Air conditioning
in Derrimut
A 30 square metre living room in Derrimut needs about 4.14kW, against 3.11kW for the same room in Portland, because Derrimut peaks at 35.2°C where Portland hits 29.1°C. Installed prices here run $1,450–$3,700 for splits.
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Why Derrimut is different
Sizing advice written for "Melbourne" is written for an average that barely exists. Derrimut accumulates 49.7 cooling degree days a year and clears 35°C about 11.3 days. Portland, across Melbourne, accumulates 10.1 and clears 35°C about 2.5 days — roughly 5 times the cooling demand. That is why the same room needs a different size unit depending which suburb it sits in, and why a number quoted for the metro average will be wrong for most addresses in it.
What size you need in Derrimut
| Room | Approx size | Cooling needed | Unit to look for |
|---|---|---|---|
| Bedroom | 12m² | 1.66kW | 2kW |
| Large bedroom or study | 18m² | 2.49kW | 2.5kW |
| Small living area | 25m² | 3.45kW | 3.5kW |
| Living room | 30m² | 4.14kW | 4.2kW |
| Open-plan living and dining | 45m² | 6.22kW | 6kW |
| Large open plan | 60m² | 8.29kW | 8kW |
Sizing follows the design criterion AIRAH uses in its FairAir calculator: the 3pm outdoor temperature exceeded on no more than 10 days a year. We compute that figure for each suburb from Bureau of Meteorology daily records, then split the cooling load into a component that scales with how far the design temperature sits above 24C and a component from sun and internal heat that barely varies by location. That 50/50 split is our assumption, not AIRAH's. Treat the result as a starting point for a quote, not a substitute for an installer measuring your rooms.
What it costs to run here
Derrimut sits in the Powercor network area, where a residential flat rate currently runs about 28.22c per kilowatt hour (AGL Residential Standing Offer). On that rate, a typical 3.5kW split system used normally costs roughly $30 a year in cooling, against about $6 for the identical unit in Portland.
That figure starts from the appliance's registered energy rating data, then scales it by Derrimut's own cooling degree days against the reference site the rating uses (Richmond RAAF (067105), 113 degree days). Without that adjustment every suburb in the zone would show an identical number, which would tell you nothing.
On a time-of-use plan in this network, electricity between 15:00 to 21:00 costs about 40.4c against 23.65c off-peak. Cooling the house hard through that window is where summer bills actually get away from people. Pre-cooling before it starts costs noticeably less.
What changes a quote in Derrimut
You probably need less than you think
Derrimut accumulates only 49.7 cooling degree days a year and clears 35°C about 11.3 days. Oversizing is the common and expensive mistake here: too much capacity short-cycles, dehumidifies poorly and costs more upfront and to run. If an installer quotes a size up without measuring the room, ask them to justify it.
Winter matters as much as summer
With 1400 heating degree days and July mornings averaging 5.7°C, a reverse-cycle system in Derrimut earns its keep for heating at least as much as cooling. Ask for the heating output at low ambient temperature rather than the nominal kW, because that figure is what determines whether it copes on the coldest mornings.
Ducted is genuinely viable here
94.8% of Derrimut dwellings are separate houses, most with accessible roof space, which makes ducted a real option rather than a stretch. The check that matters is roof cavity height and insulation condition, and any installer can confirm both in a five-minute inspection before quoting.
Family-sized homes, zoned systems
Average household size in Derrimut is 3.5 people, and with mostly detached housing that usually means bedrooms in use at different times to living areas. Zoning a ducted system, or splitting across multiple heads, tends to beat one large unit on running cost even where the upfront number is similar.
Newer housing, simpler installs
A median age of 32 alongside 94.8% detached housing points to a suburb built relatively recently. That usually means modern switchboards with capacity to spare, accessible roof cavities and straightforward pipe routes, all of which keep labour down. It is one reason quotes in Derrimut tend to cluster more tightly than in older areas.
Installed prices
| System | Installed price | Suits |
|---|---|---|
| Small split (2.5kW) | $1,450–$2,300 | Bedrooms and studies |
| Large split (5-7kW) | $2,400–$3,700 | Living areas |
| Ducted, whole home | $8,000–$16,000 | Houses with roof access |
Melbourne pricing. See the full Melbourne cost guide for a breakdown by job, or the Victoria rebate guide for discounts that come off these figures.
About Derrimut
Around 8,859 people live in Derrimut (estimated for 2025), across roughly 2,368 occupied dwellings. The housing mix is 0% apartments, 94.8% separate houses and 3% semi-detached or townhouses, which is what drives most of the practical differences in getting a system installed here.
Nearby suburbs
Climate, network tariffs and housing stock shift across a metro, so the sizing and running costs on these pages differ from Derrimut's.
Sources and data
- ABS Census 2021, dwelling structure and medians by postcode
- ABS Regional population 2024-25 (population estimate basis)
- Bureau of Meteorology, Climate Data Online
- Australian Building Codes Board climate zone map
- AIRAH FairAir cooling calculator (design condition methodology)
- Consumer Data Right energy plan data (network and tariffs)
- energyrating.gov.au product database (zoned energy rating data)
Based on Australian Bureau of Statistics data. Source: ABS, Census of Population and Housing 2021 and Regional population 2024-25, licensed under CC BY 4.0. Climate data: Bureau of Meteorology, licensed under CC BY 4.0. Climate zones: Australian Building Codes Board. Tariffs: Consumer Data Right energy plan data. Appliance energy data: energyrating.gov.au (E3 Program).
Population is an estimate derived from the 2021 Census count scaled by regional growth to 2025, not an official count. Climate figures come from LAVERTON RAAF (087031). Nearest qualifying station (still reporting, 82.9 yr of record, 99% complete). LAVERTON RAAF (087031) is 6.6 km away, 5.5 km from the open coast against the suburb's 9.2 km, at 20 m against the suburb's 46 m. Stations are ranked on distance plus penalties for differing coastal exposure, elevation, NCC climate zone, record length and marine (sand-spit or island) siting.