Air conditioner size is measured in kilowatts, not the physical box on the wall, and getting it right is the difference between a system that quietly keeps up and one that costs more and never feels comfortable. Here is how to go from your room to a sensible kW range, what shifts that number, and how we size it against real systems.
What 'size' actually means
When we talk about the size of an air conditioner we mean its capacity in kilowatts (kW) of cooling or heating, not how big the unit looks on the wall. A small bedroom split might be 2.5kW; a large open-plan living area might need 7 to 8kW; a whole home on ducted is sized differently again.
As a rough starting point, cooling capacity scales with floor area. In Mitsubishi Heavy Industries' sizing chart an insulated room needs about 120 to 140 watts per square metre, from 100 for a shaded, insulated room facing south to about 170 for one with little insulation and western sun. The list below works it out for you.
That gets you a ballpark, but it is only ever a starting point, because two rooms of the same size can need quite different systems once you account for the things below.
A rough room-by-room guide
These are the most floor area each size cools in that chart, for an insulated brick or brick-veneer room with average windows and internal blinds. The larger figure is for a room facing north, the smaller for one facing east. The chart does not give a ceiling height; Rinnai's similar chart is for insulated ceilings up to 2.6m.
- Bedroom or small study (up to about 18 to 21m²): 2.5kW
- Main bedroom or small living room (up to about 25 to 29m²): 3.5kW
- Living or family room (up to about 36 to 43m²): 5kW
- Large living room (up to about 45 to 50m²): 6kW
- Large open-plan living and kitchen (up to about 51 to 59m²): 7kW
- Bigger open plan (up to about 57 to 67m²): 8kW, and past that a multi-split or ducted
If your room is close to the figure for its size, or has any of the factors in the next section, it is usually safer to step up a size, but not by miles, because oversizing has its own problems.
Why the right size beats 'bigger is safer'
It is tempting to just buy big to be safe. It backfires. An oversized system cools the air quickly then switches off, short-cycling on and off without running long enough to pull humidity out, so the room feels cold and clammy and the compressor wears faster.
An undersized system has the opposite problem: it runs flat out on the hottest days, never quite gets there, and uses more energy doing it. The right size runs steadily, holds the temperature, dehumidifies properly and draws the least power. Size is the single biggest comfort decision you make, before brand, before features.
What actually moves the number
Floor area gets you in the ballpark; these push it up or down from there:
- Ceilings: anything above standard height is more air to condition.
- Glass: large windows, and especially west-facing ones that cop the afternoon sun, add a lot of heat load.
- Insulation: a well-insulated room holds temperature; an uninsulated top-floor room under a hot roof does not.
- Orientation and shade: north and west rooms run hotter than shaded southerly ones.
- The room's job: kitchens, and rooms full of people or electronics, carry more heat.
This is why a chart alone is never quite right, and why we ask about these when we size a system rather than just measuring the floor.
How we size it, against real systems
Most articles stop at a chart. Ours does not. Our capacity calculator asks six quick questions, including the floor area and how well the room is insulated and shaded, works from the same chart as the list above, and gives you a sensible kW range for your space.
Then it does the part a chart cannot: it matches that range to real systems we actually install and shows each one's running cost from the official energy figures, so you are choosing between systems that genuinely fit, not guessing. It takes about thirty seconds.
From a kW range to the right system
Once you have your range, two things narrow it down. Efficiency: two systems of the same size can use noticeably different amounts of power, which you can line up on our compare page. And type: a single split for one room, a multi-split for a few, or ducted for a whole home.
If you would rather not work it out yourself, send us a few photos of the space and we will size it and recommend a system that suits. If the job needs a look first, we book a site visit, and you get a fixed price in writing before any work starts.
Sources
- Mitsubishi Heavy Industries, Split Systems brochure, the Air Conditioning Sizing Chart on page 7: the most floor area each size cools in four room classes, from AS/NZS 3823 data, and the advice to have a licensed installer check the heat load
- Rinnai T Series brochure, page 7: area coverage for insulated ceilings up to 2.6m
