Buying guides
What size air conditioner do I need? A kW guide for Brisbane homes
A simple guide to sizing a split system: kW by room size, how ceiling height and sun change the answer, and what goes wrong if you pick too big or too small.
By The All In One teamPublished 4 min read
For a typical living room of about 30 m² with a standard 2.4 m ceiling, you need roughly 4.5 to 5 kW of cooling. As a rule of thumb, allow about 150 watts of cooling per square metre of floor area, and more if the ceilings are high or the room cops a lot of sun.
Quick guide: air conditioner size by room size
A rough guide, based on floor area:
| Room size | Suggested capacity |
|---|---|
| Up to 20 m² | 2 to 2.5 kW |
| 20 to 40 m² | 2.5 to 5 kW |
| 40 to 60 m² | 4 to 6 kW |
| 60 to 80 m² | 5 to 7 kW |
| 80 m² and over | 6 to 9 kW |
It’s a ballpark only. It assumes an average room, and a Brisbane lounge with west-facing glass isn’t one.
How to work out your own number
A simple formula gets you closer. Multiply the floor area of the room (in m²) by a figure that depends on your ceiling height, then divide by 1,000 to get kilowatts.
| Ceiling height | Watts per m² |
|---|---|
| 2.4 m | 150 |
| 2.7 m | 160 |
| 3.0 m | 175 |
For example, a 30 m² living room with a 2.4 m ceiling comes to 30 × 150 = 4,500 watts, or 4.5 kW. A 5 kW unit is the closest standard size.
If you’re cooling an open-plan living and kitchen area, measure the whole space the air can flow into, not just the part around the couch.
What makes you need a bigger unit
The formula assumes an ordinary room. In Brisbane, these push the number up:
- West-facing windows, because afternoon sun puts a lot of heat into a room.
- Poor insulation. A well-insulated home needs less cooling capacity than a poorly insulated one, and older houses without ceiling insulation lose their cool air quickly.
- Lots of glass, including large windows and skylights.
- High or raked ceilings, which mean more air to cool.
- A kitchen in the same space, since cooking adds heat.
- A room that regularly fits a crowd. It needs more capacity than one used by two people.
Heat also reduces what an air conditioner can deliver. Capacity falls once the outdoor temperature goes above 35°C, so on the hottest days a unit works harder to do less. That’s one reason not to size right on the minimum.
What happens if the unit is too big
Bigger isn’t automatically safer. An oversized unit runs in frequent short cycles. That can leave the room too hot or too cold, remove less humidity, use more power and wear out parts sooner.
In Brisbane the humidity part matters. A system that cools the air quickly and then switches off hasn’t run long enough to dry it, so the room can end up cold and clammy.
Installers and online calculators often overestimate the capacity needed. A better rule is to choose a model with equal or slightly greater capacity than the room needs. A room that needs 6 kW, for example, should suit a unit rated at about 6 to 6.5 kW.
What happens if the unit is too small
An undersized unit runs flat out on hot days and still struggles to reach the temperature you set. That means higher running costs and more wear on the compressor. If you’re choosing a bedroom unit at the bottom of its range, think about whether the room gets afternoon sun.
Should I get a bigger unit just in case?
Only a little. Going up one size is reasonable for a west-facing room, high ceilings or poor insulation. Going up two or three sizes isn’t. If you’re unsure, a proper assessment beats guessing. A good installer looks at the room size, ceiling height, windows, which way the room faces and how well the house is insulated, then recommends a size.
Check the electrical side
A larger unit draws more power, and your circuit and switchboard have to be able to supply it. When we install an air conditioner we check this as part of the job, because we’re licensed electricians as well as refrigerant handlers. If the existing wiring isn’t suitable, we tell you before you’ve committed to a unit.
One room, several rooms or the whole house?
Everything above is for a single split system serving one room or open-plan area. To cool several rooms you can fit a separate split system in each, use a multi-head split system with one outdoor unit, or install ducted air conditioning. The sizing rules change for those, because the rooms aren’t all cooled at once and the capacity is shared. We’ll go through the options when we assess your home.
Talk to us about sizing
Tell us which rooms you want to cool and we’ll size the system for your home, not for an average one. We install Daikin, Mitsubishi Electric, Samsung, Panasonic and Kelvinator units across Brisbane’s south and south-west. If you’d like an idea of the price first, read our split system installation cost guide.
See our air conditioning installation service, or call us on 0412 229 069 for a quote.
Written by The All In One team at All In One Electrical & Air Conditioning, licensed electrical contractors (licence 90313) and refrigerant handlers (ARC L142566) in Brisbane. This guide is general information; for advice on your property, ask us for a quote.
