BTU (British Thermal Units) measures an air conditioner's cooling power. Too few and it can't keep your room cool; too many wastes money and makes more noise. Here's the simple version.
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BTU by room size
A good rule of thumb is roughly 350–400 BTU per m². Use this chart:
| Room size | Recommended BTU | Typical room |
|---|---|---|
| Up to 15 m² | 7,000–9,000 BTU | Small bedroom, study |
| 15–25 m² | 9,000–12,000 BTU | Bedroom, home office |
| 25–35 m² | 12,000–14,000 BTU | Living room |
| 35 m²+ | 14,000+ BTU / monobloc | Open-plan, large lounge |
Add BTU if…
- The room gets strong afternoon sun — add ~10%.
- It's a top-floor or attic room — heat rises, size up.
- Several people or a kitchen/electronics in the space — each adds heat.
- High ceilings (above ~2.7 m) — you're cooling more air volume.
Don't oversize "to be safe"
A too-powerful unit short-cycles (blasts, stops, repeats), which cools unevenly, runs louder, and costs more. Match the room and only bump up for the factors above.
Know your number? Filter portable ACs by BTU on Amazon.de:
See portable ACs on Amazon.de →For a bedroom, also check the noise rating — see best portable AC for a bedroom. Got tilt-and-turn windows? Read how to vent a portable AC with tilt-and-turn windows first.