What size air conditioner do I need for my room?
The table our estimate uses to size a room, what kW and BTU actually mean, and the things that push a room up a size.
- REFCOM certified, REF1025662
- Daikin registered installer
The short answer
For most bedrooms in a London house, a 2.5kW unit (about 8,500 BTU an hour). A large bedroom or a loft room usually wants 3.5kW (about 11,900 BTU), a living room 5.0kW (about 17,100 BTU), and an open plan kitchen diner 7.1kW (about 24,200 BTU). Those are starting points by room type and floor area. A lot of west or south facing glass, a room directly under the roof, or a kitchen can move a room up one size, which is why the final figure is checked at a survey rather than read off a chart.
The sizing table our estimate uses
This is the rule the instant estimate on this site applies, by type of room and rough floor area: small is up to about 12 square metres, medium is about 12 to 20 square metres, large is more than 20 square metres.
| Room | Up to 12 sq m | 12 to 20 sq m | Over 20 sq m |
|---|---|---|---|
| Bedroom | 2.5kW | 2.5kW | 3.5kW |
| Larger or loft bedroom | 3.5kW | 3.5kW | 5.0kW |
| Living room | 3.5kW | 5.0kW | 7.1kW |
Open plan spaces start at 7.1kW, or 6.5kW on the Samsung Cebu S2. Fully installed from £1,995 at 2.5kW, £2,095 at 3.5kW, £2,595 at 5.0kW and £3,395 at 7.1kW.
kW and BTU are the same thing in different units
The size of an air conditioner is its cooling output. In the UK it is quoted in kilowatts. Older units, portable units and a lot of American advice quote British Thermal Units per hour instead. One BTU per hour is 0.293 watts, so 1kW is about 3,412 BTU per hour.
That makes the common sizes easy to translate. 2.5kW is about 8,500 BTU, 3.5kW about 11,900, 5.0kW about 17,100 and 7.1kW about 24,200. If a quote or a product page gives you one, you can check it against the other.
The kW figure is cooling output, not the electricity the unit uses. A 2.5kW unit does not draw 2.5kW from the wall. What it draws depends on its efficiency, which is a separate number on the energy label.
What each size can do, from Daikin's own data
The Daikin Comfora as an example, because Daikin publishes the full figures. Nominal cooling is measured with 35 degrees outside. Heating at minus 10 is the output Daikin declares for its seasonal heating rating.
| Cooling size | Heats, nominal | Heats at minus 10 | Cost to cool |
|---|---|---|---|
| 2.5kW | 3.0kW | 2.09kW | 9p an hour |
| 3.5kW | 4.0kW | 2.35kW | 13p an hour |
| 5.0kW | 6.0kW | 3.15kW | 18p an hour |
| 7.1kW | 8.2kW | 4.24kW | 30p an hour |
Running cost is the size divided by Daikin's published seasonal efficiency (SEER) for that size, at the Ofgem price cap of 26.32p per kWh. An estimate, not a quote.
What pushes a room up a size
Floor area gets you close. These are the things we look for at a survey that move the answer.
Big windows facing south or west
Sun through glass can put more heat into a room than anything else in it. A bedroom with a west facing bay window in late afternoon behaves like a much bigger room than its floor area suggests.
Rooms directly under the roof
Loft conversions and top floor flats take heat through the ceiling all day. That is why the estimate treats a loft bedroom as a larger bedroom whatever its floor area.
Kitchens and open plan spaces
Cooking, appliances and more people all add heat. An open plan kitchen diner is sized well above a living room of the same floor area.
A home office
Computers and screens give off heat for as long as they are on. A small study used eight hours a day can need more than its size suggests.
Wanting it to heat as well
Heating output falls as it gets colder outside. If the unit is going to be the main heat in a room through winter, size for the cold end of the table above, not the nominal figure.
High ceilings
A tall Victorian or Edwardian room holds more air than its floor area implies. We measure the height as well as the floor.
Why bigger is not always better
Modern units are inverters. They do not just switch on and off, they slow the compressor down to match the room. Daikin's data shows the 2.5kW and 3.5kW Comfora both turn down to 1.3kW, so stepping up from 2.5 to 3.5kW for a sunny bedroom costs you very little in how the unit behaves.
Going much bigger than the room needs is different. The lowest output rises with the size, to 1.7kW on the 5.0kW and 2.3kW on the 7.1kW. When a room needs less than that, the unit has to stop and start rather than tick over, which is noisier and less even. It also costs more to buy and fit, and from 5.0kW up it needs its own circuit from your fuse board rather than a fused spur.
So the aim is the size that covers the room on a hot afternoon with a little in hand, not the largest unit that fits the wall.
How we confirm it
The estimate gives you a size and a starting price from three questions. At the free survey we look at the room itself: which way the windows face, what is above it, the ceiling height, and how you want to use it. If the answer is different from the estimate, we tell you before you agree to anything, and the quote is written for the size we would actually fit.
The ranges this applies to
Every price is fully installed and includes the unit, the installation, the electrical work and VAT.
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