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What The Air Conditioning BTU Calculator Works Out
- Cooling needed: floor area x about 120 watts of cooling per square metre, the working UK figure, nudged up 15% for sunny, south-facing or top-floor rooms, up 30% for conservatories (effectively glass boxes) and down 10% for shaded, well-insulated rooms. 3,412 BTU equals a kilowatt, so a 20 m2 bedroom wants around 2.4kW or 8,200 BTU.
- Recommended unit: the nearest standard class above the calculation (5,000, 7,000, 9,000, 12,000, 15,000, 18,000 or 24,000 BTU). Undersized units run continuously without reaching temperature on the hottest days; oversized ones cool the air faster than they dry it and short-cycle into a clammy chill, so buy the class above, not the biggest.
- Typical running cost: cooling kilowatts divided by a real-world efficiency near 2.8, priced at the current Ofgem price cap unit rate, for an hour of flat-out cooling. A 2.4kW load draws under a kilowatt, roughly 20p to 25p an hour, and far less averaged across a day of holding temperature.
- Occupants and kitchen (advanced): add roughly 500 BTU for each regular occupant beyond the first two and 1,000 BTU for a room with cooking appliances, both of which add heat the floor area cannot see.
- Cooling per m2 and efficiency (advanced): edit the watts per square metre and the unit's efficiency ratio. Portables sacrifice 5% to 10% of their effect to the hot exhaust hose and the window it leaks through (single-hose designs pull warm air back in; sealed window kits help) and manage around 2.5 to 3; fixed splits reach 3.5 or more.
- Cost per summer (advanced): the electrical draw x average load once the room is cool x hours per hot day x hot days a year, priced at your unit rate. Thirty days of six hours at half load is a typical UK summer; a heatwave year might double it.
- Fixed split system instead (advanced): the same cooling at a split's efficiency of 3.5, with the summer saving. The refrigerant work is legally an F-gas certified installer's job, putting a single-room install near four figures, so the saving is about quiet and efficiency more than payback.
- Winter heating from a split (advanced): a split runs in reverse as an air-to-air heat pump, delivering about 3 kW of heat per kW of electricity, so heating the same room costs a third of a plug-in electric heater. As UK summers trend hotter, the heating-capable split increasingly beats the portable on whole-year arithmetic.