What Size Air Conditioner Do You Actually Need?

Air conditioner sizing is the single most important decision in the whole purchase, and it is the one most often gotten wrong. An undersized unit runs flat out on a forty-degree afternoon and never quite gets there. An oversized one costs more to buy, cycles on and off constantly, wears out sooner and leaves rooms feeling clammy because it never runs long enough to pull moisture out of the air. The sweet spot is a unit matched to the actual heat load of your space — not a guess based on floor area alone.

The rough rule, and why it is only a start

You will often see a shorthand of roughly 130 to 150 watts of cooling capacity per square metre of room. By that measure a 20 square metre bedroom needs around 2.5 to 3 kW, and a 45 square metre open-plan living area lands somewhere near 6 to 8 kW. That gets you in the neighbourhood, but two identical-sized rooms can have wildly different real loads, which is why professional sizing looks well beyond the tape measure.

What actually drives the load

Ceiling height matters — a room with three-metre ceilings holds a third more air than the same footprint at 2.4 metres. Insulation matters even more: a well-insulated ceiling can halve the heat pouring in on a summer afternoon. Then come windows, which are the big variable in most Australian homes. Large west-facing glass cops the full brunt of the afternoon sun, and uncovered windows can add a kilowatt or more to the required capacity. Finally, think about occupancy and appliances — a kitchen and living zone with six people, an oven and a television carries a genuinely higher load than a quiet bedroom of the same size.

Heating changes the maths in Victoria

Because most systems sold here are reverse-cycle, sizing has to work in winter too. Melbourne's cold mornings can demand as much heating capacity as summer demands cooling, and heating output drops slightly on very cold days just when you need it most. A unit sized only for summer can leave you short in July, so we always check both figures on the specification sheet, not just the headline cooling number.

Get it measured, not guessed

The difference between a 5 kW and a 7 kW unit might be a few hundred dollars at purchase, but the wrong choice costs you every single day in comfort or running costs for the next fifteen years. A proper assessment takes under an hour: we measure the rooms, note the glazing and orientation, check the insulation and look at how you actually live in the space. That is the number worth buying — and the assessment costs you nothing.