How to size a split system for your bedroom (and not get oversold)
If you've ever rung three aircon installers and got back three different kW recommendations for the same bedroom, you're not imagining things. Sizing is part science, part "depends what we have in the warehouse this week". Here's how to size honestly, and how to spot the upsell.
The honest sizing rule
For a standard south-east Melbourne home (single storey, R3 ceiling insulation, average windows), allow roughly 125–150 watts of cooling per square metre. Insulated double-storey or west-facing rooms push that up; north-facing bedrooms with curtains drawn drop it down.
| Room size | Best kW | Typical model |
|---|---|---|
| Up to 14 m² (small bedroom) | 2.5 kW | Kaden KS25 / Mitsubishi MSZ-AP25 |
| 14–22 m² (master, study) | 3.5 kW | Kaden KS35 / Mitsubishi MSZ-AP35 |
| 22–34 m² (open-plan + bedroom) | 5.0 kW | Kaden KS50 / Mitsubishi MSZ-AP50 |
| 34–50 m² (lounge / living) | 7.0 kW | Mitsubishi MSZ-AP71 |
| 50 m²+ or whole-floor | Multi-head / ducted | Quote individually |
Three common upsells to watch
1. "You need 5kW for that bedroom"
For a 12 m² bedroom, a 5 kW unit is roughly twice the capacity needed. Oversized units short-cycle (fast on/off), don't dehumidify properly, and cost 30–40% more upfront. Stick to the table above.
2. "You need the premium brand or it won't last"
Mitsubishi Electric is the gold standard, but Kaden punches well above its price for everyday Australian conditions. We've installed 800+ Kadens in SE Melbourne; failure rate is comparable to mid-range Daikin. Pay the premium if you want whisper-quiet operation or app control — not because you've been told the cheaper unit will die.
3. "Quote includes a 10-year warranty"
Most split systems carry a 5-year manufacturer warranty. "10 years" usually means 5 years on the compressor and 5 on parts — the marketing reads as one number. Always ask for the written warranty terms.
Position matters more than capacity
A 3.5 kW unit pointed the wrong way will struggle in a 16 m² room. Indoor heads need:
- Clear airflow — not blocked by wardrobe doors or curtain pelmets
- 300+ mm clearance from the ceiling for return air
- A position that throws air across the room, not into a corner
- Outdoor condenser within ~10 m pipe run for full efficiency
What we actually install in bedrooms
For Pakenham, Officer and Berwick bedrooms our default recommendation is the 3.5 kW Kaden KS35 or Mitsubishi MSZ-AP35. Quiet on low fan, R32 inverter, ARC-licensed install, $1,899–$2,499 supplied and fitted depending on pipe run.
If the room shares a wall with the kid's room and noise matters, step up to Mitsubishi (19 dB on low fan). If the budget's tight, the Kaden gets you there cleanly.
FAQ
Can a single split system cool two bedrooms?
Through a closed wall — no. Air won't redistribute without ducts. A multi-head system splits one outdoor unit between 2–4 indoor heads (one per room). Usually cheaper than two separate splits if both rooms need cooling.
What about heating with the same unit?
Reverse-cycle splits heat and cool. In SE Melbourne winters a 3.5 kW unit heats a bedroom to 22 °C from 5 °C in about 15 minutes. Cheaper per kWh than a gas heater after the first year.
Do I need a permit?
Not for a standard split system install in a freehold home. Strata properties usually need owners' corporation sign-off — we provide the install drawings if needed.