How To Choose The Best Air Con Size For Your Home Or Business

Last updated Monday, 9 November 2020
For anyone living and working in Perth, air conditioning is a wonderful invention of the modern world. True, a roof over your head and food to eat are the real essentials, but it’s a definite problem if you lack a functioning air conditioning system during the summer months.

Perhaps you already have an air conditioner in place, but are you getting the most out of your current system? Most property owners assume that as long as they hear their system turning on and feel it blowing cool air, all is well. Unfortunately, countless systems are functioning far below optimal capacity.

How does this happen? They did not take the size of their air-con seriously. With air conditioners, you have to be careful in choosing the appropriate system based on the size of your space. From there, you can determine the size of your AC.


Acsis air installation.

Why Correct Sizing of Your Air Con is Important

Size matters in air conditioning systems – that is, finding the right size of air-con for your space. Before you purchase the system, you should first consult a professional HVAC installer. That way, you can determine the best size for your home or business.

The biggest benefit of having the correct size of air-con for your building is that it encourages the system to run a full cycle properly. At the same time, your space stays cooler for a much longer time. That means lower electric bills, allowing you to save money in the long run.

A common misconception is that having an oversized system is the way to go. In reality, it does not provide any benefits other than cooling the home quicker. However, it results in over-cooling the space, which leads to higher electric bills. Plus, the system tends to shut off sooner than required.

If you have bigger-than-required air-con, you will notice that the building structure will not be cooled adequately. We’re talking about the walls, furniture, wood, and others, which, when not cooled, will only allow heat and humidity to come back in. That is when the AC will start again and again.

The consequence is that these large systems run for an incredibly short time more often than you need it. That not only costs you a lot but also tends to kill the system faster.

So, does it mean going for a smaller system is better? No. Having an undersized system will not cool your home effectively. It also does not offer any savings because you have to keep it running for a longer time, which means energy and money wasted.

To understand what unit size your building requires, you may need to consult a professional. That way, you will be provided guidance in sizing your aircon properly and correctly.

The professional will typically look at certain factors that affect the space’s cooling, which include:

  • The total volume of the living space of the home or business – and we’re not just talking about the square meterage
  • The area of exterior walls that are exposed to the sun
  • The orientation of the windows, their number and current condition
  • The amount of sunshine that enters the home
  • Degree of air sealing
  • Insulation, ventilation, and ductwork
  • The age of the house or commercial building
  • The shade from shrubs or trees

Until then, here are some basic factors that you can look into on your own to determine the size of your unit:


1. Building Size

The acsis airconditioning van ready at a home.Building size can be calculated based on the number of square metres that make up the floor space. For most accurate measurement of how much air conditioning capacity you require, find out how many square metres make up your home’s floor plan, including multiple levels.

Once you’ve calculated your home or building’s total square meterage, you can then proceed with the total volume. Floor space is essential to know about, but so is the total volume. When you have a room that has high ceilings, it means that you need more energy to cool compared to a space with the same size but with a low ceiling.

Then, you can choose a corresponding size and level of efficiency for your system.

Here is a rough guide that will help you in selecting the air-con capacity based on the room size:

  • A small room measuring up to 20 square metres, such as a small kitchen or bedroom, will require a 2 to 2.5kW system.
  • For a medium-sized room, measuring from 20 to 40 square metres, such as a small lounge or an ensuite bedroom, we recommend a 2.5 to 5kW system.
  • Large rooms that span 40 to 60 square metres will require a four to 6kW system.
  • For rooms that are bigger than 60 square metres, the recommended aircon size is from five to 9kW.

The above numbers are just a guide. Your AC will still depend on your location, as well as the other factors listed in this blog post.


2. Property Location

As mentioned above, the location of your home or business affects the size of your air-con. If you have a 30 square metre room, your system requirement in Perth will differ from someone who lives in Brisbane. Since Perth has warmer summer months, you will need a 2.8kW system. Meanwhile, for homes in Brisbane with the same size, they require a 2.2kW system, just like for those in Sydney. Homeowners in Hobart need a smaller system, which is around 1.1kW.

Please note that these calculations are based on a corner room typically used by two people. It measures 5 x 6 metres with a 2.4-metre ceiling. The windows on the north and east walls measure 1×2 metres and have blinds. This brick veneer house is insulated, along with the tile roof, but the suspended timber floor is not.

Once again, it’s a ballpark guide. The numbers and sizes may not apply to your current floor space. Additionally, it will depend on the type of air con you will use. For example, if you want a ducted air conditioner, you need to compute all the sizes of the rooms you will have the system on at the same time. You will need a professional to have a look at your space to get an accurate calculation.


3. Window Condition

They say that the windows are the eyes of a house. They do not merely exist to beautify your home but also contribute to the overall comfort you feel. But what most people do not know is that their windows are directly connected to unconditioned air. Their state weighs heavily on the type and size of unit you need.

Some of the things to consider when it comes to your windows are the following:

  • Number of windows in the building
  • Orientation to the sun
  • Window area

As you think about these window traits, create a list of all the things that may cause you to require a larger air conditioning unit. For instance, a high number of windows that face the sun will yield a higher kilowatt requirement for your system.

If you have old, rundown windows, they will decrease your home’s energy efficiency. They are also responsible for adding up to 30% of your electricity charges. You can splurge on an energy-saving air conditioner, but it will not give you the benefits you seek if your windows are dull and drafty.

It’s vital to ensure that the room is not leaking, meaning warm air from outside should not come in. That’s why the doors and windows should be closed when the AC is running. If there are small gaps near the windowsill, they should be fixed to avoid letting cool air out.

If these simple issues are not solved, the air conditioner will work excessively. It can result in more significant problems, including malfunctioning of the system. The air con will also struggle to maintain temperature, which will easily lead to a damaged unit. When the system works harder, it consumes more energy.


4. Amount of Sunshine and Shade

The amount of sun that hits your building’s exterior makes a difference, too. Before deciding on the system size, make sure you are aware of how much sunshine and shade your exterior walls receive. As you make your assessment, remember to factor in the shade from nearby trees and neighbouring houses and buildings.

Direct sunshine is damaging because it heats up the air conditioner, particularly its enclosure. Also, the UV rays are not exactly friendly to an electrical system like your AC. The sun’s rays can break up paint, rubber, and plastic molecules, which are in the washers, clips, and hoses.

The compressor of the unit will work longer to keep the room cool when it is exposed to harsh sunlight. Given that Perth enjoys warm summers, you should think about this critical detail before installation of your system. The compressor tends to deal with a higher initial temperature in its housing because of sun exposure. As a result, it faces extra wear and tear compared to units in a shaded area. It can even lead to a quicker loss of refrigerant liquid and, of course, higher utility bills.

The size of the system should allow you to determine where the location of the unit should be. Place it in an area that gets the most shade from trees. If there are not a lot of trees around, consider the angle of the sun, especially during afternoons. A wall or another part of the house should block the sun when the temperatures are at their peak.


5. Existing Ventilation and Insulation

An Acsis staff member preparing ventilation for an air con insallation. Insulation is synonymous to restricting airflow in and out of the room. It is one of the best ways to increase the effectiveness of your air conditioner. Often, however, it is not as easy to limit the movement of air, especially if the area is quite large.

But the good news is that many homes in Australia have already been built with good insulation. When you run the AC, one of the things that you need to do is to ensure all doors that lead outside the house, as well as your windows, are all firmly shut. In doing so, you stop the cool air from escaping.

Another way for air to escape is through uninsulated lofts and walls. They are thin and often have small cracks, which result in air leakage, rendering your AC ineffective. Before you have the air-con installed, it is best to invest in good insulation first. Otherwise, your home or office will not have an effective barrier, which means cool air will escape, and warm air will keep entering the building.

If you find that your home does not cool enough despite having a functional air con unit, you may want to check your ductwork. Leaky ducts, as well as dust and grime build-up in there, could be the main culprit. That is why you should visit your attic, which can seriously affect the ability of your home to be cool in the summer and warm in the winter.

A well-ventilated attic controls the moisture levels in your building. If it is vented and insulated correctly, moisture will not build up inside, allowing the AC to work efficiently.

Again, insulation plays an integral part in how drafty your attic becomes. Proper insulation protects your home from getting too cold and too hot. Usually, attics get very warm in the winter. Then, cool air tries to find a way to displace warm air, which will then leak into your attic. The same thing happens during summer, making your air con useless.

Ask your local HVAC provider to take a look at your existing system and ductwork. If there are problems or easy places for air to seep, this knowledge will help you make an informed decision about the system you need.


More Tips for Air Conditioner Sizing

Drastic oversizing and under-sizing can happen, but the trick is to avoid trying to save by going smaller or going bigger to make your home feel like a freezer. It does not work that way. Bear in mind that bigger is not always better and smaller is not the most economical option when it comes to AC systems. Instead, aim for the sweet spot.

Here is a sample of how you can calculate the power of your air-con if you plan to use a split system:

  1. Calculate the room area to determine how much cooling the space requires. This room area is where you will have the split system installed.
  2. The next step is to multiply the area by the wattage for each square metre. If you have a 2.4-metre ceiling, multiply it with 150 watts. The wattage will depend on the ceiling height. For example, if you have a 2.7-metre ceiling, multiply it by 160 watts. For three-metre-high ceilings, multiply by 170 watts. The number you get is the number of watts that the system should have to provide enough cooling capacity to the room.
  3. Change watts to kilowatts by moving the decimal point three places to the left. So, if you have a 100 square metre room and you multiplied it by 150 watts, you get 15,000 watts or 15kW. Therefore, this particular room requires a 15kW split system.

Another thing to understand is that all air conditioners come with a cooling rating. It means they are rated to cool a particular room or space. With that rating, you can determine the efficiency or cooling capacity of the system based on the size of the room or the entire building. So, if you install a unit in a bigger place, it will work harder, causing it to use more electricity. Cooling will, of course, be ineffective.
You can help the unit work less by minimising the stress you place on it. One thing that you can do is to make sure heat will not enter the room you are trying to cool. At the same time, that room should keep cool air inside, preventing leaks.

Acsis aircon split system installation with the unit on the ground.


If you have a small room, the best practice is to go for an air conditioner with a slightly larger cooling rating. It helps cool the room faster while keeping energy costs down and staying at a moderate temperature. You should never oversize in this case because you could easily spend unnecessary money for an AC with a capacity that’s too large for your space.

The degree of existing insulation also contributes to the success of your air-con system. As you gather information, you’ll know if your current system is oversized or too small to meet the current requirements.

Keep in mind that an oversized system continually shuts off and turns on, while an undersized one runs continuously. Both scenarios waste energy and cost you more than necessary, so consult with an HVAC professional today.

Do you need guidance on how to select your system size? Let our professionals help you out. Contact ACSIS Air before having us install your system. That way, we can provide expert advice on the correct air-con sizing for your home or business in Perth.