Sizing is the single decision that makes or breaks a home air conditioning installation. Get it right and the unit purrs along quietly, sipping electricity. Get it wrong in either direction and you've bought an expensive disappointment. Here's how sizing actually works — the quick rule of thumb, the factors that change it, and why we always measure before we quote anywhere from Swindon to Salisbury.
Too small and the unit runs flat out on the hottest afternoons and still never gets the room to temperature. It works hardest precisely when conditions are worst, wears out faster, and you're paying full whack for half a result.
Too big is the mistake people don't expect. An oversized unit blasts the air temperature down so quickly that it switches off before it has run long enough to wring the moisture out of the air. This is called short-cycling: the room ends up cold but clammy, the compressor suffers constant start-stop wear, and you've paid extra for a unit that performs worse. Bigger is not better — correct is better.
For a typical UK room with average glazing and ceiling height, these are the sizes we'd expect a survey to confirm:
| Room | Floor area | Unit size | Approx. BTU |
|---|---|---|---|
| Small bedroom / box room | Up to 15m² | 2.0kW | ~7,000 BTU |
| Double bedroom / home office | 15–20m² | 2.5kW | ~9,000 BTU |
| Lounge / large bedroom | 20–30m² | 3.5kW | ~12,000 BTU |
| Large open-plan living space | 30–45m² | 5.0kW | ~18,000 BTU |
| Very large open plan | 45m²+ | 6–7kW+ or two units | ~21,000 BTU+ |
On the biggest spaces, two smaller units at opposite ends often beat one giant unit — better air distribution, quieter, and you can run just one when only half the space is in use. That's a judgement call we make room by room, and it feeds into the choice between a single split or multi-split installation.
The table above assumes an ordinary room. Several things quietly add heat load, and each one nudges the required size upward:
UK installers talk in kilowatts of cooling; retailers and imported units often quote BTU (British Thermal Units, confusingly an American convention). The conversion is simple: 1kW ≈ 3,412 BTU. So a "12,000 BTU" unit is a 3.5kW unit — same machine, different label. One caution: the kW figure on the box is the cooling output, not the electrical input. A 3.5kW unit typically draws only around 1kW of electricity — that efficiency is the whole point of refrigerant-based cooling, and it's covered properly in our running costs guide.
Every system we fit is a heat pump that heats in winter as well as cooling in summer. Heating loads in a UK winter can exceed cooling loads in a UK summer, especially in older, draughtier properties — of which Wiltshire has no shortage. If you're planning to use the unit as the main heat source for a garden office or extension, the winter heat-load figure may be what actually sets the size, not the summer one. We check both.
A rule of thumb is a starting point, not a specification. At the free survey we do a proper heat-load calculation: room dimensions, glazing area and orientation, insulation, occupancy and equipment. It takes minutes, it's free, and it's the difference between a system that's quietly right for twenty years and one that's subtly wrong from day one. It also means the quote you get is for the unit you actually need — not the biggest one on the price list.
Not sure which row of the table your room falls into? Don't guess — send us a message with your town and rough room size, and we'll arrange a free survey with a proper heat-load calculation and a fixed, no-obligation price. We cover the whole of Wiltshire, from Salisbury up to Swindon.
It runs flat out on the hottest days and still never brings the room down to temperature. It works hardest exactly when conditions are worst, wears faster, costs more per degree of cooling achieved, and leaves you uncomfortable — the one outcome you paid to avoid.
No. An oversized unit cools the air so fast it short-cycles — switching off before it has dehumidified properly. You get a cold but clammy room, extra start-stop wear on the compressor, and higher running costs. Correct sizing beats generous sizing.
Often roughly double the standard rule of thumb, because solar gain through all that glazing is enormous. A conservatory that would take a 2.5kW unit as a normal room may genuinely need 5kW — exactly where a proper heat-load calculation earns its keep.
Tell us your town and which rooms you'd like cooled or heated — we'll do a proper heat-load calculation at a free survey and come back with a fixed price.
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