Air conditioning
What size air conditioner do I need?
There is a rule of thumb for sizing a room and it gets you close in about thirty seconds. It will not account for the west-facing glass in your lounge room, your ceiling height or a room that opens onto the kitchen. Start with the figure below, then adjust for the six things that move it.
6 min read
The starting figure
Why getting this right matters more than getting it big
The instinct is to buy a bit more capacity than you need and run it gently. That works for most purchases but it is the wrong instinct here.
An air conditioner does two jobs at once. It drops the temperature and it pulls moisture out of the air. The second job only happens while the system runs, because moisture needs time to condense on a cold coil. A unit too powerful for the room hits your setpoint in a few minutes and switches off before it has touched the humidity. The wall says 23 degrees and the room still feels close and sticky.
Every start brings the heaviest electrical draw and the most wear, so a unit that starts and stops all afternoon works harder than one that runs steadily. Undersizing is the more obvious failure: the system runs flat out through a February afternoon and never quite catches up.
What the rule of thumb gives by room size
The 0.125 to 0.15 kW per square metre figure worked through for a standard room with 2.4 metre ceilings, reasonable insulation and average glass. Use it to know what you are shopping for, then adjust for the factors underneath.
| Floor area | Typical room | Rule of thumb gives |
|---|---|---|
| 10 m² | Small bedroom or study | 1.25 to 1.5 kW |
| 15 m² | Typical bedroom | 1.9 to 2.25 kW |
| 20 m² | Main bedroom or home office | 2.5 to 3.0 kW |
| 30 m² | Living room | 3.75 to 4.5 kW |
| 40 m² | Lounge and dining together | 5.0 to 6.0 kW |
| 60 m² | Open-plan kitchen, living and dining | 7.5 to 9.0 kW |
| 70 m² and up | Large open plan or a whole floor of a house | Ducted or multi-head territory |
Capacity figures only. Nothing here says anything about what a system costs to buy or install, which depends on the job and not the kilowatts.
The six things that move the number
The rule of thumb assumes an average room and very few rooms are average. Each of these can shift the requirement by a real margin. Two or three of them together can take a 30 square metre room from 4 kW to 6.
- 1
Ceiling height
The rule uses floor area, but you are cooling a volume of air. Standard is 2.4 metres. At 3 metres a room holds a quarter more air. A raked ceiling in a renovated Federation house can add half as much again. Scale the figure by roughly the same proportion.
- 2
Windows and which way they face
Glass is where the heat gets in. West-facing glass takes the low afternoon sun through the hottest part of the day and in most Australian houses it is the biggest single load. East-facing glass gets the morning version. A decent eave shades the high summer sun off north-facing glass. South-facing glass adds the least. A wall of west-facing windows with no eave means you size up.
- 3
Insulation
A well-insulated ceiling and wall cavity can cut the requirement by a fifth or more. Plenty of older Illawarra houses are light on ceiling insulation. A fibro or weatherboard place with a dark roof and an empty cavity is a very different load to a modern brick veneer with batts.
- 4
Whether the room is really one room
An open-plan kitchen, living and dining space is one volume of air even if you think of it as three areas. The kitchen adds heat every evening. Doorways without doors, stairwells and double-height voids all send the cooled air somewhere you did not count.
- 5
People and appliances
A person at rest gives off around 100 watts of heat. Six people in a living room add most of a kilowatt before you switch anything on. A room that regularly holds a crowd or runs a big television and a few computers carries a load the floor area does not show.
- 6
Where you are
The Illawarra coast has a milder summer peak than western Sydney but far more humidity, so the moisture side of the job matters more here. Inland and western suburbs see higher peak temperatures. The same house can need a different size in each.
Ducted is a different calculation entirely
Everything above sizes a split system for one space. Adding up every room in the house to size a ducted system gives you a unit far bigger than you need.
You almost never run every zone flat out at the same moment. Bedrooms want cooling overnight and living areas want it through the afternoon and evening. A good design sizes for the realistic worst case. Get that wrong and you end up with a system that cannot hold the house on a bad day or one that short-cycles through every mild week.
Duct size and layout matter as much as the unit, which is why we measure the house and not just the floor plan.
A word on the numbers in the brochure
What this costs
Sizing and price are separate conversations. Two houses needing the same 6 kW unit can be very different jobs. One has an easy external wall, a nearby switchboard with a spare circuit and a straightforward run. The other needs a longer pipe run, a switchboard upgrade, scaffolding or work around a heritage facade.
We do not put a figure on a website for that reason. Any number we published would be wrong for most readers and would turn into something our installer has to argue against on your driveway. We come and look, tell you what the job involves and give you a fixed price before anything starts. Our air conditioning installation page explains how that runs. The NSW Government rebate page covers the discount an eligible upgrade attracts.
Common questions
- What size air conditioner do I need for a room?
- Start with about 125 to 150 watts of cooling for each square metre of floor, which assumes 2.4 metre ceilings and average insulation. A 30 square metre lounge room comes out at roughly 3.75 to 4.5 kW. Treat it as the middle of a range, because ceiling height, window area and orientation all move it.
- How many kW do I need for a bedroom?
- A bedroom of 12 to 15 square metres works out at roughly 1.5 to 2.25 kW on the rule of thumb, which puts it among the smallest split systems on the market. Bedrooms usually sit at the low end because they have less glass and you cool them at night. A west-facing bedroom that bakes all afternoon wants the top of the range.
- Is it better to get a bigger air conditioner than I need?
- No. An oversized system hits the setpoint quickly and shuts down before it has pulled the humidity out, so the room ends up cold and clammy. The constant stopping and starting also adds wear. A correctly sized system is more comfortable to live with.
- Do I need a load calculation or is the rule of thumb enough?
- The rule of thumb tells you whether you are shopping for a 2.5 kW or a 7 kW unit. It will not choose between two models a kilowatt apart. For a ducted system the whole-house load and how you use the zones both matter, so anyone quoting ducted without measuring the house is guessing.