What Size Air Conditioner Do I Need? How To Get The Most Out Of Your Purchase

It’s a sweltering day in the middle of summer, and you’ve just returned home after a day at the beach. After cooling down in the ocean, you walk inside your home to be greeted by a stifling environment that makes you wish (again) that you had air conditioning. So rather than just wishing, you’ve decided to do something about it and finally get an air conditioning system installed. But what do you need to consider?

Obviously, the brand and model of a unit are important, and you’ll also need to decide if you want a split system or a ducted system. But another important factor is the size of the unit, particularly if you are after efficiency and potential savings on your power bill. So, if you’re asking the question, what size air conditioner do I need, here is a general guide on how air conditioning works and how to get the most out of your purchase.

How does an air conditioner work

Air conditioning essentially works by using refrigeration to cool the air indoors via a process called phase conversion. Basically, air conditioners force refrigerant compounds to evaporate and condense repeatedly in a closed system of coils.

The process starts with the unit sucking in warm air from inside a room through a filter. The warm air flows over a set of coils that contain cold refrigerant. As the air flows over the coils, the fluid becomes hotter while the air becomes colder. A fan then blows the air back into the room. Over time, this cycle will cool the room to your ideal temperature. The refrigerant fluid then needs to be cooled down (it absorbs all the heat, which makes the air colder), so it flows through the compressor. As the pressure increases, the fluid is cooled down through the condenser unit. In terms of how an air conditioner works, there are four major components:

1. Evaporator – receives the liquid refrigerant where it evaporates. Cooling coils remove the heat and humidity from the air using the refrigerant.
2. Condenser – facilitates the heat transfer by condensing the gas back to liquid form. Hot coils release the collected heat into the outside air.
3. Expansion valve – regulates the flow of refrigerant to the evaporator.
4. Compressor – pressurises the refrigerant and acts as a pump that moves the refrigerant between the evaporator and the condenser to chill the indoor air.

The “cold side” of an air conditioner is located inside a home and contains the evaporator and a fan to blow air over the chilled coils and into the room. The “hot side” is located outside and contains the compressor, condenser and another fan to vent the hot air from the compressed refrigerant to the outdoors. The expansion valve regulates the amount of compressed refrigerant moving to the evaporator between the two sets of coils.

Image from “Home Cooling Infographic” from energy.gov

Why do I need to consider size when choosing an air conditioner?

A range of issues can result from choosing the wrong-sized air conditioner for your home or business. They include:

• Short cycling – An air conditioning system that works on consistent cycles will keep your house cool and comfortable. When it works as intended, the unit will turn on and run until the set temperature is reached. But if you are using an oversized unit, the system will shut off too soon without adequately cooling or heating the entire area.
• Inconsistent temperatures – If your air conditioner is the wrong size, it will have difficulty maintaining a comfortable temperature in your home’s interior. Certain areas of your home will remain warm, whereas others will be cool (and vice versa). This is because the system has to work harder than required, which can lead to frequent breakdowns requiring repairs.
• An increase in power bills – A system that experiences short cycling not only compromises your comfort, it can also increase your electricity bills.
• High humidity – an oversized air conditioner won’t be able to dehumidify your home property. This is because it is probably not getting enough time to remove the vapour from the air, particularly if it’s short cycling. When your home is too humid for a prolonged period, it can promote mould growth and result in health issues.

What size air conditioner should I choose?

Buying an air conditioner should never be an impulse purchase, particularly because it’s an expensive, long-term commitment that can have huge implications on your comfort levels and energy bills. If you’re going to use it in winter as well, getting it right is extra important. Research is a must!

Given how hot it can get in summer, you may be tempted to buy the biggest unit you can find, however, bigger isn’t always better, and there are a variety of factors to consider. On the other hand, don’t be tempted to buy a unit that’s too small, as it won’t cool and/or heat rooms adequately.

Room size is probably the most critical factor, so you should note down the size of the room (length and width) you are looking to cool and/or heat. You should also consider floor space and ceiling height, as a room with high ceilings generally requires more energy to cool or heat. Other factors to consider include:

• Insulation. Are the walls and ceilings insulated? What’s on top of and underneath the room? Insulation will make your home thermally efficient, meaning your unit won’t have to work as hard to cool and/or heat.
• Location. Where you live will affect the temperature range you experience throughout the year. If you live in a hotter climate, you may need a more powerful unit compared to if you live in a location with a cooler climate. Here is a rough guide to the air conditioner size you may need for a 30m2 room based on where you live:

• Orientation. Which way does the room face? A west-facing or north-facing window can often let in much more heat during summer, whereas a shaded, southern-facing window will usually be cooler.
• Budget. The other thing to factor in when choosing an air conditioner is not just the initial cost outlay but also the cost to have it regularly serviced (which is vital to keep it efficient). A larger unit may be more expensive not just to install but also to service.

What size air conditioner do I need calculator

Here is a rough guide to the air conditioner capacity (size) you’ll need to cool different room sizes, according to Choice magazine. Choose a model that’s either just the right size or slightly more than you’ll need for the room. For example, if the room needs a 6kW model, then go for a unit that’s 6 to 6.5kW. It’s also worth mentioning that you can use different-sized units throughout your home for maximum efficiency. For example, a larger unit in the lounge room and smaller units in the study and bedrooms.

Some installers offer online calculators that provide only a simplistic analysis, and they tend to recommend a larger capacity than you really need. You need to find a calculator that takes into account all the details we’ve mentioned, including the room’s size, window size, shading, window coverings, insulation, local climate etc. The more information you can include, the more accurate your calculation will be. A couple of options:

• The cooling load and heating load calculators on Fairair, offered by the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH), are worth using. They allow you to factor in a lot of details.
• Omnicalculator also has a size calculator that’s worth trying.

Energy rating labels

So obviously, it is a significant investment, and you should do your research, but on the plus side, there are minimum energy performance standards (MEPS) that residential air conditions (split systems) need to meet to be sold in Australia. This means you can be assured that any new model you buy will be reasonably efficient. Basically, when a manufacturer registers a model with the government’s Energy Rating System, the air conditioner gets a star rating label for heating and cooling based on its test results against the Australian standard. This gives you a quick and easy way to compare different models.

The more stars, the more efficient the model is and the less it should cost to run. A model with five or more stars will clearly be more efficient, but it may also be more expensive to buy. But it’s worth the investment in the long run!

It’s also worth mentioning that the new Zoned Energy Rating Label (ZERL), which was introduced in 2020, shows three cooling star ratings and three heating star ratings based on the climate zone where the unit is installed. (The old star rating label only had one rating for cooling and one for heating). This is important because an air conditioner that is good at cooling in an average climate zone (see below) might not be the best choice for someone living in a hot zone, for example. The three climate zones are:

• Hot – northern Australia, including Brisbane and Darwin.
• Average – the middle zone of Australia, including Adelaide, Sydney and Perth.
• Cold – southern Australia, including Melbourne, Canberra, Tasmania, southern Western Australia and New Zealand.

References

• Alice Richard & Ashely Iredale, What size air conditioner do you need? Choice
• 2018, How does air conditioning work? A simple breakdown, Quality Air.
• 2021, 4 Problems Caused By Installing An Oversized AC Unit, Epic Air
• Chris Barnes, 2023, What to know before buying an air conditioner, Choice