Guides & Blog

What size portable air conditioner do I need? (UK room chart)

Buy too small and it runs flat-out without ever cooling the room. Buy too big and you've wasted money and energy. The UK rule of thumb is simple: about 350 BTU per square metre, plus 15–25% for south-facing rooms, big windows or loft conversions.

Measure your room (length × width in metres), multiply by 350, and round up to the nearest class. South-facing with a bay window? Add 20%.

Shortcut: use our interactive BTU calculator — it does the maths in five seconds.

Portable air conditioner running costs UK 2026 — real numbers

The honest answer most retailers won't give you: a typical 9,000 BTU portable AC draws about 1.0–1.1 kW, which at current UK electricity prices works out to roughly 20–32p per hour.

• 4 hours a day through a 60-day summer: around £50–£75 for the season.
• Evaporative air cooler: 3–6p per hour — a fraction of the cost, but it cools you, not the whole room.
• Tower or pedestal fan: 1–3p per hour. Running one all night costs less than a biscuit.

Three ways to cut AC costs without suffering: seal the window kit properly (an unsealed window can waste a large share of the cooling), pre-cool the room in the evening rather than fighting peak heat at 3pm, and close the door — a portable AC cools one room, not a house.

Rule of the house: cool the smallest room you actually live in, not the biggest room you own.

Air cooler vs air conditioner: which works in humid British heat?

They look similar; they are not the same machine.

An air conditioner has a compressor and refrigerant. It genuinely lowers the temperature and pulls moisture out of the air — which matters, because British heatwaves are often muggy. That dehumidifying effect is half the relief.

An evaporative air cooler blows air across water. It creates a lovely cool breeze in dry heat, costs pennies to run, and needs no window hose. But in humid heat it adds moisture to already-muggy air, which can make a sticky night feel stickier.

• Sticky 30°C+ nights, need to sleep: compressor AC.
• Dry warm days, desk or sofa relief, tight budget: evaporative cooler.
• Any budget, any weather, moving air feels 3–4° cooler: a decent fan — never underestimate one.

How to fit a portable AC hose to a UK casement window

British casement windows (the side-hinged kind) confuse every AC manual written for American sliding sashes. Here's the five-minute method:

• 1. Open the casement to its first latch position.
• 2. Fit a fabric window seal kit around the frame and the opened pane — the zip tape sticks to the frame, the fabric wraps the window.
• 3. Zip the seal closed, leaving just the hose outlet open.
• 4. Push the exhaust hose through the outlet and zip snug around it.
• 5. Keep the hose as short and straight as possible — every bend costs you cooling.

No drilling, no permanent fixings, completely renter-friendly — and your landlord will never know.

Where to check current portable air-conditioner availability

Cool Isle does not maintain retailer inventory, take orders or process checkout. A seller's own page is the source of truth for current price, stock, delivery and returns.

Use the AC Locator to open official pages at established UK retailers and manufacturers. The links contain no affiliate IDs. Compare the stated room coverage, energy label, noise, hose size and included window kit before ordering.

If compressor AC is unavailable or unsuitable, start with passive shade and night ventilation, then compare a fan or air cooler with realistic expectations: an evaporative cooler is not an air conditioner and may be less comfortable in humid weather.