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Choosing the right sized air conditioner is a big decision that can turn your house into a refuge from the sweltering Australian summer. However, choosing the wrong size can result in huge power bills for an air con unit that can’t efficiently cool your house.

There are many things you should consider before choosing the size of your air conditioner. Including room or house size, positioning of the unit, cooling efficiency, and your surroundings.

Room Size

One easy way to get an idea of how powerful your air conditioning unit should be is to measure the room or area of the house that needs to be cooled. For a single room, simply measure the length and the width of the room and multiply them to get the total area of the room.

In average conditions with an assumed standard ceiling height of 2.4m it takes 0.15 kW to cool 1 square metre. this will give you a rough idea of the size of air conditioner you will need.


How To Calculate What Size Air Conditioner You Need

Step 1: Measure the width of the room the air conditioner will be in. (In metres).

Step 2: Measure the length of the room the air conditioner will be in. (in metres).

Step 3: Multiply the length and the width of the room to find the area of the room.

Step 4: Multiply the area of the room by 0.15kW to find the ideal sized air conditioner for the room.


Room Length x Room Width = Room Area

Room Area x 0.15kW = Ideal Size Air Conditioner (in kW)


Note: All measurements should be in metres. Only applicable for a standard ceiling height of 2.4m.

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For example, if you have a room that is 8 metres long and 5 metres wide you would have an area to cool of 40 metres squared. If you multiply 40 by 0.15 you will find that you need an air conditioner with a cooling capacity of 6kW.


Step 1: Room width is 5 metres.

Step 2: Room length is 8 metres.

Step 3: 5 x 8 = 40. Room Area is 40 metres squared.

Step 4: 40 x 0.15 = 6. Ideal air conditioner size is 6kW.


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However, if your house doesn’t have standard ceiling heights or standard insulation and window placement, you will need to consider the additional factors listed below.

Air Conditioner Positioning

The positioning of your air conditioner plays a big role in the energy and cooling efficiency of the unit. This is because of two factors; the in-built thermostat in any air conditioning unit and the fact that heat rises.

Most air conditioners are designed to turn off once the desired room temperature has been reached. Because the thermostat is located inside of the air conditioner, the air conditioner can only measure the temperature of the air surrounding the unit.

As the air is cooled, hot air in the room rises leaving the cooler air in the lower areas of the room. This means that the air conditioner needs to be placed as high as possible in the room to ensure that the entire room is cooled before the unit turns off. A low set air conditioner will result in an only partially cooled room with high energy use from the aircon constantly switching back off and on as the temperature changes.

Cooling Efficiency – Bigger Isn’t Always Better

It may seem like a good idea to buy a bigger air conditioner than you need to make sure the room cools down quickly. However, it is important to select an air conditioning unit that is sized to cool the area with the most efficiency.

The common mistake of choosing a too-powerful unit can result in high energy bills and clammy conditions. This is because most air conditioning units are optimised to cool a room gradually over time. Alongside cooling the room, air conditioners are built to remove excess humidity from the room to create comfortable living conditions. If an air conditioner is too big it will cool the room so quickly that it will turn off before the whole cooling cycle is completed. This can leave you feeling cool yet uncomfortable due to the excess moisture in the air.

A too big air conditioner will also use more power than a correctly sized unit. This is because it will turn off and on more frequently as it will quickly cool the room and turn off only to turn on again to quickly cool the room as the temperature inevitably rises.

Conversely, if you choose an air conditioner that is too small for the room it is cooling it will be both expensive to run and ultimately, ineffective.

If the unit does not have enough power to cool the room, it will be constantly running at a higher level to cool the room. This will result in a hefty power bill and a partially cooled room.


When determining the best size air conditioner to buy, you will need to take into account your surroundings. This includes the insulation of the house, the number and size of windows, and o course, the climate.

Insulation – The insulation of a house affects its ability to maintain the temperature whether cool or hot. This means that when you are cooling a house with an air conditioning unit, the insulation is what keeps the cool air inside and the warm air outside.

A poorly insulated house will require more cooling and therefore, a bigger air conditioner.

Windows – The calculations to find the cooling capacity required to cool a room mentioned to the beginning of this article is based on average window size and placement. This means that if your house does not have average window size and placement, you will have to take your windows into consideration.

Insulation in the walls of the house helps to maintain the temperature inside the house. So it stands to reason that windows will do the opposite of this as they are not insulated. This means that when cooling your house, heat is lost through the windows.

In terms of air conditioner size, the more windows there are in the room that you intend to cool, the more cooling power required by the air conditioner.

Climate – The final consideration of your surroundings is an obvious one – climate. It will take more time and energy to cool a room that starts out with an air temperature of thirty degrees than a room with a temperature of twenty six degrees.

For example, an air conditioner used to cool a room in northern Queensland would have to be a lot larger than an air conditioner used to cool a similarly sized room in Tasmania.

If you take all of these factors into consideration you should get a rough idea of the right sized air conditioner for you. If you’re still not sure, why not ask the experts? For quality advice and a free quote from air conditioning experts, call Advanced Climate Solutions on 07 3206 3542 today.