How to size a bedroom air conditioner for a UK home

For a well-insulated UK bedroom with average solar gain, the working rule is roughly 100 to 120 watts of cooling per square metre of floor area. A standard 12 to 14 m² double bedroom lands in the 1.5 to 2.0 kW band; a larger 16 to 20 m² room lands at 2.0 to 2.5 kW. This rule holds because British bedrooms are usually shaded for at least part of the day, have a single external wall, and rarely run continuously above 26°C even in a heatwave.

This sits inside the wider cooling picture, where the goal is a comfortable house rather than a fully air-conditioned one; the bedroom is almost always the room where a fixed split earns its place first.

First, west or south-west facing bedrooms with unshaded glazing gain heat all afternoon and hold it into the evening. Add roughly 500 W of cooling capacity for a room in this orientation with more than 2 m² of unshaded glass.

Second, loft conversions almost always need one band up. The roof is directly above, the insulation is often thinner than a first-floor ceiling, and the room shape traps heat under the ridge. A 12 m² loft bedroom that would take a 1.5 kW unit at ground-floor level typically needs a 2.5 kW unit in the loft.

Third, rooms shared by two adults have a higher latent load than single-occupancy rooms; the difference is small but real. Nudging from 2.0 kW to 2.5 kW handles this without over-specifying.

Fourth, single-glazed sash windows in an unimproved Victorian bedroom can lose the cooling as fast as the unit produces it. In this case, secondary glazing or draught-proofing is a better first pound than a bigger AC unit.

An oversized split short-cycles: it hits the target temperature quickly, switches off, and comes back on a few minutes later. Each cycle is audible, the room temperature swings by a couple of degrees, and humidity control gets worse rather than better. A 2.5 kW unit in a 10 m² box room will feel less comfortable than a 1.5 kW unit in the same room.

If the manufacturer's data sheet lists a minimum modulated output above the room's typical cooling demand, size down. Modern inverter units modulate well but not infinitely; the useful lower bound is usually about 25 percent of nominal capacity.

A competent MCS-registered installer will do their own room-by-room calculation before quoting, but the more you can give them up front the better the quote. Room dimensions, orientation, glazing type, whether the room is above another heated room or a cold garage, and any known overheating pattern from previous summers. If the bedroom is genuinely uncoolable any other way, a proper site survey is what turns the calculation into a working install.