The most common mobile-AC mistake isn't buying a bad unit — it's buying one that's the wrong size for the room. Get the BTU right and a modest unit keeps you comfortable easily; get it wrong and even an expensive one disappoints. Here's how to size it without overthinking.
What BTU actually means
BTU is a measure of cooling power — how much heat the unit can pull out of a room each hour. A higher BTU rating handles a bigger or hotter space. The key insight: you want the right BTU for your room, not the highest. Too little and the unit can't keep up; too much and it short-cycles and wastes energy. Sizing is about matching capacity to the heat load of your specific space.
Start with room size
Room area is the baseline: larger rooms need more BTU. A small bedroom needs far less cooling power than an open-plan living-dining space. Measure (or estimate) the floor area of the room you'll actually cool — remembering you should cool one closed room, not the whole flat — and use that as your starting point before adjusting for the factors below.
Adjust up for heat-adding factors
Several things mean you need more BTU than floor area alone suggests:
- Sun exposure: west- and south-facing windows that get hours of direct sun.
- Glass: large or numerous windows.
- Top floor: a flat directly under a hot roof (the classic Dachgeschoss problem).
- Heat sources: a kitchen, computers, or other appliances.
- People: more occupants add body heat.
If several of these apply, size up. If your room is shaded, north-facing, and used by one person, you can size more modestly.
Why bigger isn't always better
It's tempting to "future-proof" by buying the most powerful unit, but a badly oversized AC cools the air quickly then shuts off before it removes much humidity, leaving the room cool but clammy and the unit cycling on and off — which wastes energy and wears it out. A correctly sized unit runs steadily and comfortably. Aim to match, not overshoot.
Reduce the BTU you need
You can shrink the cooling load before you even buy: shade windows from outside, close the door to contain the cooled space, switch off heat-generating appliances, and ventilate at night so the room doesn't start the day hot. A well-prepared room lets a smaller, cheaper, quieter unit do the job.
Putting it together
Estimate your room's area, nudge the BTU up for sun, glass, top-floor heat, appliances, and people, then choose a unit in that range. When you know your target, check which correctly-sized models are in stock near you and reserve one — the right size you can actually buy beats the perfect size that's sold out.