🗓 Updated July 9, 2026 · EcoBack editorial team
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.
The one number every purchase hangs on — it is already worked out below. Adjust the room size if it is off.
Compiled from public tests & customer reviews — not tested by us. Prices vary; check the current price on Amazon. Illustrations, not product photos. One thing first: without a sealed window every portable AC loses most of its effect — hot air gets pulled straight back in. Amazon.de ships to most EU countries, with site and checkout available in English.
De'Longhi Pinguino PAC EX105
Strong cooling, often in the test-winner line.
✓ Strong cooling, good efficiency
✕ Big & heavy
Comfee MPPH-09CRN7
Affordable for small bedrooms & offices — but clearly audible.
✓ Cheap, compact
✕ Clearly audible (~63 dB)
AEG ChillFlex Pro
Comfortable in the bedroom, well equipped.
✓ Very quiet, well equipped
✕ Higher price
De'Longhi PAC N90 ECO Silent
Monoblock test winner at Stiftung Warentest, silent mode, R290 refrigerant.
✓ Quietest monoblock in its group
✕ Pricier than the value pick
Klarstein Kraftwerk Smart 12K
Strongest cooling of its group in public comparisons.
✓ Strongest cooling of its group
✕ Audibly loud
It draws in warm room air, cools it and returns it — the waste heat leaves through the window hose. Real cooling, not just moving air around.
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.