Portable AC vs fan vs evaporative cooler: what works in a UK flat

Three very different machines get sold under the vague banner of "cooling": fans, evaporative coolers and refrigerant air conditioners. Only one of them lowers the temperature of a room. Knowing which is which — before a heatwave, ideally — saves you from the most common summer purchase regret in the UK: an "air cooler" that turns a hot room into a hot, humid room.

A fan moves air. It does not cool the air at all — a running fan actually adds a small amount of heat from its motor. What it does is help your body's own cooling: moving air strips away the layer of warm, humid air next to your skin so sweat evaporates faster. Fans cool people, not rooms.

An evaporative cooler (often marketed as an "air cooler") blows air through a wet pad. Evaporating the water absorbs heat, so the air leaving the unit is cooler — but also wetter. Every degree of cooling is paid for in added humidity, and the effect depends entirely on the incoming air being dry. In a desert climate this works well. The UK is not a desert climate.

A refrigerant portable air conditioner runs a compressor and a refrigeration circuit, the same technology as a fridge: it absorbs heat from room air and pumps it outside through an exhaust hose. It is the only one of the three that lowers the room's temperature, and it also removes moisture as it runs. The price of that is a vent requirement, more noise, more weight and more electricity.

Evaporative cooling has a hard physical ceiling: it can only cool towards the "wet-bulb" temperature, which is close to the air temperature when humidity is high. UK summer air is frequently humid — muggy, storm-building heatwaves especially so — which is exactly when you want cooling most. In those conditions an evaporative cooler delivers a degree or two at best while pumping extra moisture into the room, making it feel stickier and encouraging condensation and mould in a flat that is closed up against the heat.

Indoors makes it worse. Run an evaporative cooler in a closed room and the humidity it generates accumulates, steadily choking off its own cooling effect. The instructions for these devices quietly tell you to keep a window open — which, on a 33 °C afternoon, means importing heat to feed a machine that barely cools.

Fans are cheap, quiet enough, and cost pennies to run — as a labelled illustration, a 50 W fan running for 10 hours uses 0.05 kW × 10 h = 0.5 kWh, which at an assumed 27p per kWh (check your own tariff) is about 14p. For a healthy adult in a room in the high 20s, a fan plus sensible shading and night ventilation is often all that is needed.

• Air temperature is below the low-to-mid 30s — moving air still helps sweat evaporate and feels cooling. As air temperature approaches skin temperature (around 35 °C), a fan's benefit shrinks, and in very hot, dry air it can even add to heat stress.
• You mainly need help sleeping — a fan over the bed, with windows opened once the outside air is cooler than inside, deals with many UK summer nights.
• You can create a cross-breeze — a fan pushing air out of one window while cooler evening air enters another moves far more heat than a fan just stirring the room.
• The hot spell is short — for two or three warm days a year, a fan and the passive measures in our flat-cooling guide are a proportionate response.

There is a point where moving air around a hot room stops being an answer. Signs you are past it:

• The room stays at 28 °C or above deep into the night, so there is no cool outdoor air to purge with and a fan is just stirring heat.
• Someone in the household is more vulnerable to heat — babies, elderly people, pregnancy, heart or respiratory conditions, some medications.
• You work from home in the room through the afternoon, or it contains heat-producing equipment you cannot switch off.
• The flat is a top-floor or loft conversion that heat-soaks for days during a heatwave and no longer cools down overnight.
• Humidity is part of the misery — an air conditioner dehumidifies as it cools, which no fan and no evaporative cooler will do.

In those situations a properly sized, properly vented portable air conditioner is the only device in this comparison that changes the room rather than your perception of it. Sizing and venting each have their own guide — both are worth reading before you buy, because a badly vented AC gives up much of its advantage.

• Fan: cheapest to buy and to run, minimal noise, zero installation. Cools people only; useless against a genuinely hot, humid room.
• Evaporative cooler: cheap to run, but in UK humidity delivers little cooling and raises moisture. Needs water refills and pad cleaning. The most commonly regretted purchase of the three.
• Portable AC: highest purchase price, meaningful running cost (roughly £1–£3 per day of heavy use depending on unit and tariff — see the running-costs guide for the arithmetic), compressor noise similar to a loud conversation, and it must be vented through a window or wall. The only option that actually cools and dries the room.

A reasonable UK strategy is layered: shading and night purging first, a fan for ordinary warm spells, and a refrigerant portable AC for the rooms and the weeks where those stop working. If you decide AC is the right layer, decide your size band early — mid-capacity units are the first to disappear from stores when a heatwave is forecast, and knowing exactly what you need lets you move quickly when a unit shows up nearby.