What size air conditioner do I need?

Choosing the right air conditioner can be a daunting task when you’re searching for the most energy-efficient unit to cool an area of your home. The first thing you need to get right is to buy the correct size unit.

When shopping for a new air conditioning unit there is more to consider than just the cost of the unit itself. The kilowatt capacity for the size of the room that it’s tasked with cooling is much more of a priority. You could be saving yourself the heartache of a high electricity bill by carefully choosing the best air conditioner for the type of space it’s intended for.

There are four main factors to take into account when considering which air conditioning unit size you’ll need:

  • Room size: including the length, width, and ceiling height
  • Insulation: how well your ceiling and walls are insulated
  • Location: the climate zone you’re living in
  • Orientation: the location (and size) of your windows and glass doors (and the direction they face)

We’ll cover each factor in more detail below.

Room size

Each individual room in your home will have different air conditioning capacity requirements. Generally speaking, you’ll need about 0.12 to 0.15 kilowatts of power per square meter of floor area. This is calculated by multiplying a room’s length in meters by its width in meters. You’ll also need to take into account ceiling heights in your calculations as rooms with higher ceilings will always need more energy to cool.

What size air conditioner do I need for each room?

Here we match the right air conditioner for the size of the room in your home:

Room size Room examples Air con kilowatt capacity
10-25m2 A small kitchen, bedroom, study, small lounge, small office 2.5kW
25-35m2 Bedroom, small lounge, small room with a high ceiling, office, mid-sized kitchen 3.5kW
35-60m2 A large bedroom, bedroom with an ensuite, mid-sized lounge 5-6kW
60-80m2 Large lounge, large open plan area, small shop, office 7-8kW

General Guide Only


Insulation is a material located within your home’s walls and ceilings designed to slow or prevent heat flow. In other words, insulation makes your home thermally efficient − it helps to keep heat inside your home in winter and outside your home in summer. The performance of any insulation product (how well it resists heat flow) is known as its R value. The higher the R value, the higher the level of insulation.

You’ll need to consider your home’s level of insulation when deciding on which air con size you need because if your house is poorly insulated (has a low R value), your air conditioner will need to work much harder to keep the place cool or warm. Reflective insulation and bulk insulation are the most common forms of insulation used in Australia today.


The geographical location of your home will also impact your cooling needs and air conditioning capacity requirements. If you live in a subtropical climate zone characterized by hot and humid summers, like Cairns or the Sunshine Coast, chances are you’ll need a more powerful air conditioner than someone living in a temperate climate zone (i.e. Melbourne, Tasmania, etc.).


The orientation of your home, as well as the size and orientation of windows and glass doors in each room, will impact your choice when it comes to settling on the right AC unit size. Do you know which rooms the midday sun hits? As a general rule, southern-facing rooms will be cooler which means they won’t require as much cooling capacity as north-facing rooms that receive sun for the longest period of the day. The ideal orientation for your home will also depend on your climate zone.

How to calculate the air conditioner size you need

how to calculate air con size

There are calculators online that allow you to figure out the cooling or heating capacity you need for your home. Depending on which calculator you use, the capacity takes into account a variety of factors such as:

  • Type of air conditioner
  • Whether the roof is insulated
  • Window and window glass type
  • Room height
  • Room width
  • Room length
  • The amount of sunlight the room is usually exposed to
  • Whether the room directly faces the afternoon sun
  • Climate.

Some aircon installers also provide online calculators on their websites and offer a few common size options if you’re unsure of measurements or just want a quick guesstimate before diving into specifics.

What size split system or ducted system do I need?

Split systems and ducted systems come with similar cooling capacities, so you can apply the principle of kilowatt per square meter of floor area to find your ideal unit size for both air con types. What will differ, however, will be your total running costs per system.

  • Window air conditioners are usually ideal for rooms measuring up to 36m² and this can cost 23-70c to run per hour.
  • Split system air conditioners are often recommended for spaces no larger than 75m², which can attract an hourly cost of up to 97c.
Type of air conditioner Estimated hourly running costs Size of room
Split system and window air conditioners 23c-29c Rooms up to 12m2
Split system and window air conditioners 48c-70c Rooms up to 36m2
Split system and window air conditioners 76c-97c Rooms up to 50m2
Ducted evaporative air conditioners 46c-62c (energy) / 8-9c (water) Spaces up to 125m2
Ducted refrigerative air conditioners $2.37-$3.39 Spaces up to 125m2
Portable air conditioners (evaporative) 7c (energy) / 1-2c (water) Rooms up to 20m2
Portable air conditioners (refrigerative) 40-60c Rooms up to 20m2

General Guide Only

What size portable air conditioner do I need?

Portable air conditioners are compact and mobile, meaning they can be placed anywhere in your home as long as there’s a power socket. When it comes to portable air con size, the same rule applies as for other AC unit types. You’ll need roughly about 0.15 kilowatts of cooling capacity per square meter.

What size window air conditioner do I need?

Window air con size

You’ll need a window air conditioner with around 1-1.5 kilowatts of cooling capacity per every 10 square meters of floor space. As the name suggests, window air conditioners are designed to be mounted in a window, but can also be installed in a wall. This type of AC unit is best suited to small and medium-sized rooms such as bedrooms or home offices.

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How much will the air conditioner cost?

Canstar Blue research has found that Australian households spend an average of $2,163 on the air conditioner model itself, with the majority opting for a split system air conditioner. When you’re sizing up rooms for the right air conditioner, you’ll want to make sure that you’re going to get value for money in terms of both purchase price and running costs. Keep in mind these costs include both portable air conditioners and wall-mounted air conditioning systems. Here’s a breakdown of how much an aircon unit will cost on average, based on capacity size:

Kilowatt capacity Average price
2.5kW $600 to $1,800
3.5kW $650 to $1,900
5-6kW $1,100 to $3,000
7-8kW $1,600 to $4,000

General Guide Only

Energy costs of running an air conditioner

Many people believe that running an air conditioner is essentially burning money the longer you run it, but you might be surprised to find the average energy costs of running an air conditioner are relatively low. For example, the average reverse cycle air conditioner costs around $0.25 – $0.35 per hour to run for cooling purposes, depending on the size of the room. When you first install your AC, you’ll also incur some air con installation costs.

How can an air conditioner be more efficient to run?

The efficiency of your air conditioner depends on a number of factors, including the effectiveness of the components inside and how warm it is outside. Keeping your air conditioner well-maintained will enhance its efficiency. If the air conditioner’s coils become trapped with dirt, then the airflow will reduce and it will be unable to absorb heat. By cleaning the condenser and evaporator coils once a month, during the hotter months, it will run more effectively.

Check the energy rating on the unit before you buy – the more stars, the more energy-efficient it will be to run.

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