How an evaporative cooler works

It works the same way sweat cools your skin. Turning liquid water into vapour takes energy, and the water takes that energy from the air around it as heat. An evaporative cooler (often sold as an "air cooler") has a water tank, a pad or filter that soaks up the water, and a fan. The fan pulls room air through the wet pad, some of the water evaporates, and the air that comes out the front is a little cooler and more moist.

Nothing carries heat outside, the way an air conditioner's exhaust hose does. The air the cooler blows out is cooler than the air it took in, but the room as a whole gains moisture with every hour it runs.

Why humid air makes them struggle

Water evaporates fastest into dry air. When the air already holds a lot of moisture, less water evaporates from the pad, so less heat is taken out and the air coming out is barely cooler than the air going in. The US Department of Energy, writing about evaporative coolers for American homes, says they are only suitable for areas with low humidity and shouldn't be used in humid climates, because they add humidity to your home.

There's a second problem. Humid air also slows down the evaporation of sweat from your skin, which is how your body cools itself. So as the cooler raises the humidity in a closed room, you can end up feeling stickier and no cooler.

The damp problem

Adding water to the air of a closed bedroom is what the rest of this site warns against in winter. The US Department of Energy notes that if windows aren't open far enough, humidity builds up in the home. Run an evaporative cooler with the windows shut and the humidity in the room climbs, and humidity that stays high for days is what lets mould grow.

Too dry Comfortable
40–60% Damp Mould risk
above 70%

A hygrometer tells you where your room sits on that scale. If it already reads in the damp range on a hot day, an evaporative cooler will push it higher. The ideal humidity for a home explains the ranges.

When one might help

An evaporative cooler is worth considering only when all of these are true:

  • your hygrometer shows the air in the room is dry on hot days, towards the low end of the scale above
  • you can keep a window open while it runs, so the extra moisture has somewhere to go
  • you'll sit close to it, in the airflow, rather than expecting it to cool the whole room
  • you're happy to fill the tank, and to empty and clean it as the manual says so the water doesn't go stale

Compared with a fan or an air conditioner

The evaporative cooler in our list is rated at 60W, which costs 1.6p an hour. That is more than the fans, which average 35W, or 0.9p an hour. The portable air conditioners average 847W, or up to 22p an hour.

Costs use 26.32p per kWh, the Ofgem price cap, Direct Debit average for England, Scotland and Wales for 1 October to 31 December 2026. Your own tariff may differ. How we calculate.

  • A fan gives you the moving air without the water. On a humid day that is most of what an evaporative cooler does for you, and a fan adds no moisture to the room. See the best fan for a bedroom.
  • A portable air conditioner takes heat out of the room and sends it outside, and dries the air as it goes. It costs much more to run, but it is the only one of the three that brings the room temperature down whatever the humidity. See air conditioner or fan.

If you still want one

This pick is chosen by its specs. We haven't tested it, and on a humid UK summer day expect it to feel much like a fan.

For dry, hot days with a window open

Midea Sensicool 7L Air Cooler

Type
Evaporative
Power
60W
Noise
28dB
Tank
7 litres
Cost per hour
1.6p
  • Timer
  • Remote control

It is rated at 60W, or 1.6p an hour. It has a 7 litre water tank. It has a timer, so it can switch itself off. The maker gives its noise as 28dB sound power, which isn't directly comparable with the fan figures.

View on Amazon Manufacturer specs

Sources

Checked 15 October 2026.