Air conditioning
in St Marys
A 30 square metre living room in St Marys needs about 4.58kW, against 3.15kW for the same room in Kiama, because St Marys peaks at 37.8°C where Kiama hits 29.3°C. Installed prices here run $1,350–$3,500 for splits.
Get St Marys quotes
Up to 3 licensed installers who work your postcode. Free, no obligation.
Why St Marys is different
Sizing advice written for "Sydney" is written for an average that barely exists. St Marys accumulates 153.5 cooling degree days a year and clears 35°C about 22.8 days. Kiama, across Sydney, accumulates 21.5 and clears 35°C about 1.8 days — roughly 7 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 St Marys
| Room | Approx size | Cooling needed | Unit to look for |
|---|---|---|---|
| Bedroom | 12m² | 1.83kW | 2kW |
| Large bedroom or study | 18m² | 2.75kW | 2.5kW |
| Small living area | 25m² | 3.82kW | 3.5kW |
| Living room | 30m² | 4.58kW | 4.2kW |
| Open-plan living and dining | 45m² | 6.88kW | 7.1kW |
| Large open plan | 60m² | 9.17kW | 9.5kW |
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
St Marys sits in the Endeavour network area, where a residential flat rate currently runs about 30.66c per kilowatt hour (ENGIE NSW ENGIE Standing elec). On that rate, a typical 3.5kW split system used normally costs roughly $100 a year in cooling, against about $14 for the identical unit in Kiama.
That figure starts from the appliance's registered energy rating data, then scales it by St Marys'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 16:00 to 19:59 costs about 42.34c against 32.45c 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 St Marys
Sizing for 37.8°C, not for the average
St Marys reaches 37.8°C on its hottest days and clears 35°C about 22.8 days a year. Every system loses output as the outdoor temperature climbs, so a unit specified against the nominal rating will disappoint exactly when you need it. Ask what the unit actually delivers at 40°C rather than at its test conditions.
Newer housing, simpler installs
A median age of 34 alongside 70% 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 St Marys tend to cluster more tightly than in older areas.
Installed prices
| System | Installed price | Suits |
|---|---|---|
| Small split (2.5kW) | $1,350–$2,000 | Bedrooms and studies |
| Large split (5-7kW) | $2,100–$3,500 | Living areas |
| Ducted, whole home | $6,500–$12,000 | Houses with roof access |
Sydney pricing. See the full Sydney cost guide for a breakdown by job, or the New South Wales rebate guide for discounts that come off these figures.
About St Marys
Around 14,229 people live in St Marys (estimated for 2025), across roughly 12,481 occupied dwellings. The housing mix is 8.7% apartments, 70% separate houses and 21% 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 St Marys'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 PENRITH LAKES AWS (067113). Nearest qualifying station (still reporting, 31.0 yr of record, 98% complete). PENRITH LAKES AWS (067113) is 9.7 km away, 57.1 km from the open coast against the suburb's 47.5 km, at 25 m against the suburb's 26 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.