BTU (British Thermal Units) is the cooling power figure you’ll see on Amazon listings. Too little BTU and the room never settles; too much and you waste money on a louder, bulkier unit.
Rule-of-thumb for UK homes (2026)
| Room size | Typical BTU | Examples |
|---|---|---|
| Under ~20 m² | ~9,000 BTU | Box bedroom, small office |
| ~20–28 m² | 10,000–12,000 BTU | Main bedroom, average lounge |
| 28 m²+ / open-plan | 12,000–13,000+ BTU | Through-lounge, kitchen-diner |
Measure length × width for a rough m² figure. Loft rooms and south-facing glass often need the higher end of the band.
Other factors that change the answer
- Ceiling height — Victorian rooms with tall ceilings hold more heat.
- Sun load — bay windows and skylights push demand up.
- Occupancy — people and gadgets add heat.
- Insulation — poorly insulated rental stock needs more headroom.
Hose and seal still matter
A correctly sized portable AC fails if the window kit leaks. Hot exhaust spills back in and the unit runs forever. Budget time for sealing when you unbox.
Pick by room, then by features
Once BTU is clear, filter for noise (bedrooms), Wi-Fi (commuters), or budget. Use our compare table or quiz to narrow the shortlist, then check live Amazon UK prices.