What size air conditioner do you need? The honest kW guide for Australia

Air conditioners are sold by their cooling power, in kilowatts — the kW number on the box. And here’s the thing about that number: the same insulated 30 m² room needs about 1.8 kW in Hobart, 2.3 kW in Sydney and 2.6 kW in Brisbane — figures CHOICE produced by running the industry’s own load calculator. Same room, same insulation, and the city changes the machine by nearly half as much again. Sunny Perth, in CHOICE’s words, needs “a much larger unit” than Tasmania again.

Now the part that costs people money. CHOICE’s air conditioning expert, on that same 30 m² room: “Most commercial calculators or guides will recommend 3.5 to 5 kilowatts for a room of this size, which is often going to be a lot more capacity than you really need.” The generic size charts — the ones on retailer websites and brochures — are recommending close to double the calculated answer.

A good local installer knows your climate; that’s their patch. It’s the one-size-fits- Australia chart that’s guessing — and, as CHOICE notes, some installers quote off a simplistic analysis too. Either way, the guess always errs big, and big is not free.

Why bigger isn’t “safe” — it’s just more expensive twice

Oversizing is the industry’s comfortable default. Nobody ever rings to complain that their air conditioner is too powerful, and the bigger unit costs more — so nobody on the selling side talks you down. Here’s what you actually get for the extra money:

• A bigger bill up front. Capacity is what you’re paying for: a size class is hundreds of dollars on a split system and thousands on ducted — we’ve priced both.
• A worse room afterwards. An oversized unit gets the room to temperature fast, shuts off, waits, starts again — short bursts all day instead of a steady cruise. CHOICE’s testers list what those short cycles do: the room overshoots hot and cold, the unit doesn’t dehumidify the air enough so the room feels muggy rather than crisp, power use and running costs go up, and the system wears out faster. You paid extra for that.

Too small has the opposite problem. An undersized unit has to run flat out much of the time — CHOICE again: it dries the air too much, wears faster, and uses more power — and on the couple of scorching afternoons a year you most wanted it, it can run all day and still not quite get there.

So the goal isn’t big and it isn’t small — it’s right, with a little headroom. CHOICE’s rule: if the room needs 6 kW, buy 6–6.5 kW. A touch bigger is fine. A class bigger is a mistake you pay for twice.

The sanity-check table (for checking quotes, not designing systems)

The trade’s rough rule works in watts of cooling per square metre of floor — how much machine per square metre of the room. It moves a lot with climate:

Where you live	Rule of thumb (cooling)
Tropical north (Darwin, Cairns)	~160–180 watts per m²
Subtropical (Brisbane, coastal QLD)	~140–160
Temperate (Sydney, Adelaide, Perth)	~125–150
Melbourne and similar	~100–130
Cool zones (Canberra, Hobart, alpine)	~80–110

Worked through: a 20 m² bedroom in Sydney sanity-checks at roughly 2.5–3 kW (20 × 125–150, then divide by 1,000 to get kilowatts). A 40 m² living area, about 5–6 kW. If a quote says 8 kW for that living room, that’s not proof anyone’s wrong — but it’s absolutely a question to ask.

Three things push the number up, and quotes often skip them:

• No ceiling insulation adds about a quarter — CHOICE’s worked example jumps 26% for the same Sydney room. Good insulation trims 10–15% the other way. If your ceiling has none, batts cost far less than the extra machine you’d otherwise buy: fix that first.
• High ceilings: add ~10% at 2.7 m, 20–30% at 3 m or more — you’re cooling the air above your head too.
• Big west-facing windows: add 10–20%. Or shade them, and don’t.

The proper answer is free, and it’s the industry’s own tool

A rule of thumb checks a quote. Only a heat-load calculation — a room-by-room tally of the heat actually coming in through your walls, ceiling, windows and people — should size the system. The industry body AIRAH puts its calculator online for anyone, free, no sign-up: FairAir. You’ll need a tape measure and about ten to fifteen minutes: room dimensions, what the walls and ceiling are made of and whether they’re insulated, and your windows — how big, which way they face, how they’re shaded.

Worth knowing: Daikin and Mitsubishi Electric publish no size-by-floor-area charts at all. Both say to get a heat-load calculation. When the people who make the machines won’t guess from floor area, an installer quoting off “it’s a four-bed house” shouldn’t either. Run FairAir before you get quotes and you’ll know within one conversation which installers measured your house and which ones guessed.

Sizing ducted: you never run every room at once

Ducted systems have their own trap. Add up what every room needs and buy that total, and you’ve paid for something that never happens — every room, full blast, at the same time. Real houses don’t work like that: bedrooms get cooled at night, living areas by day. That’s what the zones are for. So installers size zoned systems to a “diversity factor” — around 60–80% of the all-rooms total in published installer practice. A quote for a fully zoned system sized at 100% of the room-by-room sum deserves one polite question: what did you assume about diversity? At current ducted prices, the answer is worth thousands.

In a cold-climate home? Size for winter, not summer

In Melbourne, Canberra, Hobart and the cool zones, it’s the heating that sets the size — reverse-cycle is the cheapest way to heat a room, and winter works the machine harder than summer there. Use FairAir’s heating calculator instead (the Victorian government’s ducted-heating guidance works on about 110–140 watts per m²), and check the unit’s rated heating capacity at low outdoor temperatures — the spec where cheap units quietly fall away.

The verdict

• The city sets the size. Hobart to Brisbane is nearly half as much again for the same room — and generic size charts recommend close to double what a calculation says. Distrust any chart that didn’t ask where you live.
• Right-sized, small headroom. Need 6 kW? Buy 6–6.5. A class bigger costs more up front, then short-cycles into a muggier room with a higher running bill.
• Fix insulation before kilowatts. An uninsulated ceiling adds about a quarter to the machine you need — and batts are cheaper than kilowatts.
• Ten to fifteen minutes on FairAir before you get quotes tells you which installers measured and which guessed.
• Ducted: ask about the diversity factor. Paying for every room at once is the polite way installers oversize.

City sizing examples, the 3.5–5 kW comparison, the oversizing and undersizing effects and the sweet-spot rule: CHOICE, “What size air conditioner do I need?” (checked against the live article, August 2026). Climate rules of thumb: published Australian installer guidance (2023–2026) — the ranges vary between sources, which is rather the point. Heating anchor: Victorian government (VEU) ducted-heating guidance. FairAir is AIRAH’s public calculator, checked live August 2026. Rules of thumb sanity-check a quote; only a heat-load calculation designs a system — and your house, not this article, is the final authority.