Cooling power is the decision that most determines whether you sit cool or resign yourself to an expensive fan. It is measured in BTU per hour, and getting it right is a simple sum plus a couple of adjustments. This guide gives you the BTU-per-room rule, a ready-to-use table in m² and sq ft, and the factors that force the number up or down, so you neither fall short nor waste money.
The base rule: around 340 BTU per m²
For a UK home with normal insulation and 2.4 m ceilings, allow roughly 340 BTU per m² — about 30 BTU per sq ft. A portable needs a little more than a fixed split because the vent hose and the slightly open window cost some cooling.
So a 12 m² bedroom (about 130 sq ft) is comfortably covered by a 7,000-9,000 BTU unit, and a 30 m² living room (about 320 sq ft) lands squarely on 12,000 BTU. The rule is deliberately generous to give you headroom on the hottest days.
Quick table by room size
- Up to 15 m² / 160 sq ft (bedroom, study): 7,000-9,000 BTU · 2.0-2.6 kW.
- 15-25 m² / 160-270 sq ft (medium room): 9,000-12,000 BTU · 2.6-3.5 kW.
- 25-35 m² / 270-375 sq ft (living room): 12,000 BTU · 3.5 kW.
- 35-45 m² / 375-485 sq ft (large room, studio): 14,000-18,000 BTU.
- Over 45 m²: 18,000 BTU or two units sharing the load.
The Midea PortaSplit (electriQ PortaSplit in the UK) delivers 3.5 kW / 12,000 BTU, landing right in the sweet spot for the average British living room of 25-35 m². That is one reason it sells out so fast in a heatwave.
Factors that push the power up
Floor area is only the starting point. These factors mean you need more BTU than the basic sum suggests:
- Direct afternoon sun or large windows: add 10-20%.
- Ceilings above 2.7 m: work it out by volume, not floor area.
- Open-plan kitchen or heat-producing appliances: add about 10%.
- Top-floor flat or loft conversion under the roof: add up to 20%.
- Several people in the room regularly: about 600 BTU per extra person.
Why going too big is also a mistake
Over-sizing feels like the safe option, but it is not. A unit that is too powerful hits temperature very quickly and switches off, then restarts: that short-cycling cools in fits and starts, dehumidifies worse (the room ends up cold but clammy) and wears the compressor.
The ideal is a unit that runs steadily for a good stretch, drying the air and holding a stable temperature. That is why it pays to size it correctly rather than overdo it.
Frequently asked questions
How many BTU is 1 kW of cooling?+
1 kW is roughly 3,412 BTU per hour. So a 3.5 kW unit is about 12,000 BTU and a 2.0 kW unit about 7,000 BTU. Manufacturers use both units, so it helps to be able to convert.
What if my room gets a lot of sun?+
Add 10-20% to the calculated power. A west-facing room with a large window takes on a lot of heat through the afternoon, exactly when it is hottest, so the extra headroom is worth it.
Can one portable cool a whole flat?+
A portable is designed for one room. Cold air does not turn corners or travel down hallways well, so for several rooms the sensible approach is one per zone, or moving the unit to wherever you are.
What happens if I buy a unit with too many BTU?+
It cools very fast and shuts off quickly, repeating short cycles that dehumidify poorly and leave the room cold but clammy. It also wears the compressor. Sizing correctly beats over-sizing.
Read also
- How to choose a portable air conditionerThe practical guide to getting the power, noise and running cost right
- Portable air conditioner running costsWhat it really costs to run, and how to read the label to pay less
- Split-type portable vs monoblocTwo ways to cool without installation, with very different results
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