What Size Air Conditioner Do I Need? Perth Guide

The most common question I get on a quote isn’t about price — it’s “what size air conditioner do I need for this room?” And it matters, because getting the size wrong is the single most expensive mistake you can make with a split system. Too small and it runs flat out on a 40°C Perth afternoon and never quite gets there. Too big and it short-cycles, leaves the room clammy, and costs more to buy and run for no benefit.

This is a plain-English guide to sizing an air conditioner for a Perth home: a quick room-size-to-kW chart, how sizing actually works, the five things that change the answer, and why “just go bigger to be safe” is usually the wrong call.

Want a number for your actual room right now? Pop your room size into our air conditioner cost calculator for an instant supply-and-installed estimate, or book a free measure-up and we’ll size it properly.

The quick answer: air conditioner size by room

Here’s the chart most people are after. These are cooling capacities (kW) for a standard Perth home with average insulation and ceilings around 2.4m. Use it as a starting point, not gospel — the factors further down can move you up a size.

Room size	Typical use	Recommended capacity
Up to 20m²	Small bedroom, study, nursery	2.0–2.5 kW
20–30m²	Standard to large bedroom	2.5–3.5 kW
30–45m²	Lounge, small open-plan living	3.5–5.0 kW
45–60m²	Large living or living/kitchen combo	6.0–7.1 kW
60–80m²	Big open-plan living/dining/kitchen	8.0 kW+

If a single room is bigger than about 80m², or you’re trying to cool several rooms at once, a single wall-mounted split usually isn’t the right tool. For one big room or open area, a ceiling cassette — recessed into the ceiling with four-way airflow — spreads the air more evenly than a wall head; for several rooms, a multi-head system or ducted makes more sense. More on that below.

How air conditioner sizing actually works

Air conditioners are rated in kilowatts (kW) of cooling and heating capacity — not to be confused with the kW of electricity they use, which is much lower. A 2.5kW split might only draw around 0.7kW of power to deliver that 2.5kW of cooling. That’s the whole point of a reverse-cycle system: it moves heat rather than generating it, so you get several units of cooling for every unit of electricity.

The rule of thumb sparkies and aircon techs use is roughly 0.125 to 0.15 kW of cooling per square metre of floor area. Multiply your room’s length by width to get the area, then by 0.15, and you’ve got a sensible starting capacity. A 4m × 5m bedroom is 20m², so 20 × 0.15 = 3.0kW — a 2.5kW or 3.5kW unit depending on the other factors below.

That formula assumes an “average” room. Your room probably isn’t perfectly average, which is where the next part comes in.

The 5 things that change the size you need

Two rooms with the same floor area can need different units. Here’s what moves the number:

1. Ceiling height. The rule of thumb assumes ~2.4m ceilings. Raked ceilings, high voids or a converted attic add a lot of air volume to cool — bump up a size.
2. Insulation. A well-insulated, sealed modern home holds temperature and needs less. An older uninsulated brick-and-tile place, or a room under an un-insulated roof, leaks heat and needs more.
3. Windows and which way they face. This is the big one in Perth. Large windows, and especially west-facing glass that cops the hammering afternoon sun, dramatically increase the cooling load. A west-facing living room with big windows can need a full size more than the same room facing south.
4. What the room is used for. Kitchens (oven, fridge, cooktop), rooms full of people, and home offices with lots of gear all generate heat. An open-plan living-kitchen needs more grunt than the floor area alone suggests.
5. Our climate. Perth summers are hot, dry and relentless — long runs of 35–40°C days. We size toward the upper end of the range so the unit can actually keep up when it counts, rather than running at 100% and losing the battle.

Why bigger isn’t better

It’s tempting to “just go a size up to be safe.” It’s not as risky as it used to be — almost every split system sold today is an inverter unit, meaning the compressor varies its speed and can run gently at part load rather than slamming fully on and off. That lets a modern unit cope with mild oversizing far better than the old single-speed systems did, so you don’t need to agonise over a small step up.

The catch is that every inverter still has a minimum output it can’t drop below. Grossly oversize a unit and even its lowest setting overshoots the room — so it cools fast, clicks off, drifts up and restarts. That’s short-cycling, and it brings back the old problems:

• Clammy comfort. An aircon only pulls moisture from the air while it’s running. Lots of short bursts don’t run long enough to dehumidify, so the room can feel cold but damp.
• More wear. Frequent stop-starts are harder on the compressor than steady running.
• Money up front for nothing. A bigger unit costs more to buy and install — and if it’s too big to ever stretch its legs, you’ve paid for capacity you’ll never use.

Undersizing is the opposite problem — the unit runs flat out, never reaches temperature on the hottest days, and wears out early. The sweet spot is a unit matched to the room, which is exactly what proper sizing gets you.

One big room, or the whole house?

The chart above is for cooling a single room with a split system. For a larger space or multiple rooms, you’ve got options:

• A ceiling cassette — one unit recessed into the ceiling pushing air four ways, ideal for a single large room or open area where a wall head can’t move enough air evenly (or where you’d rather keep the wall space clear).
• Multiple single splits — one in each room, controlled independently. Often the most cost-effective for two or three rooms.
• A multi-head split — several indoor heads running off one outdoor unit, good where you don’t want a row of outdoor compressors.
• Ducted reverse cycle — one system, vents through the ceiling, zoned. The neatest whole-home answer, sized to the total load of the zones you run at once.

Which one wins depends on your home and how you live in it — happy to talk it through on a quote.

Don’t guess — get it measured

A chart will get you close, but the only way to size confidently is to measure the room and account for the windows, ceiling, insulation and orientation. Our cost calculator does the first pass in about thirty seconds, and a free on-site quote nails it down. We’d rather spend ten minutes getting the size right than have you live with a unit that’s wrong for the room for the next decade.

We lead with Panasonic because it’s a genuinely premium unit — on par with Daikin on quality — that we can price sharper through our supplier, and the range covers every size from a 2.0kW bedroom unit to an 8kW+ open-plan system.

Common questions

What size air conditioner do I need for a bedroom? Most standard Perth bedrooms (around 12–18m²) are well served by a 2.5kW split. A large master with an ensuite or big west-facing windows might want a 3.5kW.

What size for a living room or open-plan area? A typical lounge of 30–45m² suits a 3.5–5.0kW unit. A larger open-plan living/kitchen of 45–60m² usually needs 6.0–7.1kW. Above that, look at ducted or multi-head.

How many kW do I need per square metre? Roughly 0.125–0.15kW per m² for a standard room with 2.4m ceilings. Use the higher end for west-facing rooms, big windows, high ceilings or poor insulation.

Is it better to oversize or undersize? Neither — match it to the room. Oversizing causes short-cycling, poor humidity control and higher cost; undersizing means it never keeps up on the hottest days. Right-sized is what you want.

Does ceiling height really matter? Yes. The standard guide assumes ~2.4m ceilings. High or raked ceilings add a lot of air volume to cool, so you’ll typically need to step up a size.

The bottom line

Sizing an air conditioner comes down to your room’s floor area, adjusted for ceiling height, insulation, windows (especially west-facing), how the room’s used, and Perth’s tough summers. The chart above gets you in the ballpark; for a real number, run your room through the calculator or book a free measure-up and we’ll size it properly the first time — no guessing, no upsell.