18 June 2026 · 5 min read
Rough sizing by room area
A bedroom of 10 to 20 square metres typically needs 2.5kW. A 20 to 30 square metre room needs 3.5kW. An open plan living and dining area of 30 to 45 square metres needs 5.0 to 6.0kW, and 45 to 65 square metres needs 7.1 to 8.0kW.
These are starting points, not a design. Two rooms of identical size can need different capacity.
What pushes a room up a size
West-facing glass, large windows without external shading, cathedral or raked ceilings, poor or absent ceiling insulation, dark roofing, and rooms that open onto stairwells all increase the load.
Kitchens attached to living areas add heat, and so does a room that regularly holds a lot of people.
Why oversizing is a real problem
An oversized system reaches temperature quickly and shuts down before it has removed the humidity, leaving the room cold and clammy, with short cycling that wears the compressor.
Undersizing means the unit runs flat out on the hottest days and never quite gets there. Correct sizing sits in the middle and runs at low inverter speed most of the time, which is the efficient operating point.
Frequently asked questions
How many kW do I need per square metre?
As a rule of thumb, allow about 150 watts per square metre for a well-insulated Sydney room with standard ceilings, then add capacity for west-facing glass, high ceilings or poor insulation.
What size air conditioner for a standard bedroom?
A typical 3 by 3.5 metre Sydney bedroom is comfortable on a 2.5kW split system.
Does a bigger unit cool faster and save money?
No. It cools faster but cycles on and off, leaves humidity behind and costs more to buy and to run over its life.