What Size Ducted Air Conditioner Do I Need?

Australia is famous for its beautiful hot and sunny weather. While the warm weather is enjoyable for many - sometimes the heat can be overwhelming. For this reason, it’s extremely common for Aussie households to have an air conditioning system installed.

One of the best and most popular air conditioning options is ducted air conditioning. This type of AC is a system where there is a central area where air is heated or cooled, and this air is then distributed through the house via ducts. 

Most residential houses - especially multi-storey houses - with air conditioning opt to go with a ducted system. Now that you’re familiar with what ducted air conditioning is, you may be wondering what sized unit you need. 

Fortunately, we’ve put together this guide to help shed some light on what sized AC system you should get in order to adequately heat or cool your house, space or area. 

Benefits of correctly calculating ducted air conditioning size

When it comes to the importance of correctly calculating the right sized ducted air conditioning for your space, energy efficiency plays a major part. After all, for many people, the main goal is to have air conditioning generate a comfortable temperature for the least amount of power used.

If you choose a ducted air conditioner that is too large for the area you’re cooling, it would raise your electricity bill significantly because the bigger a unit is, the more power it uses. While this may seem obvious, purchasing an oversized air conditioner is a common mistake made by homeowners. Some people assume that a larger ducted air conditioner will have to run for less amount of time to make a home cooler or warmer and will result in a lower power bill.

While there is some truth to this, a larger unit will actually end up using more electricity and result in more expensive power bills. For starters, a large unit uses a significant amount of energy every time it turns on. Plus, long air conditioning run cycles - like what is generated through a correctly sized AC - are far more desirable and energy-efficient than short-run cycles - which are generated through an oversized AC.

On the other end of the scale, an air conditioning unit that is too small will significantly raise power bills, too. If an air conditioner isn’t powerful enough, it won’t be able to reach the desired temperature that’s input by the owner. The system will continuously work to attempt to reach this degree, and with the unit running over a long period of time, power usage will substantially rise. Plus, when a unit runs consistently, it reduces its life span and makes it more prone to broken parts, which can result in a costly fix - or worst case scenario - an unrepairable break of the motor and/or parts.

By installing the right sized ducted air conditioner, homeowners can avoid all of these problems - and enjoy an air conditioner that cools their space adequately and comfortably.

Ducted Air Conditioning Size Calculator

Because of zoning, and varying sized rooms, it can be complex to calculate what sized ducted air conditioner is required. At Sun City Air, we have skilled technicians who can conduct an analysis of your house and inform you of the correct size you need. To find out more about how to zone an area, you can check out our guide here [insert link].

However, if you’d like to try and calculate the size for yourself, you can use the below formula.

Modern air conditioner capacity is always measured in kilowatts - and this number is the amount of power the air conditioner expends to work. So, the higher the number of kilowatts the bigger the unit and the smaller the number, the smaller the unit.

Please note: the below formula takes into account which zoned areas are used more commonly.

How to calculate ducted air conditioning size in three steps

Find the floor area of areas you commonly use in the daytime

These rooms would normally be considered to be the kitchen, living room, hallways, stairs, office and study. 

The reason this is the first step is that these areas will be where your ducted air conditioner will be working at its hardest. Daytime areas generally take longer to cool, largely due to the amount of radiant heat as well as higher ambient temperatures as compared to nighttime. 

Consider the ceiling height. 

Most houses have ceiling heights of approximately 2.4 metres. Rooms with higher ceilings will need to be multiplied by a higher wattage. You can consult the below table to find out how much wattage you should multiply the data from step one by.

If your ceiling is 2.4 metres high 

Multiply the daytime area (m2 from step 1) by 150 (watts).

If your ceiling is 2.7 metres high 

Multiply the daytime area (m2 from step 1) by 160 (watts).

If your ceiling is 3 metres high 

Multiply the daytime area (m2 from step one) by 175 (watts).

Convert watts to kilowatts

In order to convert watts (the answer from step two) into kilowatts, you can shift the decimal point to the left by three digits.

An example of this would be if your total daytime living area is 100m2 and your ceiling is 2.4 meters high; your equation will look like this

100m2 x 150 watts =15000 watts

15000 watts can then be converted to kilowatts by moving the decimal point three digits left, which is 15 kilowatts.

This answer is the capacity of the unit that you’ll require to cool the area. For the above example, a 15-kilowatt air conditioning unit will be required.

Additional factors to consider for choosing a larger or smaller capacity ducted air conditioner

We’ve already outlined some benefits of choosing the correctly sized air conditioner for the space you’re looking to cool. However, calculating the correct size isn’t completely clear cut. There are multiple factors that can affect the temperature of an area and may mean that you need a larger (or smaller) sized unit to adequately cool your space.

One of these factors is the amount of radiant heat that enters the room. If your space has large windows that have sunlight shining on them for a significant period of the day, then the general temperature of the room will likely be raised. A space like this will usually require a slightly larger system than the calculation generates.

Another scenario where a larger sized ducted air conditioner unit may be required is when the roofs in the rooms don’t have insulation. The heat from the sun will warm the roof and directly heat the room, causing a higher general temperature and, consequently, requiring more power to properly cool.

Additionally, when a ducted air conditioner is primarily used during daylight hours, it will have to combat the sun directly (and the resulting higher temperature), so a stronger unit may be required in particularly hot and/or sunny geographical locations

On the other hand, there are also scenarios where an air conditioner that's a small size than the result generated from the equation is required.

As previously mentioned, insulation has a large effect on the size of the air conditioner that's required. If you are looking to cool a room that is well insulated, a smaller system may be required. Areas that are well insulated are able to contain conditioned air very well.

We've discussed that areas with a significant amount of natural light may require higher cooling abilities, but the reverse sentiment can be applied to areas with a small amount of natural light - or those that are predominately used at night. For example, a theatre room is generally dark and has little natural lighting, a room like this will have a lower overall temperature, and a system that's smaller than the figure generated from the equation will likely be required.

Of course, the size of the air conditioner directly affects the cost of energy bills. However, something as simple as a dirty filter can also raise power consumption significantly. Cleaning a filter is quick and straightforward, and can help you avoid serious problems with your system down the track. To find out how to clean the filter of your ducted system, check out our guide here. [insert link]

Time to call the experts?

If you are unsure about what size air conditioner is best suited for your house, then it may be time to call in the experts. Our Brisbane-based air conditioning installation team are on hand to give you the assistance you need. Our technicians are able to conduct call-outs to your house to assess what sized air conditioner is ideal for your space.

Brisbane Air-Conditioning services you can trust

Getting started is easy. Be up and running in minutes.