BTU stands for British Thermal Unit, the traditional unit for measuring thermal energy. Applied to air conditioners, it indicates cooling power: how much heat the unit can remove from a room in an hour (BTU/h, often shortened to just BTU).
How it converts to watts
European spec sheets almost always list the figure in kW too, since that’s the official International System unit. The conversion is simple: 1000 BTU/h ≈ 293 W. A 9000 BTU/h air conditioner has a cooling power of about 2.6 kW, a 12,000 BTU/h unit about 3.5 kW, and an 18,000 BTU/h unit about 5.3 kW — the three most common sizes for home use.
The sizes on the market
The residential market almost always moves in steps of 9000, 12,000, and 18,000 BTU, with occasional 7000 or 24,000 BTU models for more specific needs. There’s no “custom-fit” size: you pick the closest step to your calculated need, rounding slightly up if in doubt, never down.
How to estimate the right BTU
A common rule of thumb is about 100 BTU per square meter under standard conditions (2.7 m ceiling, average insulation, no particular exposure). From that baseline, apply corrections: +10-20% for rooms heavily exposed to sun or on the top floor under an uninsulated roof, +10% for above-average ceiling height, +5-10% for each person beyond the first who regularly occupies the room. The calculation remains approximate: for precise sizing, especially in large rooms or open spaces, an installer’s site visit is worth it, since it also accounts for details like the number of windows and heat-generating appliances present.
Why it matters more than floor area alone
Two rooms of the same size can need different sizes: a first-floor bedroom with double glazing needs fewer BTU than a top-floor living room with a large south-facing window. BTU still remains the first number to check when comparing models — we cover it in detail, alongside the other selection criteria, in our complete guide on how to choose an air conditioner.