Air conditioner for 20, 30 or 40 m²: which capacity is right

20 m²: 9000 BTU

A bedroom, nursery or study up to 20–25 m² with a standard ceiling and no all-day sun: 9000 BTU (about 2.5 kW). South-facing with a large unshaded window: go to 12000 BTU.

30 m²: 12000 BTU

A living room of 25–35 m² is the classic 12000 BTU (3.5 kW) case, and the best-selling class in Bulgaria. If the room opens into a kitchen or hallway, size for the whole open space.

40 m²: 18000 BTU

A large living room, studio or office of 40–55 m² needs 18000 BTU (5 kW). Between 35 and 40 m² you are on the border: good insulation and north exposure can get by with 12000 BTU; south exposure and lots of glass call for 18000 BTU or two smaller units.

Corrections that change the answer

The base rule is 100–120 W of cooling per square metre. Adjust for:

• Ceiling above 2.9 m or attic: +15–20%
• South/west exposure with large unshaded windows: +15–25%
• Top floor with an uninsulated roof: +15%
• Kitchen or room with many people and appliances: +10–20%
• Excellent insulation, north exposure, small windows: −10–15%
• Primary heating: size by the heating output at −7°C, not the cooling figure

Why bigger is not better

An oversized unit cools fast and stops before it has dehumidified the air, cycles often and wears the compressor. An inverter forgives one step up, not double the capacity.