A BTU (British Thermal Unit) measures how much heat an air conditioner can remove per hour. Getting the number right matters: too low and the unit won't cool effectively, too high and you'll waste energy and lose comfort.
Choosing an air conditioner with the correct BTU rating is one of the most important decisions you'll make before buying.
Start with the room volume, not just the floor area. Because ceiling height affects how much air needs cooling, work in cubic metres: floor area × ceiling height = room volume (m³). A room of 20 m² with a ceiling height of 2.70 m gives you roughly 54 m³.
- Under favourable conditions (good insulation, limited sun exposure): around 30 W per m³
In this example:
- 54 m³ × 30 W = approx. 1.6 kW → roughly 5,500 BTU
A unit rated around 7,000 BTU should be sufficient.
- Under unfavourable conditions (heavy sun, poor insulation, high ambient temperatures): around 50 W per m³
Same room:
- 54 m³ × 50 W = approx. 2.7 kW → roughly 9,000 BTU
Here you'd want a 9,000 BTU model or slightly above, such as 10,000 or 12,000 BTU, which runs more efficiently at partial load.
A poorly sized unit creates real problems:
- Inadequate cooling when undersized
- Energy waste and inefficient cycling when oversized
These figures are a useful starting point. Other factors that affect your real-world requirement:
- Ceiling height: taller rooms need more power
- Sun exposure: south- and west-facing rooms heat up faster
- Thermal insulation: well-insulated homes hold temperature better
- Number of occupants: each person adds to the heat load
- Appliances: running equipment raises the internal temperature
Factor in all of these and you'll avoid sizing mistakes, cut energy costs, and keep your home genuinely comfortable all summer.