The most common mistake when buying an air conditioner is not choosing one that is too small. It is choosing one that is too big — on the assumption that more capacity means better cooling. The result is the opposite: worse cooling and a higher bill.
Multiply the room area in square metres by 600 BTU for a baseline, then adjust for sun, occupancy and ceiling height. A 20 m² bedroom needs roughly 12,000 BTU. Getting the size right matters more than the brand — and oversizing is worse than undersizing.
The formula
BTU = room area (m²) × 600Then adjust:
- Heavy afternoon sun or a west-facing wall — add 10%
- Top floor or an uninsulated roof — add 10–15%
- More than two people in the room regularly — add 600 BTU per extra person
- Kitchen — add 4,000 BTU for the cooking heat
- Ceiling above 2.7 m — add in proportion to the extra height
Sizing table
| Room area | Baseline BTU | Common size |
|---|---|---|
| 10–15 m² | 6,000–9,000 | 9,000 |
| 15–22 m² | 9,000–13,000 | 12,000 |
| 22–30 m² | 13,000–18,000 | 18,000 |
| 30–40 m² | 18,000–24,000 | 24,000 |
| Over 40 m² | Over 24,000 | Two units or a larger system |
Units are sold in fixed steps. When your calculation lands between two, take the smaller one if the room is shaded and the larger if it takes direct sun.
Why oversizing is the expensive mistake
This is counter-intuitive enough that it is worth spelling out.
An air conditioner does two jobs: it lowers the temperature and it removes humidity. The second happens gradually, while the unit runs.
An oversized unit hits the target temperature in a few minutes and shuts off — before it has pulled meaningful moisture out of the air. The thermostat is satisfied, so the room reads as cool while feeling damp and heavy. Then the temperature drifts up, the unit restarts, and the cycle repeats.
Two consequences:
Comfort suffers. Cold and clammy is less comfortable than slightly warmer and dry. This is why people who oversize often end up dissatisfied without understanding why.
It costs more. A compressor draws its highest current at start-up. Frequent short cycles mean repeated start-up surges, plus more wear on the compressor — the single most expensive component.
A correctly sized unit runs longer at lower load. That is not inefficiency — it is exactly how the machine is designed to work. Long steady cycles cool better, dehumidify properly, and cost less than short violent ones.
Inverter or not
Non-inverter: the compressor is either at full power or off. Cheaper to buy, less efficient, temperature swings more.
Inverter: the compressor varies its speed to match demand, running at low power most of the time. Uses noticeably less electricity, holds a steadier temperature, quieter.
When the inverter pays for itself: with heavy use — several hours a day over a long season. With occasional use, the price premium may take longer to recover than the unit's useful life.
What else affects the result
Sizing is only half the outcome. Two identical units perform very differently depending on the room:
- Insulation. An uninsulated room loses cooling as fast as the unit supplies it. Sealing gaps around doors and windows often does more than an extra 3,000 BTU.
- Direct sun. A closed curtain or an external blind on a sunny window cuts the heat load significantly before the unit ever sees it.
- Placement. High on the wall, with clear space in front. Not directly above a doorway or behind furniture.
- Filter cleaning. A blocked filter forces the unit to run longer for the same result — the cheapest maintenance with the largest effect.
Before you buy
- Measure the room — length × width, in metres.
- Apply the formula and the adjustments that fit your room.
- Check the energy label and compare the efficiency rating between models of the same capacity.
- Confirm the electrical supply can carry the unit, especially for 18,000 BTU and above.
- Budget for professional installation. A poor install undermines a good machine — refrigerant charge and pipe runs affect performance directly.
See also: what actually uses the most electricity at home.
Frequently asked questions
What is a BTU?
A unit of heat energy. In air conditioning it expresses how much heat the unit can remove from a room in one hour. The higher the number, the greater the cooling capacity.
How do horsepower ratings relate to BTU?
Horsepower is an approximate shorthand common in some markets. One horsepower is roughly 9,000 BTU, one and a half about 12,000, and two about 18,000. The figures are approximate and vary slightly between manufacturers.
Is bigger always better?
No — the opposite. An oversized unit cools the air quickly and switches off before it has pulled the humidity out, so the room feels cold and clammy. It also uses more electricity because of the constant start-stop cycling.
What is the difference between inverter and non-inverter?
A conventional unit runs at full power or not at all. An inverter varies compressor speed to match demand, so it uses less electricity and holds a steadier temperature. The price difference is usually recovered within a few years of heavy use.
Does ceiling height matter?
Yes. Standard calculations assume a ceiling around 2.7 metres. If yours is higher, increase the capacity by the same proportion as the extra height.
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