What size ducted air conditioner does your house need?
Published 6 min read
ActronAir's rule of thumb puts a house with 100 m² of daytime living area and 2.4 m ceilings at 15 kW. Here is that formula worked for five house sizes, what it leaves out, and the ducted sizes we actually install either side of each answer.
A first number from your floor area
ActronAir's formula on an example plan: add up the rooms you use in the day, multiply by the figure for your ceiling height, and move the decimal point three places. The rooms and their areas are made up for the example; the formula and the 100 m² answer are ActronAir's.
Ducted air conditioning is sized in kilowatts (kW). As ActronAir puts it, that is how the output of an air conditioner is measured in Australia: the cooling or heating it delivers. It is not the power it draws. A Daikin Inverter Ducted 14 kW draws 4.3 kW at its rated cooling output, by its GEMS registration.
ActronAir, one of the brands we install, publishes a quick formula for a first estimate. Measure each room in your daytime living area, multiply its length by its width, and add the rooms up. That gives you the area in square metres.
Then multiply the area by a figure for your ceiling height:
Ceiling height
Multiply the area by
2.4 m
150 watts
2.7 m
160 watts
3 m
175 watts
The answer is in watts. Move the decimal point three places to the left and you have kilowatts. ActronAir's own example is a house with 100 m² of daytime living area and 2.4 m ceilings: 15,000 watts, so 15 kW.
The formula, worked for five house sizes
The same formula as three lines, one for each ceiling height ActronAir gives, with a dot at each of the five areas in the table. The dashed line is the largest ducted unit we install. Only 150 m² under 3 m ceilings goes past it. Hover a dot for its figure.
Here is the same formula for five daytime living areas. They are examples to show how the numbers move, not a size for your house.
Daytime living area
2.4 m ceilings
2.7 m ceilings
3 m ceilings
50 m²
7.5 kW
8 kW
8.75 kW
80 m²
12 kW
12.8 kW
14 kW
100 m²
15 kW
16 kW
17.5 kW
120 m²
18 kW
19.2 kW
21 kW
150 m²
22.5 kW
24 kW
26.25 kW
Ceiling height matters. The same 100 m² comes to 15 kW under 2.4 m ceilings and 17.5 kW under 3 m ceilings.
The last cell is bigger than any single ducted unit we install, because our largest is 25 kW. A house that size is a design question for the quote, not a number to pick from a list.
Not sure what size you need?Answer five quick questions and get a kW recommendation with matching systems.Size it for me
What the formula leaves out
The nine things ActronAir's guide lists as left out of the formula, numbered in the same order as the list below, placed on one house.
ActronAir is clear that the formula is quick and easy, but also very rough, and that it does not consider many of the important factors. Its guide lists them:
how well your house is insulated
the climate where you live
the type and amount of lighting
the type and amount of ducting
how many windows you have, and how efficient they are
heat moving through the walls
how many skylights you have
how many people live in the house
the type of foundation the house sits on
Its advice is that the most accurate way to size a system is to have a professional calculate it for your home.
Hot days take capacity away
ActronAir's own example: a 14 kW system running in temperatures over 35°C might be reduced to 10 or 12 kW. The guide gives the range, not a single figure, so the lighter part of the bar is the uncertain part.
There is one more catch in ActronAir's guide. Above 35°C, an air conditioner loses efficiency and cooling capacity. Its example: a 14 kW system running in temperatures over 35°C might be down to 10 or 12 kW.
So a system sized right on the edge has the least to spare on the hottest days. The ducted cost factors guide explains why the same floor plan needs more capacity in Western Sydney than on the coast, and why capacity follows the heat load of the house, not square metres alone.
The sizes we actually install
Every ducted model we install, one dot each, counted live from our catalogue and stacked in half-kW steps. The colour is the phase on its GEMS registration. The shaded band is between 14 and 16 kW, where a 15 kW estimate lands. Hover a dot for the model.
A formula gives you a figure like 15 kW. Ducted units come in steps, so the real choice is the step either side of it. Here is what we install across our 23 ducted ranges, 130 models in all:
Size
Models
Single phase
Three phase
3.6 to 8.8 kW
42
38
0
9.5 to 10.5 kW
18
13
1
12 to 13.6 kW
21
19
1
14 kW
13
11
2
15 to 16 kW
10
8
0
17 to 18 kW
8
3
3
19 to 21 kW
11
0
6
23.5 to 25 kW
7
0
6
The phase of each model comes from its GEMS registration. 19 models have no GEMS record in our data, so they count in the Models column only.
Take ActronAir's 15 kW example. Most ranges step from 14 kW straight to 16 kW. Only four models sit in between:
The table also shows where three phase power comes in. Up to 16 kW, 89 of the 93 models with a record run on single phase. At 17 and 18 kW it is three each way, and from 19 kW every model with a record is three phase. The single or three phase guide covers what an upgrade involves.
The size in the name and the tested size
Every ducted model we install that has a GEMS record: the size in its name across, the cooling capacity it is registered at up. The shaded band is half a kW either way. The four amber dots are the ones outside it. Hover a dot for the model.
One more check when you compare sizes. The size in a model's name and the cooling capacity registered under GEMS are not always the same number. Of the 111 ducted models we install that have a GEMS record, all but four are within half a kW:
If your estimate lands close to a step, it is worth comparing the registered figure as well as the name.
How we size it for your quote
A formula gives you a range to talk about. It is not a size to buy. The right size comes from your house: the rooms, the ceilings, the insulation, the windows and the roof space.
Send us a rough floor plan with the rooms you want cooled, plus photos of the roof space and the switchboard. That is enough for us to size the system and quote it. Start a quote, or try the calculator for a first kW range.
ActronAir, What size air conditioner do I need?: updated 24 July 2025. The formula, the 150, 160 and 175 watt figures for 2.4, 2.7 and 3 m ceilings, the 100 m² example, the factors it leaves out, the 14 kW example above 35°C, and the advice to have a professional calculate it.
