As a rough guide to what size air conditioner you need, a bedroom or study up to 20m2 needs about 2kW to 3.5kW, a living area of 20m2 to 40m2 about 3.5kW to 6kW, and a large open-plan space of 40m2 to 60m2 about 6kW to 9kW. Your home can move those numbers a long way, which is why the table below is a starting point, not an answer.
Getting the capacity right matters more than getting the brand right. It is the number that decides whether the system works well, how much it costs to run, and how long it lasts.
Air conditioner capacity is measured in kilowatts (kW) in Australia. Note that this is the heating or cooling the unit delivers, not the electricity it uses, which is much less.
What size do I need for each room?
As a starting point for a typical, reasonably insulated Australian home:
| Room or space | Approximate capacity | Common system types |
|---|---|---|
| Up to 20m2 (bedroom, study) | 2kW to 3.5kW | Split system, window unit |
| 20m2 to 40m2 (small living area) | 3.5kW to 6kW | Split system |
| 40m2 to 60m2 (large or open-plan living) | 6kW to 9kW | Split system |
| Whole house | Depends on the home | Multi-split or ducted |
Treat this as a starting point, not an answer These figures assume average ceiling height, reasonable insulation and moderate window area. Change any of those and the right answer moves, sometimes by a lot.
What changes the size I need?
Six things move the answer more than floor area does:
1. Ceiling height You are conditioning a volume, not a floor. A room with 3.6 metre ceilings needs considerably more capacity than the same floor area at 2.4 metres.
2. Insulation An uninsulated ceiling can be the single biggest heat load in the house. Insulating first often means a smaller, cheaper system.
3. Windows: size, orientation and shading West-facing glass in an Australian summer is brutal. Large unshaded windows can double the cooling load of a room. Shading them is often cheaper than upsizing the system.
4. Roof type and colour A dark tiled or metal roof over an uninsulated ceiling puts far more heat into the room than a light, well-insulated one.
5. Your climate The same room in Darwin and Hobart needs different systems, and in a cold climate you must size for heating as well as cooling. These are two different numbers on the same unit.
6. What is in the room People, cooking, computers and large televisions all generate heat. A kitchen and living area combined needs more than a lounge of the same size.
Is a bigger air conditioner safer?
People often assume a bigger unit is the cautious choice. It is not.
An oversized air conditioner reaches the set temperature quickly, then shuts down. It does this over and over, which:
- Costs more up front, for capacity you never use
- Costs more to run, because repeated starts are the least efficient part of the cycle
- Dehumidifies poorly, so the room hits the set temperature while still feeling clammy. This is why an oversized system can leave a room feeling cold and damp rather than comfortable
- Wears the compressor through frequent cycling
An undersized system has the opposite problem: it runs flat out, never quite gets there, and shortens its own life.
The right way to size a system
Get a heat load assessment This is a calculation an installer does on site, accounting for all six factors above. It is the reason we tell people to insist on a site visit before accepting a quote. An installer who quotes from a floor plan or over the phone is guessing.
If you want a rough figure before you start getting quotes, AIRAH (the Australian Institute of Refrigeration, Air Conditioning and Heating) provides a free sizing calculator on its consumer site at fairair.com.au. It is a better starting point than a rule of thumb, though it still does not replace an on-site assessment.
Sizing a ducted system
Ducted is not simply the sum of the rooms. The system is sized for the load of the zones you will realistically run at once, not the whole house at maximum.
Ask your installer:
- What total capacity are you quoting, heating and cooling?
- Which zones did you assume would run together?
- What happens on the hottest day if I open every zone?
- What is the minimum number of zones that must stay open?
Getting this wrong is expensive, because a ducted system is not something you can easily swap out.
Questions to ask about capacity
- What capacity are you quoting, in kW, for cooling and for heating?
- Did you do a heat load assessment, and can I see the figures?
- What did you assume about my insulation and my windows?
- If I improve the ceiling insulation or shade the west windows, could I use a smaller system?
- For a cold climate: what is the rated heating capacity at low outdoor temperatures, not just at 7 degrees?
Compare quotes on the same basis
When your quotes come back, check they are for comparable capacity. A cheaper quote for a smaller system is not a better deal, it is a different product. If one installer has quoted noticeably smaller or larger than the others, ask them to explain their heat load figures.