Quick references (very commonly used in Portugal)
DECO provides these guidelines:
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18–20 m² → 9000 BTU/h (≈2.5 kW)
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20–35 m² → 12000–13500 BTU/h (≈3.5–4 kW)
And remember that large rooms with a lot of sunlight may require much greater capacity.
EDP also indicates 9000 BTU as typical for a bedroom/small living room (in our climate), serving as a "sanity check".
When should you increase power (even with a small area)?
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large/south-facing windows
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top floor (heated roof)
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weak insulation
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Too many people in the division or too much equipment (PCs, consoles)
BTU to kW conversion (for comparing technical specifications)
Sometimes capacity is given in kW. The typical conversion is:
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1 kW ≈ 3412 BTU/h
The classic mistake: buying "fewer BTUs to save money"
If the appliance is undersized, it will operate at its limit for longer, potentially consuming more energy and providing less comfort. The goal is to get the power right while also achieving good efficiency (SEER/SCOP).
Useful tools
If you want to go beyond "m²", use calculators that ask for solar exposure and insulation (e.g., BTU calculator).
FAQ
Would a 9000 BTU unit fit in a living room?
For small rooms it may be sufficient; for medium-sized rooms, DECO generally recommends higher ranges (12,000–13,500).
Can I use the same power for heating?
It depends on the equipment (reversible) and the SCOP; compare by the label.