How To Calculate The Size (Capacity) of A Ducted Air Conditioner For Your Home

If you’re installing ducted air conditioning in your home, it’s absolutely crucial to get the size and air conditioner capacity right the first time.

After all, it’s extremely expensive (not to mention difficult and time-consuming!) to change a ducted air con system after it’s already installed.

To help you make the very best decision, our experts have whipped up this easy-to-follow ducted air conditioning size calculator, which will help you discover the best kilowatt (KW) capacity for your brand new ducted air con. Follow our steps on how to calculate air conditioner size for a house (personalised to suit your home’s dimensions) to stave off future buyer’s remorse.

Why is it important to choose the right size air con?

It all comes down to energy efficiency. Air conditioners run best when they are designed to fill the space efficiently. An energy-efficient air con will gradually cool your house in a sustainable way, while a poorly-sized AC will be stressed, need to work much harder, may short-cycle and will certainly need more repairs.

If your air con is too large, you can expect your air con to:

  • Cool your house rapidly then abruptly shut off on repeat
  • Use up lots of unnecessary power
  • Cost you significantly more in electricity

An undersized air conditioning unit will:

  • Need to work much harder to reach the set temperature on very hot days or very cool days
  • Be in constant overdrive, causing your air con’s parts to wear out faster
  • Result in inefficient cooling and a higher electrical bill

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What Sized Air Conditioning Do I Need?

Knowing what sizing you want for your ducted air conditioning can be tricky. Air con ducting sizes vary widely, but the important question to ask yourself is, ‘How many kw (kilowatts) do I need?’

Modern air conditioners are almost always rated in kilowatts (KW). This KW rating measures the output of the air conditioner. Both your air conditioner’s cooling and heating functions are measured in KW. =

A few technical things to note: The ratio of the KW output to the electrical input is called the COP (coefficient of performance), also known as an EER (energy efficiency ratio). This is a measure of your air con’s efficiency. The higher the COP, the more efficient your air conditioner will be.

Here’s a simple example: Say a Daikin 12.5 KW standard inverter has a cooling capacity of 12.5 KW and a heating capacity of 15 KW. This means you have a cooling COP of 2.99 and a heating COP of 3.35, respectively. In this case, you know that your air con’s heating cycle is more efficient than its cooling cycle.

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Air Conditioner Size Calculator

Our air conditioning calculator helps determine the right unit size for your needs in two easy steps.

How To Calculate The Air Conditioner Size For A House

To calculate your ideal ducted aircon size for your house, follow these simple steps:

1. Calculate the total floor area of your daytime living areas

Your AC calculations should take into account room size to determine the total floor area. Daytime living areas include kitchens, family rooms, lounges, media, study entrances, hallways, stairs – any area of your home that will be used during the daytime, except for utility rooms.

Why is this helpful? Calculating this area will help you determine the necessary capacity for your air con. This is based on the assumption that your bedrooms are not used during the day. At night, the direct radiation your house receives from the sun decreases and the heat load comes off the house.

2. Multiply the daytime living area by ceiling height

The rule of thumb calculation is to multiply the total daytime living area by the ceiling height. Naturally, you’ll find that a house with higher ceilings requires an air conditioner with a larger capacity because air conditioning is all about the volume of air that needs to be cooled or heated.

For a house with a ceiling height of 2.4 metres – Multiply the total daytime living area (m²) x 150 (watts)
For a house with a ceiling height of 2.7 metres – Multiply the total daytime living area (m²) x 160 (watts)
For a house with a ceiling height of – Multiply the total daytime living area (m²) x 175 (watts)

These calculations will give you the amount of watts you need. Once you have this number, you can move the decimal point three points to the left to convert it to kilowatts.

As an example, let’s calculate the ducted air conditioning for a 4 bedroom house. The average-sized 4-bedroom house in Brisbane has a daytime living area of approximately 85 square metres with 2.7 metre high ceilings. In this case, 85 x 160 watts = 13.6 KW. Therefore, the home needs an air conditioner with a capacity around 13.6 KW.

When Do I Need A Larger Capacity Ducted Air Conditioner?

You might want to consider a larger capacity air conditioner than this ducted air conditioner size calculator suggests if:

  • You have large areas of single glass windows (particularly facing west)
  • Most of your bedrooms and living areas are used during the day
  • You have uninsulated roof space

When Do I Need A Smaller Capacity Ducted Air Conditioner?

You might want to consider a smaller capacity air conditioner than this air conditioner size calculator suggests if:

  • You have a large amount of high-rated insulation in your roof space and walls
  • You have the ability to close off and not use daytime living areas on very hot or cold days
  • You have plenty of roof space ventilation, for example eave vents and whirly birds
  • You have high rated duct insulation, such as ultra duct

The Most Accurate Way To Decide Your Air Conditioner Capacity

There are many factors beyond this in evaluating the right air conditioner for your home. The best way to know for sure is to seek an air con specialist who can visit your home and evaluate all the relevant factors. This will give you a more accurate estimate of the best KW for your needs. Getting an accurate estimate will help your air conditioner run more efficiently and save you money on power bills.

To ask us a question about ducted air conditioning sizing or request a visit, contact Crown Power today on 0427 175 654 (Brisbane & Sunshine Coast) or 0421 376 620 (Gold Coast). We have years of experience to help you select the best air con for your space.