What does BTU mean and how does it relate to kW?

BTU per hour is another unit of cooling capacity, often used by retailers. One kW is 3,412 BTU per hour. So a 9,000 BTU unit is 9,000 / 3,412 = about 2.6kW, and a 12,000 BTU unit is about 3.5kW. UK installers usually quote kW, so convert before you compare.

Should I size for the hottest possible day?

No. Sizing for a rare extreme day usually gives an oversized unit that cycles on and off and leaves the room clammy the rest of the time. Size for typical hot conditions in your area. In Leeds, the Met Office average July maximum is 21.4°C, and well-sized inverter units can still push harder on hotter days.

Can one unit cool two rooms through an open door?

Only partly. Some cool air will spill into the next room, but you won't get a controlled temperature there, and the unit will work harder. If you want two rooms properly covered, a [multi-split](/guides/single-split-vs-multi-split) with an indoor unit in each room is the reliable approach, and on a semi or terrace it keeps you to one outdoor unit.

What size do I need for a 20 square metre bedroom?

Using Daikin's rule of 0.1kW per square metre, about 2kW for a typical room with a 2.4m ceiling. Daikin suggests a poorly insulated, sunny [bedroom](/guides/best-air-conditioning-bedroom) of that size might need 2.5kW. A loft bedroom under the roof, common in Leeds back-to-backs, may need more, so the survey figure is the one to go by.

Does an open-plan kitchen need a bigger unit?

Usually, yes. Daikin's guide adds 10% to 20% for kitchens because of heat from cooking and appliances. In an open-plan room, measure the whole space, apply 0.1kW per square metre, add for the kitchen area and any sunny glazing, and expect the installer to check whether one indoor unit can cover the shape of the room.

What Size Air Conditioner Do I Need? UK kW Guide (2026)

Start with the rule of thumb

Daikin's UK air conditioner size guide gives a starting figure of about 0.1kW of cooling capacity per square metre of floor area, for a typical room with a 2.4m ceiling and average insulation. So a 20 square metre room starts at about 20 x 0.1 = 2kW, and a 35 square metre room at about 3.5kW.

To use it, measure the length and width of the room in metres and multiply them to get the floor area. Multiply the area by 0.1 to get a first figure in kW. Then adjust it for the factors below.

Size here means cooling capacity, not how big the unit looks on the wall. Units are sold in set capacities, so the installer picks the model whose rated capacity best matches your figure.

kW and BTU

UK installers usually quote capacity in kW, but many retailers and online guides use BTU per hour. One kW is 3,412 BTU per hour. To convert, multiply kW by 3,412, or divide BTU by 3,412.

So 2kW is 2 x 3,412 = 6,824 BTU per hour, and a unit sold as 12,000 BTU is 12,000 / 3,412 = 3.5kW. Checkatrade's 2026 cost guide says a 12,000 BTU system is usually suitable for an average-sized bedroom, which is why that size comes up so often in price comparisons. Our Leeds air conditioning installation cost guide breaks down a typical quote line by line.

Factors that change the figure

The rule of thumb assumes an ordinary room. Daikin's guide lists the main reasons to adjust it.

Ceiling height: capacity rises in proportion to volume, so a 3m ceiling adds 25% over 2.4m, since 3 / 2.4 = 1.25.
Sunny windows: large south or west facing windows add 10% to 20%.
People: add about 0.18kW for each person regularly in the room above two.
Kitchens: heat from cooking and appliances adds 10% to 20%.
Insulation: a well-insulated 20 square metre bedroom might need 2kW, a poorly insulated sunny one 2.5kW.
Climate: Daikin gives 3kW for a 30 square metre room in Scotland against 3.6kW in the South East.

A worked example: a Leeds terrace living room

Take a through-terrace in Headingley with a living room 5m by 4.2m, a 2.9m ceiling, a west-facing bay window and a family of four using it in the evening.

Floor area is 5 x 4.2 = 21 square metres, so the basic figure is 21 x 0.1 = 2.1kW. The ceiling is 2.9m against the 2.4m assumed, so the volume factor is 2.9 / 2.4 = 1.21, and 2.1 x 1.21 = 2.54kW. The bay faces west, so add 10%: 2.54 x 1.1 = 2.79kW. Four people is two more than the rule assumes, so add 2 x 0.18 = 0.36kW, giving 2.79 + 0.36 = 3.15kW. In BTU, that is 3.15 x 3,412 = about 10,750 BTU per hour.

That is a starting point for the conversation with the installer, not the answer. A solid stone wall, a cellar below and the house next door on each side all change how the room behaves, and a heat-gain calculation at survey takes those into account.

Why oversizing is a real problem

It is tempting to go up a size to be safe, but it often backfires. An oversized unit cools the air quickly and then switches off, before it has run long enough to take moisture out of the air. The room reaches the set temperature but feels damp and clammy, and the constant stopping and starting is less efficient and harder on the equipment.

Modern inverter units can turn down below their rated output, which helps, but only within a range. Choosing the right size matters more than adding a margin.

Undersizing has its own problem. A unit that is too small runs flat out on hot afternoons and still can't keep up, which is most likely in loft rooms and conservatories.

Home offices and small workplaces

Rooms with several people and computers need more than the floor area suggests. People give off heat, and Daikin's adjustment of about 0.18kW for each person above two shows how quickly it adds up.

Worked example: a small office above a shop in Leeds city centre, 6m by 5m. The floor area is 6 x 5 = 30 square metres, so the basic figure is 30 x 0.1 = 3kW. Six people work there, four more than the rule assumes, so add 4 x 0.18 = 0.72kW, giving 3.72kW. Computers, screens and a printer add heat too, and the installer should ask about them. Larger offices usually move to ceiling cassettes or ducted units, which our commercial page covers.

A home office is simpler. One person and a laptop in a 10 square metre box room comes to about 10 x 0.1 = 1kW, and a south-facing window would add 10% to 20%.

Common mistakes

Most sizing problems come from a few habits that are easy to avoid.

The first is using floor area alone for a room that isn't ordinary, such as a loft bedroom, a conservatory or a kitchen diner with bifold doors. The second is comparing BTU on one quote with kW on another without converting. The third is buying a portable unit by its BTU rating and expecting it to match a split system of the same number. The fourth is going up a size just in case. Ask the installer to show you the figures behind the size they recommend.

Heating as well as cooling

Almost every split system fitted in UK homes is an air-to-air heat pump, so it heats as well as cools. In a UK home, the heating need in winter can be larger than the cooling need in summer, so the installer should check both. Daikin calls a SCOP, the seasonal heating efficiency, above 3.5 high performance.

At the Ofgem price cap for October to December 2026, electricity is 26.32p per kWh by Direct Debit. With a SCOP of 4.0, heat costs 26.32 / 4 = 6.6p per kWh. A correctly sized unit runs closer to its best efficiency for more of the year.

Leeds and West Yorkshire homes

Housing stock in Leeds varies a lot, and that shows up in sizing. Leeds City Council counts about 19,500 pre-1919 back-to-back houses. Their attic bedrooms sit under the roof with one outside wall, so they often need more than the floor area alone suggests. Victorian terraces in areas such as Headingley, Chapel Allerton and Armley have high ceilings downstairs, which the ceiling factor accounts for.

Newer homes on estates around Garforth, Morley and Rothwell are usually better insulated, so they may come in at or below the rule of thumb, unless large glazed areas face south or west.

The climate here is mild by UK standards. The Met Office 1991 to 2020 averages for Leeds Weather Centre give a mean daily maximum of 21.4°C in July. Daikin's climate adjustment works the same way, needing less capacity in cooler regions than in the South East, which is another reason not to oversize.

What a proper survey should cover

A good survey goes beyond measuring the floor. Expect the installer to check these.

Room size and ceiling height, measured, not estimated.
Windows: size, direction and glazing type.
Walls, roof and insulation, including any loft room or conservatory.
How the room is used and by how many people.
Heating needs in winter as well as cooling in summer.
Where the indoor and outdoor units can go, and the pipe route between them.

Several rooms on one system

If you want more than one room covered, each room is sized on its own, then the installer chooses an outdoor unit that suits the combination. On a semi or terrace, permitted development in England allows only one outdoor unit, so several rooms usually means a multi-split. Our single split versus multi-split guide explains the differences.

Our partner installers are MCS certified and carry out the sizing at survey. Domestic installations of air-to-air heat pumps are zero-rated for VAT until 31 March 2027 under HMRC's VAT Notice 708/6.

Sources

Guidance is general and indicative. Always confirm details with a qualified F-Gas registered installer. Get a free quote.

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