What size portable air conditioner do you need? BTU and kW for UK rooms

Portable air conditioners are sold by cooling capacity, and capacity is the one specification you cannot fix after buying. A unit that is too small will run flat out through a heatwave without ever reaching a comfortable temperature; one that is far too large costs more, weighs more and cools less evenly than the number on the box suggests. Sizing well takes about ten minutes with a tape measure.

Cooling capacity is quoted in BTU (strictly BTU per hour — British thermal units of heat moved out of the room each hour) or in kilowatts. The conversion is fixed: 1 kW of cooling is about 3,412 BTU. So a "9,000 BTU" unit moves roughly 2.6 kW of heat, and a "12,000 BTU" unit roughly 3.5 kW.

Do not confuse cooling output with electrical input. A 9,000 BTU unit does not draw 2.6 kW from the socket; refrigeration moves several units of heat per unit of electricity, so its input power is typically closer to 1 kW. The input figure matters for running costs, the output figure for whether the room actually gets cool. Both are on the rating plate and the energy label — read both before buying.

One more wrinkle: capacity figures depend on the test method behind them, and single-duct portables sold in the UK are rated under conditions stricter than the methods some overseas listings still quote — which is why two apparently identical units can claim very different BTU numbers. When comparing, prefer the figure on the UK energy label over a larger number in the marketing copy.

Measure the room's floor area — length times width in metres. The bands below are the commonly used starting points for UK rooms with standard ceilings of around 2.3–2.4 m. They are rules of thumb, not physics: treat them as the starting band, then adjust using the next section.

• Up to 15 m² (a typical UK double bedroom or small lounge): 5,000–7,000 BTU, roughly 1.5–2.0 kW of cooling.
• 15–25 m² (a larger lounge or open lounge-diner): 7,000–10,000 BTU, roughly 2.0–2.9 kW.
• 25–35 m² (a big open-plan living space): 10,000–12,000 BTU, roughly 2.9–3.5 kW.
• 35–45 m² (large open-plan flat living areas): 12,000–14,000 BTU, roughly 3.5–4.1 kW — at this size, consider whether one portable unit can realistically cover the space at all.

If your ceilings are noticeably higher than 2.4 m — Victorian conversions, warehouse flats — the room holds more air than its floor area suggests, so move up a band.

British housing stock adds heat in ways the generic bands do not capture. For each of the following that applies to your room, consider moving up one band, or picking the top of your current band:

• South- or west-facing glazing with no external shading — afternoon solar gain through glass is usually the single biggest heat load in a UK room.
• Loft conversions and dormer rooms — heat rises into them from the whole building and the roof above them bakes all day.
• Top-floor flats generally, for the same two reasons.
• Single glazing or a poorly insulated 1960s–80s build — heat leaks in as readily as it leaks out in winter.
• Kitchens or open-plan rooms containing a fridge-freezer, oven or other appliances that run during the day.
• Regular occupancy above two people, or a home office full of computing equipment — people and electronics are heaters.

These adjustments compound loosely, not precisely. A 14 m² south-facing loft bedroom is not a "5,000 BTU room" just because of its floor area; it behaves more like a room half as big again. When in doubt between two bands under UK heatwave conditions, take the larger one — with one caveat, covered next.

An undersized unit is the worse mistake. On a 32 °C day it will run continuously, never cycle off, never reach its setpoint, and cost you full running power for a room that is merely less hot rather than cool. Because heatwaves are exactly when you bought it, an undersized portable fails precisely when you need it.

An oversized unit has softer downsides: it costs more to buy, is heavier to move between rooms, and cools the air so quickly that it cycles off before it has dehumidified properly, which can leave the room cool but clammy. It will also draw more power while running — though for less time. Modest oversizing is a reasonable hedge; buying double the band is wasted money and a wetter-feeling room.

Single-duct portables — the common type, with one exhaust hose — add a quirk worth knowing: the air they blow outside has to be replaced, so warm air is drawn into the room through every gap in the building. Part of the rated capacity is always spent fighting this self-inflicted load. It is another reason to avoid the bottom edge of your band, and a reason sealing the window opening around the hose matters as much as the hose itself.

Take a 20 m² lounge in a top-floor flat with a west-facing window and two occupants most evenings. Floor area alone says 7,000–10,000 BTU. Top floor plus west glazing are two size-up factors, so aim at the top of the band or just above it: a 9,000–10,000 BTU (roughly 2.6–2.9 kW) unit is a sensible target, and 7,000 BTU would likely disappoint in a proper heatwave. The same 20 m² room on the ground floor, shaded and north-facing, would be fine at the bottom of the band.

Once you know your target band, availability becomes the real constraint — in a hot spell the popular mid-size capacities are the first to go. It is worth deciding your acceptable range in advance (say, 8,000–10,000 BTU) so you can act quickly when a unit in that range shows up near you, rather than re-deriving the answer while stock moves.