How many BTU your AC needs

How it works

The power depends on square meters, height, sun exposure and the people present. Too low won't cool, too high wastes energy and cycles on/off constantly.

Reference

Roughly 300-400 BTU per m²: this estimate helps you choose between models (9000, 12000, 18000 BTU…).

What a BTU is, and why it is still used

The BTU is an imperial unit of energy, and what appears on air conditioners is really a power: BTU per hour. The conversion is easy to remember, because 3,412 BTU/h make about one kilowatt: a 9,000 is 2.6 kW, a 12,000 about 3.5, an 18,000 about 5.3.

Careful not to confuse it with electricity drawn: a 9,000 does not consume 2.6 kW, it consumes about a third of that, because a heat pump does not produce cold, it moves it. It is the ratio between the two numbers, measured over a whole season, that gives the SEER and SCOP figures on the label.

Why too big is worse than too small

This is the least intuitive point. An oversized unit reaches the set temperature quickly and switches off, then restarts, and in those short cycles it has no time to dehumidify: the room ends up cold and damp at once, which is the worst sensation there is, while the compressor wears and consumption rises.

A correctly sized unit instead runs nearly always at reduced output and quietly, which is exactly what inverter technology was invented to do. If you must err, err slightly low: the few extreme days a year cost a few minutes of waiting, not a wrong installation for ten years.

What shifts the sum far more than floor area

Orientation: a south-facing room with large windows can need fifty per cent more than an identical one facing north. The top floor under an uninsulated roof is the worst case there is. Windows: recent double glazing changes the demand more than any square metre. And the heat produced inside: people, the oven, a computer on all day, a fridge.

This is why the rule of 300 to 400 BTU per square metre is a starting point and not an answer: in a west-facing loft it can be short by half, in a flat between two heated floors it can be generous.

Before buying, three things that matter as much as power

A multi-split does not add powers up: two 9,000 indoor units do not require an 18,000 outdoor unit, because they almost never run at maximum together, and the sizing is done by the manufacturer with their own tables. The position of the indoor unit matters more than it seems: cold air falls, so high up and not pointed at people. And the refrigerant: new systems use low-impact blends, and the installer must be certified and issue the paperwork.

Finally noise, which is the commonest source of regret in a bedroom: on the data sheet what matters is the figure at minimum speed, not the average.

Nearby tools

The room dimensions come from Room area and volume. For what it costs to run there is Electricity cost, for how much insulation would save Insulation savings, and for condensation and mould Dew point.