What size air conditioner do I need?
Most bedrooms need a 2.5 kW unit, most living rooms 3.5 kW, and big open-plan rooms or lofts 5 kW. Those are starting points. Glass, sun, ceiling height and how many people use the room all push the size up or down, and a survey confirms the right answer.
- Daikin's starting point for UK rooms
- 0.1 kW per m²
- for big south or west-facing windows
- Up to a fifth more
- 2.5 kW room, fitted, inc. VAT
- From £1,850
- to confirm the right size
- Free survey
Size means output, not the size of the box
The kW figure on an air conditioner is how much cooling it delivers, not how big the unit is or how much electricity it uses. A 2.5 kW unit moves 2.5 kW of heat out of the room at its rated output, while drawing much less power to do it.
Some sellers quote BTU instead. 1 kW is about 3,412 BTU an hour, so a 2.5 kW unit is about 8,500 BTU, a 3.5 kW unit about 12,000 BTU and a 5 kW unit about 17,000 BTU.
Inverter units also speed up and slow down. Daikin lists the 2.5 kW Perfera (FTXM25A) cooling anywhere from 0.9 to 3.8 kW, which is why a well-chosen size copes with a heatwave and still runs gently on a mild day.
The rule of thumb
Daikin's UK size guide gives a starting point of about 0.1 kW per square metre for typical UK homes, with average insulation and a standard 2.4 m ceiling. So a 20 m² bedroom starts at about 2 kW, and a 35 m² living room at about 3.5 kW.
Daikin also points out that the South-East is warmer than the rest of the UK. Its example is a 30 m² room that might need 3 kW in Scotland but could need 3.6 kW in the South-East, about a fifth more. That matters across London and the Home Counties, and it's why we rarely size a room below the simple rule.
Room size to unit size
A quick guide for a room with a 2.4 m ceiling, ordinary windows and average insulation. The starting point uses Daikin's 0.1 kW per m².
| Room | Floor area | Starting point | What we usually fit |
|---|---|---|---|
| Box room or small study | Up to about 10 m² | Up to 1 kW | 2.0 or 2.5 kW |
| Double bedroom | About 10 to 20 m² | 1 to 2 kW | 2.5 kW |
| Large bedroom or home office | About 20 to 25 m² | 2 to 2.5 kW | 2.5 or 3.5 kW |
| Living room | About 25 to 35 m² | 2.5 to 3.5 kW | 3.5 kW |
| Open-plan kitchen and living | About 35 to 50 m² | 3.5 to 5 kW | 5 kW, or two units |
| Loft room | Any | More than the area suggests | Often 3.5 or 5 kW |
Add about a fifth for the South-East, then allow for the factors below. Very small rooms often get 2.0 or 2.5 kW because that's near the smallest size made.
What pushes the size up
Each of these adds heat to the room. Figures are from Daikin's UK size guide.
- Large south- or west-facing windows: Daikin suggests a tenth to a fifth more capacity
- High ceilings: a 3 m ceiling holds 25% more air than the standard 2.4 m
- More than two people using the room: add roughly 0.18 kW for each extra person
- Kitchens: often a tenth to a fifth more than a living room of the same size, because of cooking and appliances
- Poor insulation and sun: Daikin's example is a 20 m² bedroom needing about 2.5 kW instead of 2 kW
- Rooflights and glass extensions, which take the full midday sun
Glazing and south-facing rooms
Windows are usually the biggest single factor. Sun through glass heats the room directly, and south- and west-facing rooms get it in the hottest part of the day, so bifolds, full-height glazing and glass extensions can push a room up a whole size. North-facing and shaded rooms often sit at the starting point, and good blinds or external shading reduce the load.
Loft rooms and top-floor flats
Loft rooms are the rooms most often undersized. The roof sits directly above, so on a sunny day the ceiling itself warms up, and sloping ceilings mean there's little loft space above to buffer the heat. Rooflights let in strong overhead sun, and warm air rises from the floors below.
For a loft bedroom we often fit 3.5 kW where the floor area alone suggests 2.5 kW, and 5 kW for a large loft suite. Wall space can be tight under the slope, so a floor-standing console is sometimes the better fit.
Why bigger isn't better
It's tempting to buy a size up just in case. Daikin's guide warns that oversizing leads to higher running costs and noisier operation, while an undersized unit runs flat out all the time, which raises bills and shortens its life.
Noise is a real cost in bedrooms. On Daikin's spec sheet, the 2.5 kW Perfera runs at 19 dB(A) on its quietest setting, but the 5 kW model can't go below 27 dB(A). Mitsubishi Electric's MSZ-LN shows the same pattern. The right size is the quietest size that still copes on the hottest day.
Sizing for heating as well
Every home system we fit heats as well as cools, and the heating output is usually higher than the cooling rating. Daikin's 2.5 kW Perfera gives 2.8 kW of heat at its nominal rating, and Mitsubishi Electric's MSZ-LN25 gives 3.2 kW.
If you'll heat with it through winter, we size for whichever is bigger: the summer heat gain or the winter heat loss. A cold glass extension can need more for heating than for cooling.
What each size costs to fit
Home prices include VAT. Two rooms can share one outdoor unit on a multi-split, which often costs less than two separate systems.
- from£1,850inc. VAT
A bedroom or study
2.5 kW wall unit, 5 m pipe, fitted in a day
- from£2,050inc. VAT
A living room
3.5 kW wall unit
- from£2,450inc. VAT
Open plan or a loft
5 kW wall unit
- from£3,650inc. VAT
Two rooms, one outdoor unit
Multi-split
How we confirm the size at the survey
Measure
Floor area and ceiling height, including slopes and vaulted sections.
Look
Window directions, glazing, rooflights, insulation, and what's above and below.
Ask
How many people use the room, any computers or cooking, and whether you'll heat with it.
Quote
A fixed written price naming the exact model and size, so you can compare like with like.
Sizing questions
What size air conditioner do I need for a bedroom?
Usually 2.5 kW. Most double bedrooms are 10 to 20 m², which Daikin's rule of thumb puts at 1 to 2 kW, and 2.5 kW leaves room for sunny days and the warmer South-East. A loft bedroom or a very sunny main bedroom may need 3.5 kW.
What size air conditioner do I need for a living room?
Usually 3.5 kW for a living room of about 25 to 35 m². A large open-plan kitchen and living space of 35 to 50 m² usually needs 5 kW, or two smaller units at opposite ends.
Is it better to get a bigger air conditioner?
No. Daikin warns that oversizing raises running costs and makes the unit noisier, and bigger units are louder at their quietest setting. Pick the size that copes with the hottest day, and no bigger.
Does a loft room need a bigger air conditioner?
Often, yes. The roof takes the sun directly, sloping ceilings leave little buffer, and rooflights add strong overhead sun. We often fit 3.5 kW in a loft bedroom where a normal bedroom of the same size would take 2.5 kW.