How to size an air conditioner
An air conditioner has to remove heat as fast as the room gains it on a hot day, and add heat as fast as the room loses it on a cold night. That rate is the room’s heat load, measured in kilowatts (kW). This calculator adds up the main sources:
- Walls, windows and ceiling. Heat moves through each surface depending on its insulation and the temperature difference. Under a roof, the ceiling sits below a hot roof space, which is why top-floor rooms need more cooling.
- Sun through glass. West- and east-facing windows let in the most sun in summer. North windows with eaves are much easier.
- Air leaking in. Older houses with gaps around doors and windows let in hot or cold air.
- People and appliances. Each person adds about 75 W, and kitchens add much more.
Then it adds an allowance for humidity on cooling, and for warm-up time on heating if the unit isn’t left running.
Why not just use “150 watts per square metre”?
That rule of thumb is a reasonable average, but rooms vary a lot. A shaded south-facing bedroom might need half that, while a west-facing living room with big glass doors under a hot roof can need much more. An undersized unit runs flat out and still can’t keep up. An oversized one costs more and cycles on and off, which is less efficient and less comfortable.
Cooling or heating: which decides the size?
In most of Australia, cooling decides it. In colder places, such as Tasmania, Canberra and much of New Zealand, heating often does. This calculator works out both and recommends a unit that covers the larger one.
Common questions
What size air conditioner do I need for a 30 m² room?
Often around 3.5 kW, but it depends on the windows, insulation, ceiling height and what’s above the room. A west-facing room under the roof can need 5 kW or more. Enter your room above for a better estimate.
Is a bigger air conditioner better?
Not really. An oversized unit costs more to buy, cycles on and off more, and can leave the air feeling clammy because it doesn’t run long enough to remove moisture. Aim for the size that covers your room’s load.
Does insulation change the size I need?
Yes, a lot. Ceiling insulation is the biggest single improvement for a room under a roof, and it reduces both the size of the unit and its running cost.
Should I get split systems or ducted?
Split systems suit cooling a few rooms. Ducted suits whole-house heating and cooling, and a ducted system is usually sized for about 80% of the sum of the rooms, because they don’t all peak at once. Our full Calcs tool does the whole house room by room.