 to 4 degrees, and that small drop fades as the unit pumps more moisture into the room. Run one for long enough in a closed room and you can end up with air that feels clammy rather than cool, which is the opposite of what you wanted.
The fix that makes them work, which is keeping a window or door open so dry outdoor air keeps flowing through, is also the thing that undoes a fan's comfort. You cannot have a sealed, cool room and an effective evaporative cooler at the same time. For more on managing a hot home without refrigeration, see our guide to keeping cool without air con.
What an evaporative cooler costs to run
This is where evaporative coolers genuinely shine. A domestic unit is mostly just a fan and a small water pump, so the wattage is low, typically 60 to 150 watts. That puts running costs in the same territory as a powerful fan rather than an air conditioner.
| Appliance | Typical power | Cost per hour | Cost for 8 hours |
|---|---|---|---|
| Evaporative cooler | 100 W | 2.6p | 21p |
| Large pedestal fan | 70 W | 1.8p | 15p |
| Tower fan | 50 W | 1.3p | 10p |
| Portable air conditioner | 1,000 W | 26.1p | £2.09 |
At 26.11p per kWh, Ofgem cap July to September 2026. Wattages are typical figures; check your own unit's rating plate.
So an evaporative cooler costs roughly a tenth as much per hour as a portable air conditioner. Run for eight hours a day across a hot fortnight, that is about £3 against roughly £29. If your only comparison is the cost column, the cooler wins easily. The problem is that the air conditioner in that table will actually drop the room temperature on any day, and the cooler may not. You are comparing the cost of cooling that works against the cost of cooling that might not. Our breakdowns of fan running costs and air conditioning running costs set out both ends of that range in detail.
One running cost the table hides is water. Evaporative coolers consume water, often a few litres on a hot day, and you may add ice for a short boost. On a metered supply that is a small extra cost, though usually pennies.
When an evaporative cooler is a sensible buy
They are not useless. There are a few situations where a UK household gets genuine value from one.
- You want a better fan, not an air conditioner. On a dry, hot day the moving air plus a few degrees of cooling feels noticeably nicer than a plain fan, for similar running cost.
- You are cooling yourself, not the room. Pointed at a person, the breeze does most of the work, the same as any fan. The evaporation is a small bonus.
- You have a draughty or open space. A conservatory with the doors open, a garage, a workshop or a garden gazebo gives the unit the constant supply of drier air it needs.
- Your budget rules out refrigeration cooling. A decent evaporative cooler costs far less to buy and run than a portable air conditioner, and it needs no window kit.
Where they disappoint is the most common use case: a sealed bedroom on a muggy night, hoping for air-conditioner comfort. There they add humidity and barely move the temperature. If overnight comfort is the goal, our guide on how to sleep in a heatwave without air con has cheaper tactics that actually work.
Evaporative cooler or portable air conditioner?
If you have decided a fan is not enough, the honest choice for most UK homes is between an evaporative cooler and a portable air conditioner, and it comes down to what you are willing to trade.
- Choose an evaporative cooler if you want low running cost, no window hose, and you mostly need a person-directed breeze in an airy space on dry days.
- Choose a portable air conditioner if you need a real temperature drop in a closed room regardless of the weather, and you accept the higher purchase price, the window kit and roughly ten times the running cost.
One thing to avoid is using an evaporative cooler in the same room as a dehumidifier, since one adds moisture and the other removes it, so you pay to run both while they fight each other. Our side-by-side guide to a portable air conditioner versus a fan covers the full comparison if you are still weighing the two.
The bottom line
Evaporative coolers do work in the UK, but only on the days the UK behaves least like the UK: hot and dry, with air moving through the room. On a humid summer day in a closed room they behave like an expensive fan that also makes the air feel sticky. The cheap running cost is real and the cooling on a good day is real, just smaller and less reliable than the marketing implies.
If your honest need is to feel a breeze and shave a couple of degrees on the hottest, driest afternoons, a cooler is a reasonable budget buy. If you need a room to actually be cold on demand, a portable air conditioner is the only one of the two that delivers, at roughly ten times the running cost. For everything in between, a good fan plus the cheap heat-management habits in our summer energy checklist will get you most of the way for the least money.