How to Choose the Right Size Air Conditioner for Your Home
Choosing the right size air conditioner is one of the most important decisions you will make when buying a new system. Get it wrong and you will either waste money on energy bills or struggle to cool your room properly.
Why Size Matters
An undersized air conditioner will run constantly trying to reach the set temperature. It will never quite get there, your energy bills will be higher than expected and the unit will wear out faster from the extra workload.
An oversized unit creates different problems. It will cool the room too quickly, cycling on and off frequently. This short-cycling means the room never properly dehumidifies, leaving it feeling clammy. The constant starting and stopping also increases wear on the compressor.
The Basic Calculation
The standard rule of thumb for Australian homes is 125 to 150 watts of cooling capacity per square metre of floor area.
To calculate your room’s cooling needs:
- Measure the length and width of your room in metres
- Multiply length by width to get the floor area in square metres
- Multiply the floor area by 150 (watts per sqm)
- Divide by 1,000 to convert to kilowatts (kW)
For example, a 5m x 4m room is 20 square metres. At 150 watts per sqm, you need 3,000 watts or 3.0 kW of cooling capacity.
Factors That Affect Sizing
The basic calculation is just a starting point. Several factors can increase or decrease the capacity you need.
Insulation Quality
A well-insulated room with ceiling insulation, wall insulation and good seals around doors and windows needs less cooling capacity. You can reduce the base calculation by about 10%.
A poorly insulated room (older home, no ceiling insulation, single-glazed windows) may need 20 to 30% more capacity than the base calculation.
Window Exposure
Large windows facing north or west let in significant amounts of heat during summer. If your room has heavy sun exposure, add 10 to 20% to the base calculation.
East-facing windows get morning sun which is less intense. South-facing windows get minimal direct sunlight and do not need adjustment.
Ceiling Height
The base calculation assumes a standard ceiling height of 2.4 metres. For every additional 30cm of ceiling height, add about 10 to 15% to the required capacity.
Rooms with cathedral ceilings or double-height spaces may need significantly more capacity.
Climate Zone
Where you live in Australia makes a difference. Darwin and Cairns have hotter, more humid climates that require more cooling capacity than Melbourne or Hobart.
As a rough guide:
- Hot and humid (Darwin, Cairns): add 20%
- Hot and dry (Adelaide in summer): add 10%
- Moderate (Sydney, Brisbane, Perth): add 5%
- Mild (Melbourne, Hobart): use base calculation
Common Room Sizes and Recommended kW
| Room Size | Floor Area | Recommended kW |
|---|---|---|
| Small bedroom | 10-15 sqm | 2.0-2.5 kW |
| Medium bedroom | 15-20 sqm | 2.5-3.5 kW |
| Living room | 20-30 sqm | 3.5-5.0 kW |
| Open plan living | 30-45 sqm | 5.0-7.0 kW |
| Large open plan | 45-65 sqm | 7.0-9.0 kW |
Use Our Free Calculator
For a personalised recommendation that accounts for all these factors, use our free AC Size Calculator. Enter your room dimensions, insulation quality, window exposure and climate zone for an accurate kW recommendation.
Frequently Asked Questions
What size split system do I need for a bedroom?
Most bedrooms are 10 to 20 square metres. A 2.0 to 3.5 kW split system is typically sufficient. Smaller units (2.0 to 2.5 kW) suit standard bedrooms, while larger bedrooms or those with poor insulation may need 3.0 to 3.5 kW.
Is it better to go slightly bigger or smaller?
If you are between sizes, going slightly larger (one step up) is usually the safer choice. A slightly oversized unit running at low speed is more efficient than an undersized unit running flat out. However, going significantly oversized creates the short-cycling problems described above.
Do I need a different size for heating vs cooling?
Reverse-cycle air conditioners have different capacities for heating and cooling. In most Australian climates, the cooling capacity is the primary concern. However, if you live in a cold area (Melbourne, Hobart, Canberra) and plan to use the system for winter heating, check the heating capacity as well.
How do I know what kW my current air conditioner is?
Check the label on your indoor or outdoor unit. It will list the model number and capacity in kW. You can also check the original documentation or search the model number online.
Need help choosing? Browse our range of split systems or contact our team for expert advice. We can recommend the perfect system for your space.
Need expert advice?
Our team can help you choose the right air conditioning system for your space.