Online BTU calculators ask for a room's length and width and give you a number. That number is a reasonable first guess for an ordinary room with an ordinary window. It can be well out for a loft bedroom under a hot roof, a conservatory, or a kitchen with a wall of glass facing south.
This guide explains what goes into sizing so you can make sense of the figures you see, and why a survey is the point where the guess becomes a decision.
kW and BTU: the same thing in different units
Air conditioner capacity is quoted either in kilowatts (kW) or British Thermal Units per hour (BTU/h). They measure the same thing: how much heat the unit can move. One kilowatt is roughly 3,412 BTU/h, so a 2.5 kW unit is around 8,500 BTU/h and a 3.5 kW unit around 12,000 BTU/h.
Manufacturers usually quote separate cooling and heating capacities, and the heating figure is often a little higher. If you want the unit to heat the room in winter as well as cool it in summer, both figures matter.
What pushes the size up or down
Room size and ceiling height
The volume of air and the surfaces it touches.
- Larger floor area needs more capacity
- High or vaulted ceilings add volume
- Open-plan spaces count as one room
Glazing and orientation
Sunlight through glass is often the largest heat gain.
- South and west facing rooms gain most
- Large windows, bifolds and roof lights add load
- Blinds and shading help
Insulation and construction
How quickly the room gains and loses heat.
- Well-insulated rooms need less
- Rooms under a roof run hotter in summer
- Older solid walls behave differently from new builds
How the room is used
People and equipment produce heat too.
- Home offices with computers
- Kitchens with cooking appliances
- Bedrooms used mainly at night
Loft rooms and conservatories run hotter
Rooms directly under a roof pick up heat from the roof covering all day, then hold it into the evening. Loft conversions and top-floor rooms often need more capacity than their floor area suggests.
Conservatories are the extreme case. Glass gains heat quickly in sun and loses it quickly on cold nights, so the unit has to cope with both ends. Sizing them on floor area alone almost always leads to a unit that struggles.
Why too big or too small both cause problems
An undersized unit runs flat out and still fails to reach the set temperature on the hottest or coldest days. An oversized one cools the room quickly and then cycles on and off, which can leave the air feeling clammy because it has not run long enough to remove moisture. Modern inverter units cope with a range of loads better than older ones, but matching capacity to the room still gives the best comfort.
Where several rooms are served by one outdoor unit, the indoor units are each sized for their room and the outdoor unit for the combination, which is another reason to look at the house as a whole.
A sizing figure is only as good as the information behind it. Tell us about the room, and the visit will take care of the rest.
