What size split system should I buy?

At Customised Air Conditioning, one of the most common questions we get is “What Size Split system should I buy”?

Split systems are one of most efficient and effective ways to heat & cool your home but it’s crucial that you get the right size.  Too big and you’ll end up spending more on the unit & power as the unit turns itself on and off too frequently.  Too small and the system will be working on overdrive to maintain the desired temperature.

Keep on reading and we’ll explain how to select the right size unit for your space including factors to consider and why the size is important.

Is bigger, better?

As we touched on above, the whole idea of a split system revolves around being efficient.  Efficiency is estimated by a ratio of power input to rolling and heating capacity (measured in kilowatts, kW).  We often hear two different arguments - one suggesting that you should buy a bigger unit because it’ll have plenty of power to cool your space and the opposite which suggests you should buy a small split system as they’re much cheaper.  Both concepts have the merits but it doesn’t really tell the whole story.

Too big

If your system is too powerful or big for the space, it will constantly be turning itself on and off which is very inefficient!  Turning on and off uses excessive power and that leads to big power bills.  

Too small

If your unit is undersized, it will be constantly working at max capacity to achieve the right temperature.  When machines are pushed to their limits constantly, parts wear out and things break.  You might save on the upfront costs but you might be looking at a new unit much sooner than expected.

Just right

The right size split system will heat and cool your space comfortably and avoid any temperature swings.  The right size will mean a long lasting machine that doesn’t send your electricity bill sky high.

How do I pick the right size?

The main variables to consider are:

  1. Room size (cubic metres as you should consider the height of the room too)
  2. Insulation
  3. Windows & Doors
  4. Climate

Room Size

For the purpose of this article, we’ll assume you have a normal 2.1-2.7m high ceiling.  Anything higher than that will increase the cubic meters substantially and you would be best increasing the size of the split system

As a rough guide:

Small Bedroom - Up to 20m2 - 2-3kW

Large Bedroom with ensuite - 20 - 40m2 - 2.5-5kW

Large Lounge - 40-60m2 - 4-6kW

Open Plan/Apartment Space - More than 60m2 - 5-10kW


The better the insulation, the easier the air conditioner has to work to cool the space so you can lower the size if you have exceptional insulation

Windows & Doors

As per insulation, if you have double glazed windows that help reduce effect of the outside temperature, you will be able to go for a smaller unit.  If you have lots of doors and spaces that let in draughts, you’ll need a bigger unit to keep up.


We’ll speak to the Perth climate which is hot summers and mild winters.  Given most of us will use our air conditioners far more for cooling than heating, we recommend looking at the specifications to ensure the cooling capacity is adequate.  This would be different depending on where you live in the world.

How about some help?

If you’re unsure of what system you need, why not contact the professionals?  We install split systems day in, day out and we would love to help you!  No need to suffer in the heat this summer, get in touch with us Customised Air Conditioning to organise your split system.


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