How to size an air conditioner without the 20 percent upsell

A rule of thumb that is not terrible: about 100 to 140 watts of cooling per square metre in a typical insulated Australian room. A 25 square metre lounge often wants 2.5 to 3.5 kilowatts, not a 7 kilowatt monster.

West-facing glass, a raked ceiling, or no insulation will push that number up. A well-shaded brick home in Melbourne will sit at the low end. Always ask the installer what area they assumed. If they did not ask about windows, they guessed.

Oversizing feels safe. It is how you get a unit that blasts cold for eight minutes, shuts off, and leaves the room clammy. Humidity stays. The compressor short-cycles. The quote was bigger for no comfort.

For ducted, zoning matters more than the headline kilowatt. A system that can idle two bedrooms while the living zone runs will cost less to run than a bigger unit blasting the whole house because the thermostat is in the hallway.

Want a number for your house? Check the rebate on your postcode. No name required. Check the rebate on your postcode.

Or do the sums yourself first: what size you need and what it costs to run.