September 8, 2026 · Cairns Aircon Installers
It is the first question almost everybody asks, and the one that gets answered badly more often than any other. Get the size right and the system runs quietly, dries the air properly and lasts. Get it wrong in either direction and you will be unhappy with it for the next decade.
Capacity Is Measured In Kilowatts
Air conditioner capacity is quoted in kilowatts of cooling, and that is a different number from the kilowatts of electricity the unit draws. A 7kW system does not use 7kW of power. It typically moves three to five times more heat than the electrical energy it consumes, which is why a modern inverter is so much cheaper to run than the old box units.
When somebody says "a 2.5" or "a 7.1", they are talking about cooling capacity in kilowatts. Those are the numbers on the box.
Why Cairns Sizing Is Not Southern Sizing
Most sizing guides on the internet are written for Sydney, Melbourne or the UK. They assume a design day that we would call mild, and they assume the machine mostly has to deal with heat rather than moisture.
Up here the unit has two jobs. It has to pull the temperature down, and it has to pull a very large amount of water out of the air. During the build-up, latent load (the moisture) can account for a substantial share of the total work the system does. A unit sized on a southern chart will run permanently flat out in February and still leave the room feeling damp.
What Actually Determines The Size
A proper heat load calculation looks at all of this:
- Floor area and ceiling height. Volume, not just area. A raked ceiling in a Queenslander is a lot more air to condition than the floor plan suggests.
- Glazing. How much glass, which way it faces, and whether it is shaded. West-facing glass in Cairns is brutal and is the single biggest swing factor in most homes we quote.
- Orientation and shading. Eaves, verandahs, awnings, neighbouring buildings and mature trees all change the answer.
- Insulation. Ceiling insulation in particular. Plenty of older Cairns houses have very little, and it shows.
- Construction. A block home holds heat differently from a timber Queenslander on stumps.
- Occupancy and equipment. People, kitchens, computers and televisions all add heat.
- Air leakage. Open-plan spaces, stairwells and gaps to unconditioned areas.
Rough Starting Points, With A Warning
Before anyone measures anything, these are the ranges we most often end up in for Cairns homes. Treat them as a sanity check on a quote, not as a specification:
- Bedroom: commonly in the 2.5kW to 3.5kW range.
- Larger bedroom or small living area: commonly 3.5kW to 5kW.
- Open-plan living, kitchen and dining: commonly 7kW to 9kW, and sometimes more.
- Whole home ducted: almost always needs a calculation. There is no useful rule of thumb.
If your west-facing living room has a wall of glass and no eave over it, the top of that range may still not be enough. If it is shaded all afternoon and well insulated, you may do fine below it. That gap is exactly why the calculation exists.
The Problem With Going Too Big
People assume oversizing is the safe error. It is not. An oversized system reaches the set temperature quickly, then shuts down. That is called short cycling, and it causes real problems:
- The room stays humid. Dehumidification happens while the unit runs. Short bursts do not give the coil enough run time to strip moisture out of the air, so you get a cold clammy room. In Cairns this is the complaint we hear most about an oversized unit.
- Temperature swings. Hard on, hard off, rather than a steady hold.
- More wear. Starting and stopping is harder on components than running steadily.
- Worse efficiency. An inverter is at its most efficient modulating at part load, not cycling.
The Problem With Going Too Small
An undersized unit runs continuously on the worst days and never quite gets there. It will feel acceptable in July and disappointing in February, which is precisely the wrong way around. It also runs at full load constantly through the hardest months, which shortens its life.
Undersizing usually happens for one of two reasons: somebody sized on floor area alone, or somebody quoted the cheapest machine that would fit the budget without saying what the trade-off was.
Get The Calculation Done
A heat load calculation takes a technician a short visit and it is the difference between a system you forget about and one that annoys you every wet season. We do it as part of quoting, and we will tell you if the size you had in mind is wrong in either direction.
Have a look at our split system installation and ducted air conditioning pages for how each option is sized and installed, or request a quote and we will work it out properly for your place.