kW and BTU explained

In the UK, air conditioning capacity is measured in kilowatts (kW) — it tells you how much cooling or heating a unit can deliver. You’ll sometimes see capacity quoted in BTU (British Thermal Units) instead, especially on imported or older units. The two convert directly: 1 kW is roughly 3,412 BTU per hour.

Quick conversion

kW	≈ BTU/h	Typical use
2.0	7,000	Small bedroom / study
2.5	9,000	Standard bedroom
3.5	12,000	Large bedroom / small lounge
5.0	18,000	Large living room
7.0	24,000	Open-plan / large space

Room sizes here are a starting point only — see how to size a system.

Capacity vs electricity used — don't mix them up

Here’s the point that trips people up. The kW figure on an air conditioner is its output — how much cooling or heating it produces — not how much electricity it consumes. Thanks to the heat-pump effect, a unit might deliver 3.5 kW of cooling while drawing only around 1 kW of electricity from the socket.

So when you read “3.5 kW”, think “3.5 kW of cooling power”, not “3.5 kW on your electricity bill”. What it actually costs to run depends on its efficiency — see running costs.

Bigger isn’t better: it’s tempting to assume a larger kW rating is the safer bet, but an oversized unit short-cycles, controls humidity poorly and wastes energy, while an undersized one struggles on the hottest or coldest days. The right capacity is a proper match to the room — which is exactly what sizing is about.