What Size Air Conditioner Do You Need in Cairns? Getting It Right Before Summer

Most people buying an air conditioner ask what it costs. The better first question is what size it needs to be, because that decision sets how the room feels, how much the power bill is, and how long the unit lasts. Get it wrong and no amount of servicing fixes it.

In Cairns it matters more than most places. The unit is not just cooling the air, it is pulling water out of it for months on end, and a system sized off a southern rule of thumb can do one job well and the other badly. October is the month to sort it, before the installers’ diaries fill up with people whose old unit gave up in the first hot week.

What the size number actually means

The kilowatt figure on a split system or cassette is its cooling capacity: how much heat it can move out of the room. It is not how much power it uses. A larger unit can move more heat, and a modern inverter unit can also throttle itself down when the room is already comfortable, which is why a well-sized inverter is cheaper to run than an old fixed-speed unit working flat out.

The charts you see in shops match a capacity to a floor area. They are a starting point, built for an average room in average conditions. A Cairns room in December is rarely average.

What changes the answer up here

• Which way the room faces. A west-facing room with big windows takes the afternoon sun straight in and can need noticeably more capacity than the same room on the shaded side of the house.
• Ceiling height and the roof above it. Raked ceilings, high Queenslander ceilings and rooms under an uninsulated tin roof hold far more air and heat than the chart assumes.
• Open-plan living. If the lounge opens onto the kitchen and the hallway, you are cooling all of it, whether you meant to or not. The size has to match the space the air can actually reach.
• Who and what is in the room. People, ovens, computers and a big TV all add heat. A kitchen-family room and a spare bedroom of the same floor area are not the same job.
• Insulation, glazing and shading. Ceiling insulation, window tinting, awnings and a verandah on the hot side can all bring the size you need down, sometimes enough to drop a model.
• The humidity. In the build-up and the wet the unit spends a big share of its effort taking moisture out of the air. That is a large part of why a room feels comfortable here, not just the number on the remote.

Too small: the unit that never catches up

An undersized unit runs at full output from mid-morning until the sun goes down and still does not get the room to temperature. It is loud, it is working at its limit every afternoon of the summer, and that is the system that tends to fail first. The usual fix people try is setting the remote lower, which changes nothing except how long the compressor runs.

Too big: the room that is cold but clammy

Bigger is not safer. An oversized unit drops the temperature quickly, hits the set point and backs off before it has had time to pull the moisture out of the air. In the tropics that leaves a room that reads cool on the thermostat but feels damp and sticky, and over time that damp shows up as a musty smell and mould on the walls, the curtains and inside the unit itself. You also pay more upfront for capacity the room never uses.

How we size a unit

We size the unit to the room and the Far North climate rather than the cheapest box that fits. That means coming out and looking: the room size and the ceiling, which way it faces and what the sun does to it in the afternoon, what is above it, how open it is to the rest of the house, and what the room is used for. Then we tell you what size you actually need and what it will cost to run, rather than selling you up. If a smaller unit does the job, that is the one we quote.

Where the unit goes matters as much as its size. A head mounted where the air can throw down the length of the room cools it evenly. One tucked in a corner behind a door leaves warm spots, and people then buy a bigger unit to fix what was really a placement problem.

Split, multi-head, cassette or ducted

Sizing is per room, so the system type follows from it. One room, one wall-mounted split is usually the simplest answer. Several bedrooms off one outdoor unit can suit a multi-head split. Ceiling cassettes throw air in four directions and suit larger open rooms and commercial spaces. Ducted works for a whole house but needs the roof space and careful zoning. We covered that choice in more depth in our split system or ducted guide.

Before you buy, have these ready

• Rough measurements of the room, and whether it is open to other rooms.
• Which way the main windows face, and whether they get afternoon sun.
• What is above the room: a second storey, an insulated ceiling or a bare tin roof.
• What the room is used for, day and night, and how many people are usually in it.
• Photos of where you would like the indoor and outdoor units to go.

Installation is licensed work, and we are QBCC and ARCtick licensed, so the warranty on your unit stays intact. We supply and install split systems, cassettes and commercial systems across Cairns and the inner suburbs, the northern beaches from Machans Beach to Palm Cove, and south to Edmonton and Gordonvale and up the Range to Mareeba and Atherton. If you are thinking about a new unit before summer, now is the time to have it sized properly.