What Size Air Conditioner Do I Need? Room Size to kW Guide (Australia)

Getting the size right is the most important decision you'll make when buying an air conditioner — more important than the brand. Too small and it'll run flat out and never cool the room. Too big and it'll short-cycle, waste power and wear out early. Here's how to work out the capacity you actually need.

Quick answer: Air conditioners are sized in kilowatts (kW) of cooling capacity. As a rough guide, allow around 0.125–0.15kW per square metre of floor space. That puts a 20–30m² bedroom at roughly 2.5–3.5kW, an open-plan living area of 40–60m² at 6–8kW, and a whole home at a ducted 10–16kW. Ceiling height, insulation, window size and sun orientation all shift these numbers, so a professional heat-load calculation is the accurate method.

The rough sizing guide

Use this as a starting point, not a final answer:

Why "per square metre" isn't the whole story

Two rooms of the same size can need very different capacities. The factors that change the maths:

Ceiling height. A room with 3m ceilings has far more air volume to cool than one with standard 2.4m ceilings.

Insulation. A well-insulated home holds temperature; a poorly insulated one leaks it. Older brick homes and rooms under an uninsulated roof need more grunt.

Windows and orientation. Large windows, and especially west-facing rooms that cop the afternoon sun, gain a lot of heat. In South West Sydney, a west-facing living room can feel like a greenhouse by 4pm in January and needs sizing up.

Occupancy and heat sources. More people, plus appliances like ovens and computers, add heat load.

Why oversizing is a real mistake

It's tempting to think "bigger is safer," but an oversized air conditioner is a genuine problem. It cools the room so fast that it switches off before it has properly dehumidified the air, then switches straight back on — a cycle called short-cycling. The result is a room that feels cold and clammy, higher power bills, and more wear on the compressor. Right-sized beats oversized every time.

The professional method: heat-load calculation

For a reliable answer, a proper heat-load calculation accounts for all of the above — floor area, ceiling height, insulation, glazing, orientation, occupancy and local climate. This is especially worth doing in Sydney's hot inland suburbs like Hoxton Park, West Hoxton and Prestons, where summer heat regularly tops 40°C and a guess can leave you short.

Any reputable installer performs a heat-load assessment as part of quoting. If someone quotes a unit size over the phone without asking about your room, treat that as a red flag.

Match the size to the system

Once you know the capacity, it feeds directly into your choice between ducted and split and your installation cost. Bigger capacity means a higher price, which is exactly why paying for the right size — no more, no less — is the smart move.

Not sure what your rooms need? Book a free assessment with BES. We'll measure up, run the numbers, and recommend the right capacity for your home across Hoxton Park and South West Sydney.