The basic kW rule (single-storey, average insulation)
Start with floor area: roughly 125–150 watts per m² for cooling in southern Australia (Melbourne, Hobart, Adelaide), 150–175 W/m² for Sydney/Brisbane/Perth, 175–200 W/m² for Darwin and tropical Queensland. That gives you the baseline kW. A 30m² living room in Sydney = roughly 30 × 165 W = 5 kW system.
Quick reference table
Up to 20m² → 2.5 kW. 20–30m² → 3.5 kW. 30–45m² → 5.0 kW. 45–60m² → 7.0 kW. 60–80m² → 8.0–9.0 kW. 80–100m² → 10+ kW (start considering multi-split or ducted). 120m²+ → fully ducted system.
Adjustments — ceiling height
Standard 2.4m ceilings: use the table as-is. 2.7m ceilings (common in newer homes): add 5–10%. 3m+ ceilings (Queenslanders, warehouse conversions): add 15–25%. Raked or cathedral ceilings: add 20–30% and consider air flow direction carefully.
Adjustments — insulation and orientation
Poor insulation (1980s and older home, no ceiling batts): add 15–25%. Excellent insulation (R5+ ceiling, double glazing): subtract 10–15%. West-facing room with large glass: add 15–20%. North-facing in tropical Queensland: add 10–15%. South-facing room (cool): subtract 5–10%.
Adjustments — occupancy and use
Living room with 4+ people, TV, frequent cooking nearby: add 10–15%. Bedroom with 1–2 occupants: use the baseline. Home office with computer equipment: add 5–10%. Kitchen during cooking (consider a separate range hood / make-up air).
Worked example: 28m² Sydney living room, west-facing
Baseline: 28m² × 165 W = 4.6 kW → round up to 5 kW. West-facing add 15% → 5.3 kW. So a 5 kW system is right at the edge — go 6 kW (or 5 kW with proper window shading).
Why installers should always verify
These calculations are a starting point. A professional heat-load calculation considers window U-values, infiltration rates, internal heat gains and climate design temperatures. Insist on a heat-load calculation for any quote over $5,000 — quick rules of thumb are fine for single splits, but ducted systems should be properly sized.