What size air conditioner do I need?

Work out what capacity air conditioner you need thanks to this advice:


Nice article. I’ve been mizzling over which a/c for the last 4 years. I am indecisive for a number of reasons… finances, location and orientation. All my windows are on the north or west, with a couple to the east and nothing to the south. Current a/c is a window rattling “skinny” TECO in a north window, which is 4.2kw and which does a decent job of cooling the loungeroom, and I have a Close Comfort portable in the bedroom which does a brilliant job of cooling me (but not the room, it wasnt designed that way). There’s only one place I could have a decent sized split system installed, and that is at the west… the external unit would be back to back and shaded by a large canvas blind, and “pergola” (inverted commas because its really just a covered walkway) which is roofed by that awful laserlite stuff. The heat under there is really unpleasant and thats what the ext unit would have to deal with.

So I am stuck, again, and when the window unit dies I will have to make a firm decision… the article has given me some pause for thought.


A timely topic.

Two other points the Choice team might like to comment on.

Inverter driven air conditioners are supposedly more efficient. In particular compared to the older style compressors which could only perform at one speed and load? Aside from inverters being preferred for efficiency do they also have less startup inrush on the house electrics? Modern inverters also appear to be more efficient over all with a greater COP where they are re replacing older units.

The cost of installing a new install split system air conditioner can be equal to the cost of the air conditioner. What is a fair price per aircon for a standard install with the outdoor and indoor units adjacent or close by? Include a reasonable cost for the electrics without any board upgrades other than an extra circuit, and up to 20m of cable.

Our history says between $600 and $800 for smaller units, with larger than 6-7kW being a little more due to the increased cable size, with one circuit per aircon. I’ve seen lower quotes from the retailers recommended installers, however there appear to be riders that could easily add extras on the day.

We set our cooling to 26C on hotter days, or 25C when it drops below the low 30’s.
For heating 18C works with one winter layer, or light jacket.

The differential we try to achieve is based on minimising the load on the system as our house despite well insulated roof spaces has minimal wall and floor insulation.


Inverters are more efficient, using less power to present the same outcome as the older generation units did but there does lie a trap for the unwary:

Inverters can boost their cooling & heating for short periods over the rated levels, but once this short interval of boost has expired the unit then goes below the rated level for sometime (sort of into economy mode). If temps haven’t reached levels required inverters then boost again repeating the cycle ad infinitum until either the temps are stabilised or the AC is turned off.

So if getting one don’t under estimate the rating you need nor buy something under what you need regardless of what the sales person may tell you. If you work out you need 3 kW of cooling don’t buy something a little less, buy something that meets your needs or a little more than your needs eg 3.5 kW. Also don’t buy something like 6 kW when you only need 4kW, it won’t be as efficient as getting something closer to your needs eg 4.2 or 4.5 kW and buying the much bigger unit will usually cost a whole lot more. You want something that cycles (drops and increases power to keep temps stable) smoothly rather than using a lot more power and cycling harder.


And its almost dead. It been grinding a bit under the strain of being in use day and night (not looking forward to the electric bill for this quarter). so the thing is, I still dont have enough dosh for the a/c unit and an install, so I decided to bite the bullet and buy a standard self evaporative portable. I didn’t want to spend a heap so I consulted with Mr Ebay and he sold me (via OzPlaza) a Devanti unit. And I must say its remarkably quiet, although its a pest having a hose at the window which has to be taken down when it rains (no protection there). Its much colder than the TECO which I appreciate, and about 10Db quieter, according to my meter. I haven’t bitten into the savings too much (it cost $479 and is currently on sale for even less) so should have enough for a split before next summer (although, a new stove is also required, asap… so maybe Summer of 2021). Overall, very happy with the Devanti, its 22000BTU whatever that is, and it seems very efficient. Won’t really know until we get more >45º days.

[edit] Subsequent to my last post I have realised (after consulting the manual) that the TECO is actually only 2.4Kw cooling. Which means I really dont need 5Kw. Did the measurements and it seems that 3.5-4 will be sufficient. The new portable is apparently 6.4 Kw. Not sure I quite believe that but it would explain the speed with which the room cools down :slight_smile:

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I’ll cross link my comment rather than repeat it.

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Hello everyone,

Im an air conditioning technician with 25 years + experience in the industry. This is a really common question we are asked by customers who have had multiple quotes by different installers, where one company quotes one size and the other installer quotes a different size. The really basic calculation to get the correct size is (for Brisbanes climate) room length x room width x 140 (watts per square meter). For example a 20 sq meter room will require approx. 2800 watts (or 2.8kw of cooling capacity). Depending on your location, you can increase or decrease the “wattage” per square meter to suit the local climate (hotter climates require approx 160 watts per square meter while cooler climates may only need 120-125 watts per square meter.

That may be a starting point but does not take into account a specific building’s roofing type, quality of insulation, and amount of and type of glass (single or double or triple glazed).

Perhaps the equation suggests the absolute minimum for further refinement?

Brisbane housing includes timber homes elevated on stumps with/out verandahs. The metal roofs are high pitched while some retain their exposed timber framing. Under floor insulation is zero. Roof insulation and ventilation vary. A more modern house slab on ground in brick veneer or masonry block will today have a high level of insulation. Some homes have 2.4m nominal ceiling heights and others, such as ours 3.2m or more.

How can it be the same assessment applies equally to all?

A second question is given how inverter RC Air Cons function, is the sizing less critical a factor providing the system is not under sized?

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