What size portable air conditioner do I need?
The UK BTU sizing guide: match the unit to your room, avoid the two classic mistakes, and know what it'll cost to run - then check what's actually in stock right now.
BTU by room size (UK homes)
BTU (British Thermal Units) measures cooling power. A practical rule for UK rooms with standard ~2.4 m ceilings is 500-600 BTU per square metre:
| Room | Typical size | BTU needed |
|---|---|---|
| Small / box bedroom | up to 12 m² | 7,000 BTU |
| Double bedroom / office | 12-18 m² | 8,000-9,000 BTU |
| Large bedroom / small lounge | 18-22 m² | 10,000-12,000 BTU |
| Living room / open plan | 22-30 m² | 12,000-14,000 BTU |
| Large open plan / conservatory | 30 m²+ | 14,000-16,000+ BTU |
Add ~10-20% for south-facing glazing, loft rooms, conservatories, poor insulation, or several people/PCs in the room. AirconWatch tracks refrigerant units of 5,000 BTU and above - but 5-6,000 BTU suits only a small, shaded box room; for anything bigger, treat 7,000+ as the practical minimum.
The two classic mistakes
1. Buying too small
An undersized unit runs flat out, never reaches temperature, and costs you the same electricity as a right-sized unit that cycles. If you're between bands, and the room gets direct sun, go up a band.
2. Buying an "air cooler" by accident
Evaporative coolers and misting fans are often listed alongside real air conditioners but don't meaningfully lower room temperature - they add humidity. A real portable AC has a compressor, a BTU rating, and an exhaust hose. (That's also our filter: AirconWatch never lists coolers or fans.)
Single hose vs dual hose
Single-hose units exhaust hot air outside using air from the room, which pulls warm air back in through gaps - fine for bedrooms, less ideal in big spaces. Dual-hose units draw outside air for the condenser instead, cooling faster and more efficiently. Most UK stock is single-hose; treat a dual-hose model in your size band as a find.
Running costs
Reckon on roughly 1 kW for a 9,000 BTU unit and 1.2-1.6 kW for 12,000-14,000 BTU. At ~26p/kWh (the July to September 2026 Ofgem cap rate) that's about 26-42p per hour at full tilt - less in practice once the thermostat starts cycling. Inverter models (variable-speed compressors) cost more upfront and noticeably less per summer.
Noise, drainage and window kits
- Noise: 52-56 dB is typical - like a fan on high. For bedrooms, look for "sleep mode" figures under 50 dB.
- Drainage: most modern portables self-evaporate condensate through the exhaust; you'll only empty a tank in very humid weather.
- Venting: every refrigerant portable needs the hose outside - budget for a window seal kit if one isn't included.
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