What Size Air Conditioner Do I Need? Adelaide Sizing Guide
The Adelaide rule of thumb for sizing an aircon: 0.15kW per m² for a well-insulated room with normal 2.4m ceiling height. A 25m² living room sits in the 5kW band; a 50m² open-plan kitchen-living needs 7kW; a 12m² bedroom is comfortable on 2.5kW. That’s the baseline. From there, you add adjustments for the climate zone (Salisbury and the inland north get +20%; coastal Glenelg gets -10% on the cooling side), the orientation (west-facing rooms +10%), the insulation grade (poor insulation +10%), and the specific use (kitchens with cooking heat +5–10%).
This guide walks through the sizing calculation room-by-room and then sums up to whole-house ducted capacity. By the end, you should be able to put a kW number to your home before you talk to an installer — and importantly, recognise when an installer’s quoted capacity is undersized.
The base formula — kW per m²
For an insulated Adelaide room with 2.4m ceilings, in the metro band:
Required cooling kW = floor area (m²) × 0.15
Apply this to standard rooms:
| Room | Area (m²) | Base kW |
|---|---|---|
| Single bedroom | 12 | 1.8 → round up to 2.5kW |
| Double bedroom | 16 | 2.4 → 2.5kW |
| Master bedroom | 20 | 3.0 → 3.5kW |
| Small living room | 20 | 3.0 → 3.5kW |
| Medium living room | 30 | 4.5 → 5kW |
| Large living/dining | 45 | 6.75 → 7kW |
| Open-plan kitchen-living | 60 | 9.0 → 9kW |
Always round up to the next available capacity tier (2.5, 3.5, 5, 7, 9). Don’t round down — the difference between a 4.5kW theoretical requirement and a 3.5kW unit is the difference between a unit that catches up and a unit that struggles.
Adjustment 1 — Climate zone
Adelaide’s microclimate matters meaningfully on the cooling side. The adjustments:
| Zone | Suburbs | Adjustment | Why |
|---|---|---|---|
| Inland heat-load | Salisbury, Modbury, Mawson Lakes, Para Hills, Salisbury East | +20% | 41°C+ heatwave maxes, no sea-breeze relief |
| CBD / inner east / inner north | CBD, Norwood, Prospect, Walkerville, Burnside | 0% (baseline) | Urban heat-island, mild humidity |
| Coastal | Glenelg, Henley Beach, Brighton, Semaphore, West Lakes | -10% | Sea-breeze cools afternoons, lower peak max |
| Adelaide Hills | Stirling, Aldgate, Crafers, Mount Barker | -10% (cooling), +heating | Cool summer max, but heating-side capacity at 0–2°C ambient is the lead spec |
Apply to your room: a 30m² living room in Salisbury needs 4.5 × 1.20 = 5.4kW (so 7kW). The same room in Glenelg needs 4.5 × 0.90 = 4.05kW (so 5kW).
Adjustment 2 — Orientation and sun exposure
| Factor | Adjustment |
|---|---|
| West-facing room with significant afternoon sun | +10% |
| North-facing room with no shade | +5% |
| South-facing room (Adelaide southern hemisphere — minimal sun) | -5% |
| Room with external wall + roof exposure on multiple sides | +5–10% |
| Glass-heavy room (large windows, sliding doors) | +10% |
Apply: a 30m² west-facing Salisbury living room with a large sliding door = 4.5 × 1.20 × 1.10 × 1.10 = 6.5kW (so 7kW).
Adjustment 3 — Insulation grade
| Insulation | Adjustment |
|---|---|
| Modern insulated home (R-3.0+ ceiling, R-2.0+ wall, double-glazed) | -5% |
The glazing line in this table compounds fast. If you’re considering a window upgrade in parallel, get a quote from Adelaide window supply and installation specialists — premium uPVC double-glazing typically drops the calculated unit size by 25–40%, which often means a smaller, quieter, cheaper system.
| Standard 2000s+ insulated home (R-2.5 ceiling, single-glazed) | 0% | | 1980s-90s home with minimal insulation | +10% | | 1920s-1940s heritage with no insulation, single brick | +15% |
The heritage insulation premium is real. A Norwood bungalow with no ceiling insulation and single-brick walls genuinely needs 15% more capacity than the same room in a 2010-built home.
Adjustment 4 — Use and load
| Factor | Adjustment |
|---|---|
| Kitchen with cooking + appliances | +5–10% |
| Home office with high-power computers/multiple screens | +5% |
| Room with 4+ regular occupants | +5% per extra person beyond 2 |
| Sunroom with all-glass walls | +20–25% |
Worked examples
Example 1: Burnside heritage 4-bedroom
| Room | Area | Base kW | Adjustments | Final kW |
|---|---|---|---|---|
| Master bedroom (S-facing) | 20m² | 3.0 | -5% (south) +15% (heritage no insulation) = +10% | 3.3 → 3.5kW |
| Bedroom 2 (W-facing) | 14m² | 2.1 | +10% (west) +15% (heritage) = +25% | 2.6 → 3.5kW |
| Bedroom 3 | 12m² | 1.8 | +15% (heritage) | 2.07 → 2.5kW |
| Living room (W-facing, large windows) | 35m² | 5.25 | +10% (west) +10% (windows) +15% (heritage) = +35% | 7.1 → 7kW |
| Kitchen-dining | 25m² | 3.75 | +5% (kitchen) +15% (heritage) = +20% | 4.5 → 5kW |
Whole-house ducted capacity: sum of room loads + 15% diversity factor (assumes not every zone runs at peak simultaneously) = (3.5+3.5+2.5+7+5) × 0.85 = 18.3kW. So push for 18-20kW ducted capacity, 5 zones.
Example 2: Salisbury 4-bedroom modern
| Room | Area | Base kW | Adjustments | Final kW |
|---|---|---|---|---|
| Master bedroom | 18m² | 2.7 | +20% (heat-load) | 3.24 → 3.5kW |
| Bedrooms 2-3 (W-facing) | 12m² each | 1.8 | +20% (heat-load) +10% (west) = +30% | 2.34 → 2.5kW each |
| Bedroom 4 | 12m² | 1.8 | +20% (heat-load) | 2.16 → 2.5kW |
| Living room | 30m² | 4.5 | +20% (heat-load) | 5.4 → 7kW |
| Open-plan kitchen-family | 50m² | 7.5 | +20% (heat-load) +5% (kitchen) = +25% | 9.4 → 9kW |
Whole-house ducted capacity: (3.5+2.5+2.5+2.5+7+9) × 0.85 = 23.0kW. Push for 22-24kW ducted, 5-6 zones. This is what we mean by the “Salisbury home needs 18kW where Burnside needs 14kW” rule of thumb — the inland heat-load corridor genuinely runs +20% across every room.
Example 3: Glenelg coastal cottage
| Room | Area | Base kW | Adjustments | Final kW |
|---|---|---|---|---|
| Master bedroom | 14m² | 2.1 | -10% (coastal cooling) +15% (heritage) = +5% | 2.21 → 2.5kW |
| Bedroom 2 | 12m² | 1.8 | -10% (coastal) +15% (heritage) = +5% | 1.89 → 2.5kW |
| Bedroom 3 | 10m² | 1.5 | +5% | 1.58 → 2.5kW |
| Living room | 25m² | 3.75 | -10% +15% = +5% | 3.94 → 5kW |
Multi-head total capacity: (2.5+2.5+2.5+5) = 12.5kW. Going through a 4-head outdoor compressor at 12.5kW combined indoor capacity — typical Glenelg multi-split spec. The coastal-rated coil treatment is the other essential spec — see the Glenelg location page.
Ducted-specific sizing — the diversity factor
For ducted systems, you don’t simply add up the per-room loads. You apply a diversity factor of 0.80-0.85 because not every zone runs at peak load simultaneously:
- High diversity (4-bedroom home, zones rarely all run together) — factor 0.80
- Medium diversity (4-bedroom with master + living dominant) — factor 0.85
- Low diversity (open-plan with very few zones) — factor 0.90
The diversity factor lets you spec a 14kW outdoor unit on a home with 18kW of total per-room load, because the realistic simultaneous demand sits around 14kW.
Heating-side sizing — the cold-climate rule
For Adelaide Hills, foothills (Mitcham, Belair) and inner-east cold-morning installs, the cooling-side sizing isn’t enough — you need to confirm the unit’s rated heating capacity at 2°C ambient (not the 7°C nameplate).
The rule: the unit’s rated heating kW at 2°C ambient must equal or exceed the room’s cooling kW requirement.
Standard inverter splits derate roughly 25-35% from nameplate to 2°C performance. Cold-climate inverters (Mitsubishi Hyper Heating FH, Daikin Ururu Sarara) hold rated capacity to -15°C ambient — so a 7kW Hyper Heating unit delivers 7kW at 2°C.
For Stirling and Mount Barker installs, this is the spec to push on. See the Stirling location page and Mount Barker location page.
When the calculation flags an oversize problem
Oversizing is the second-most-common Adelaide install regret (after undersizing). An oversized unit:
- Cycles short — turns on, drops the temperature too quickly, turns off, room temperature swings widely
- Doesn’t dehumidify properly — runs too briefly to remove moisture from the air
- Costs more upfront and more in service
If your room calculation lands between two capacity tiers (say 4.7kW theoretical), round up to the next tier — but don’t double-up. A 5kW unit in a room that actually needs 4.7kW is fine; a 7kW unit in the same room is oversized.
Service and load adjustments — when the numbers change
The sizing numbers above are correct for new installs in current Adelaide conditions. They change in two situations:
- Existing aircon being replaced — match capacity to the existing unit only if the existing unit was correctly sized to begin with. Many Adelaide replacements are upsizing a previously-undersized unit.
- Climate change projections — Adelaide’s heat-load is increasing year-on-year. For a system you’ll keep for 15 years, building in 5-10% extra capacity headroom is a sensible hedge.
Frequently asked questions
Can I just use the room area to pick a unit? Almost — the 0.15kW per m² baseline is a solid starting point. The climate-zone, orientation, and insulation adjustments matter for accuracy but the baseline gets you 80% of the way.
What if I’m between two capacity tiers? Round up. A theoretical 4.7kW requirement gets a 5kW unit, not a 3.5kW. Undersizing is a bigger problem than slight oversizing.
Do I need to size for heating or cooling? Cooling sizing is usually the binding constraint in Adelaide — except for the Hills and foothills where heating-at-low-ambient becomes the dominant spec. For most metro installs, size for cooling and confirm the heating-side rated kW at 2°C is sufficient.
Why does Salisbury need 20% more capacity than Burnside? Heat-load. The Western Plains run 5-7°C hotter on peak summer afternoons than the CBD. A unit sized for Burnside conditions cycles constantly in Salisbury and never quite catches up. The +20% rule is real and supported by Bureau of Meteorology data.
Will you do this calculation? Yes — we run the sizing calculation as part of the free quote process. The point of this guide is so you can sanity-check the quoted capacity before you sign off.
What about ceilings higher than 2.4m? Add 5% per 0.3m of additional ceiling height. A 3.0m ceiling room needs 10% more capacity than a 2.4m equivalent.
Does the calculation work for ducted? Yes — sum the per-room loads, apply the diversity factor (0.80-0.85), and you have your outdoor unit capacity. Pair it with appropriate zone count (one zone per 2-3 rooms typically).
Ready for a written, line-itemed aircon quote with proper sizing?
Submit the quote form — we run the sizing calculation against your specific rooms, climate zone and use pattern. A written, line-itemed quote, usually within 24–48 hours.