Residential Air-Conditioning: How to Size It

When it comes to modern air-conditioners, size really does matter and that principle “the bigger, the better” simply does not apply here. Living in Australia, or any other hot region, can hardly be comfortable without a good AC system and choosing its size will have a large impact on its efficiency.

AC units run optimally only if they are sized well to the conditions, since they are designed to cool the space in slow cycles and not rapidly. So, if the unit is oversized, it will cool down the room quickly and then turn off, before it finishes the whole cycle that it was designed for. Then, the house will quickly start to warm up and the unit will start again and it will continue, on-again, off-again, throughout the entire day. This will just work up your electricity bill and waste large amounts of energy.

On the other hand, if your AC unit is too small, you will put it in constant overdrive just trying to keep up with the large space. It will run at a high level, again not in the way it was designed to, wasting energy and costing you more in the end.

Need to know the basics

Firstly, you need to know what the standard unit of measurement for AC systems is. In most countries, AC units output is measured in kilowatts (kW), which shows energy consumed over time. In order to determine the most energy-efficient air-conditioner for your house, there are several factors to consider.

The first step is to calculate the square footage of your space and you can easily do that by multiplying the width by the length and height of the room. However, this is not the only influential factor determining the size of your AC. The amount of insulation in your house is very important as it significantly affects your house’s capacity to maintain room temperature. If your home is well insulated, you will not need a unit with a very high cooling or heating capacity.

The next important factor to consider is the climate you live in. Obviously, there’ll be much greater demands in the tropical and sub-tropical regions than in the cooler ones.

Additional factors that will also influence the size of your air-conditioner are the kind of lighting you have, the number and efficiency of your windows, the number of skylights, the type of walls and foundations you have and how much heat transfers through, the type and amount of ductwork and a few more.

All of this might seem quite difficult and complicated, and if you feel you can’t do it yourself, experienced professionals can do it for you, from the calculation of your AC capacity to installation and professional maintenance.

Single room solutions

If you are looking for a simple solution to cool or heat individual rooms, mini split AC systems might just be what you are after. They are smaller than the traditional units, offering you flexible installation to best suit your needs and allowing you to create comfortable living spaces with the utmost efficiency. Check out the new range of Frigidaire mini split air conditioners to see what I’m talking about.


If you still want to have a general idea before your purchase, you can use an online calculator and make your own calculation, but it will be rough, without consideration of some factors mentioned previously.

To assess the amount of air conditioning needed for your daytime living area, you need the total area square metre footage but the formula differs depending on your ceiling height. For example, if your ceiling is 2.4m high, multiply the area in square metres by 150 watts, but if it’s 3 metres high, then you multiply it by 175 watts. The result will show you the number of watts you need, so you can move the decimal point three points to the left to convert it to kW.

For instance, if your living room area is 100m² and the ceiling is at 2.4m, the calculation is: 100m² x 150 watts = 15,000 watts, which is 15 kW, so a unit of 15kW is roughly the correct size for your house.

Choosing the right size of your AC unit is a serious task that is best left to the professionals. Having the right information is crucial in order the make the best decision possible that will also result in picking a unit which is cost-effective, protective of your family’s health and functional in the years to come.