Why your portable air conditioner underperforms its box rating
Published 17 Jul 2026 · sources: US DOE test procedure (SACC); manufacturer specifications
Portables are what Britain buys in a heatwave, and most people end up disappointed. Not because they're scams. There's a piece of physics the UK box doesn't mention.
The single-hose problem
Most UK portables have one exhaust hose. The unit takes air from your room, uses some of it to cool the coils, and blows that share out of the window. Every litre pushed out must be replaced — and it gets replaced by warm air pulled in from the hallway, the loft hatch, the gaps around your windows. It cools the room while pulling warm air back in through every gap in the house. Some of its own motor heat stays in the room too, because the whole machine is standing in it.
The result: real-world cooling meaningfully below the headline BTU figure, and a unit that runs flat out on the hottest nights while the room barely moves.
America measures this. Britain does not.
The US Department of Energy forces manufacturers to publish a Seasonally Adjusted Cooling Capacity (SACC) — a rating that accounts for infiltration and duct heat. When SACC arrived, many portables’ advertised capacity fell substantially overnight; the machines hadn't changed; the labels had. The UK has no equivalent disclosure rule, so British boxes still carry the flattering number.
What to do with this
- Prefer dual-hose units — a second hose brings in outside air for the coils, so the unit stops depressurising your room. They are rarer in the UK but they exist.
- Seal the window kit like it matters — because it matters more than 2,000 extra BTUs. Foam, tape, close the gaps.
- Right-size your expectation — one room, door closed, evening onwards. A portable is a tool for surviving tonight, not conditioning a flat.
- If you keep buying portables every summer, the maths increasingly favours a fixed solution — and the rules on those are more generous than most people think. Start with what the law actually says, or check your address below.