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No matter how hot under the collar you get, buying an air conditioner should never be an impulse purchase. 

It's an expensive, long-term commitment that has big implications for your power bill and comfort levels alike, so it's important to do your homework before installing one.

Plus, if you're going to use it in winter as well (reverse-cycle air conditioning is actually the most efficient way to heat your home), getting it right is extra important as you'll be relying on it for all your heating and cooling needs.

In a hot country like Australia, it might be tempting to just opt for the biggest air conditioner you can get, but bigger isn't always better.

So how can you find out the capacity you need? We'll walk you through everything you need to know.

On this page:

• What influences the size of air conditioner you need?
• How to calculate what size air conditioner you need
• Air conditioner room size chart: A rough guide
• Why where you live matters
• Why roof insulation matters
• Why you shouldn't go too big or too small

What influences the size of air conditioner you need?

You may assume that the size of your room dictates the capacity of air conditioner you should buy. 

But although room size is an important consideration, every home is different and there are actually many other factors that will affect your air conditioning capacity requirements.

I saved hundreds on my last air conditioner purchase by calculating that a smaller capacity model would suit my home

Chris Barnes, ÐÓ°ÉÂÛ̳air conditioning expert

"Take some time to assess the room before you on which air conditioner to buy," says ÐÓ°ÉÂÛ̳air conditioning expert Chris Barnes.

"I saved hundreds on my last air conditioner purchase by calculating that a smaller capacity model would suit my home."

These four things have the biggest impact on what size air conditioning unit you'll need. When you use an air conditioner size calculator, you'll get a more accurate estimate if you can enter as many of these details as possible.

1. Room size: What's the length, width and ceiling height? While floor space is important, so is total volume – a room with high ceilings will require more energy to cool and heat.
2. Insulation: Are the ceilings and walls insulated? What's underneath and on top of the room?
3. Location: Where do you live? A room in Darwin will need a more powerful air conditioner for cooling, compared to an otherwise identical room in Hobart.
4. Orientation: Which way does the room face? A large north- or west-facing window can let in a lot of heat in summer, whereas a shaded, southern-facing window will be cooler.

How to calculate what size air conditioner you need

Some installers and online calculators offer only a simplistic analysis and may tend to recommend a larger capacity than you really need.

Find a calculator that takes all of the room's details into account, including window size, shading, window coverings, insulation and local climate. The more information you can include, the more accurate the calculation will be.

The more information you can include, the more accurate the calculation will be

The and calculators on , 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 about the room, its insulation, window orientation and more, to get an accurate guide to the right air conditioner size.

Alternatively, you can try the calculators on manufacturer and installer websites, but we think these usually tend to overestimate the capacity you need.

Our air conditioner buying guide has lots of info about buying the best air con for your needs, and you can easily compare the cooling capacities of different models in our air conditioner reviews.

Air conditioner room size chart: A rough guide

Although you'll need to use a calculator that takes a range of factors into account before you decide the capacity you need, having a rough idea can be helpful.

Here's our guide to the approximate air conditioner capacity (size) you'll need to cool different room sizes, from a small bedroom through to a large lounge room.

Why where you live matters

As mentioned above, the climate where you live will have a big impact on your air conditioning requirements. 

You'll need a more powerful unit in Darwin than in Brisbane to deal with the extra heat and humidity, for instance. 

And obviously you'll need a much larger unit for cooling a home in sunny Perth than you will in Tasmania's cooler climate.

Why roof insulation matters

Ceiling insulation has the biggest impact on the size of air conditioner you need. 

If you don't already have it, our experts recommend you consider installing it before you invest in air con, as it can end up saving you money in the long run. 

You can see below the difference roof insulation can make on your air con capacity requirements.

Why you shouldn't go too big or too small

We've provided some rough guides in this article, but we recommend that you do an accurate calculation so you don't end up with a system that's drastically over- or under-sized. 

Don't be tempted to go smaller to save money, or larger to keep your house feeling like a fridge.

Bigger isn't always better, and smaller isn't always more economical. Aim for the sweet spot

Chris Barnes, ÐÓ°ÉÂÛ̳air conditioning expert

You can always use different sized units throughout your house for maximum efficiency – maybe a larger unit in the lounge room and smaller ones in bedrooms and the study.

Remember too that the size of the unit could affect the cost of installation and replacement.

"Bigger isn't always better, and smaller isn't always more economical," says Chris. "Aim for the sweet spot."

Here's what can happen if you super-size or skimp on your air con.

Too big

The unit may run frequent short cycles to achieve the target temperature. This can mean:

• the room gets too hot or cold
• the unit doesn't dehumidify the air enough (so the room feels less comfortable)
• power use increases
• running costs increase
• more wear and tear on the system.

Too small

The unit may have to run at maximum output more often. This can mean:

• the unit dries out the air too much
• more wear and tear on the system
• power use increases.

Stock images: Getty, unless otherwise stated.

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