I've installed a fair few air conditioners over the years — enough to know that the most common mistake people make is either going too small because they're trying to save money upfront, or going too large because they assume bigger is always better. Neither works well. Aircon sizing is one of those things where getting it right actually matters.
In Australia, and particularly in Queensland, the stakes are higher than in most countries. We're not talking about mild summer comfort — we're talking about 40-degree days, high humidity that makes the heat feel worse, and bedrooms that stay warm well into the night. An undersized unit in a Brisbane summer is genuinely miserable.
Understanding kW ratings
Australian air conditioners are rated in kilowatts (kW) of cooling capacity. This is the amount of heat the unit can remove from a room per hour. A 2.5 kW unit removes 2.5 kilowatts of heat per hour — enough for a small bedroom in a mild climate. A 7.1 kW unit is suitable for a large open-plan living area in a hot climate.
The kW rating on the box refers to cooling capacity. The unit also has a power consumption figure (what it draws from the wall) — these are different numbers. A 5.0 kW cooling unit might consume 1.5 kW of electricity to deliver that 5.0 kW of cooling. That ratio is the efficiency rating (star rating).
The basic sizing guide
As a starting point, Australian installers use roughly 125–150 W of cooling capacity per square metre of floor area for standard rooms. That gives you a baseline kW figure before adjusting for your specific conditions.
| Room size | Suggested capacity | Typical application |
|---|---|---|
| Up to 20 m² | 2.5 kW | Small bedroom, study |
| 20–30 m² | 3.5 kW | Standard bedroom, small lounge |
| 30–45 m² | 5.0–6.0 kW | Large bedroom, medium lounge |
| 45–65 m² | 7.1 kW | Large lounge, open plan |
| 65–80 m² | 8.0–9.0 kW | Large open plan, combined living/dining/kitchen |
These are starting figures for a standard Australian home with average insulation and ceiling height. Your actual requirement may be higher or lower depending on the factors below.
Free aircon sizing calculator
Enter your room dimensions and conditions to get a recommended kW range — built for Australian climate zones.
Use the aircon sizing calculator →Factors that push the number up
The basic table above assumes moderate conditions. Add capacity for any of these:
- Hot climate (QLD, NT, northern WA): Add 20–25% to the base figure. Brisbane summers are significantly hotter than Melbourne, and the calculation needs to reflect that.
- North or west facing rooms: These rooms receive the most direct afternoon sun and heat up substantially more than south-facing rooms. Add 10–15%.
- Poor insulation or older homes: Homes built before the 1990s often have minimal ceiling insulation and single-skin brick or weatherboard walls. Add 15–20%.
- High ceilings (above 2.7 m): More air volume to cool. Add roughly 10% per additional 300 mm of ceiling height above standard.
- Large glass areas (floor-to-ceiling windows, glass doors): Glass is a poor insulator and a major heat gain source. Add 10–15% for rooms with significant glazing.
- Open plan spaces that connect to kitchen: Cooking generates heat. Add 10% if the kitchen is part of the cooled space.
- Multiple occupants in the room regularly: Each person generates approximately 100 W of body heat. For a room regularly used by 4+ people, add 400 W (0.4 kW) to your capacity requirement.
Factors that allow you to go smaller
- Cooler climates (southern Victoria, Tasmania, ACT): Reduce base figure by 10–15%.
- Excellent insulation: Well-insulated homes with double-glazing can use the lower end of the capacity range.
- South-facing rooms with minimal sun: These rooms are naturally cooler and need less capacity.
- Rooms used only occasionally: A guest bedroom used a few times a year can run a smaller unit — it just needs longer to reach temperature.
The oversizing problem — why bigger isn't always better
This is the counterintuitive part. An oversized aircon isn't just wasteful — it actually performs worse in important ways:
- Short cycling: An oversized unit cools the room to the set temperature very quickly, then turns off — then the room warms up and it turns on again. This on-off cycling is inefficient, wears the compressor faster, and creates temperature swings.
- Poor dehumidification: Aircons remove humidity as a byproduct of cooling. An oversized unit that runs in short bursts doesn't run long enough to remove adequate moisture. In Queensland's humid climate this matters — a room can feel cool but still sticky and uncomfortable.
- Higher energy bills: Starting the compressor uses more energy than running continuously. Short cycling drives up electricity costs.
The right size unit runs for longer, dehumidifies properly, and maintains a consistent temperature more efficiently. In Queensland's climate especially, getting this right makes a meaningful difference to both comfort and running costs.
Split system vs ducted — when each makes sense
Split systems (wall-mounted indoor unit + outdoor compressor) are the right choice for most Australian homes cooling individual rooms or zones. They're cost-effective, energy-efficient, easy to install in existing homes, and can be sized precisely for each room. A bedroom split system and a separate living area split system gives you independent temperature control and means you're only cooling what you're using.
Ducted systems make sense for whole-home cooling in new builds or major renovations where ductwork can be installed in the ceiling. The upfront cost is significantly higher ($8,000–$20,000+) but the convenience of whole-home cooling from a single controller is appealing. Modern ducted systems with zoning allow you to cool only the active areas, which improves efficiency considerably.
For most Australian homeowners retrofitting an existing home: split systems room by room is the more practical and cost-effective approach.
Star ratings and running costs
Australian aircons are rated under the Zoned Energy Rating Label (ZERL) with separate star ratings for heating and cooling in different climate zones. For Queensland (hot climate zone), focus on the cooling star rating in the hot zone column — this is the figure that reflects your actual usage conditions.
As a rough guide, each additional star reduces running costs by approximately 10%. A 5-star unit will cost significantly less to run over 10 years than a 2-star unit of the same capacity — often enough to offset the higher purchase price within 3–5 years depending on usage.