Two numbers, and the smaller one wins
Sizing solar is not one calculation, it is two, and people only ever do the first.
The first is how much power you get through. Divide it by what a kilowatt of panels generates in a day where you live and you have the system your bill wants. The second is how many panels physically fit on the one roof face that is not shaded at two in the afternoon. Whichever number is smaller is the system you can actually have, and on a small or awkward roof it is very often the roof.
That is worth knowing before a salesperson quotes you a 10 kW system for a house with 20 square metres of north-facing tile.
Rough output per kW, by city
Averaged across a year, on a well-oriented roof. Winter is roughly half of summer everywhere, and a west-facing array gives up something to a north-facing one.
| City | Rough output | What 6.6 kW gives you a day |
|---|---|---|
| Alice Springs | 5.0 kWh per kW | ~33 kWh |
| Darwin | 4.4 kWh per kW | ~29 kWh |
| Perth | 4.4 kWh per kW | ~29 kWh |
| Canberra | 4.3 kWh per kW | ~28 kWh |
| Brisbane | 4.2 kWh per kW | ~28 kWh |
| Adelaide | 4.2 kWh per kW | ~28 kWh |
| Sydney | 3.9 kWh per kW | ~26 kWh |
| Melbourne | 3.6 kWh per kW | ~24 kWh |
| Hobart | 3.5 kWh per kW | ~23 kWh |
Two worked examples
Sydney, 18 kWh a day, 40 m² of usable roof, 440 W panels.
- Your use wants 18 ÷ 3.9 = 4.62 kW
- Panels for that 4,615 ÷ 440 = 10.5, rounded up = 11 panels
- Panels that fit 40 ÷ 2 = 20 panels, the roof is not the limit
- System 11 × 440 = 4.84 kW, using about 22 m² of your 40
- A year 4.84 × 3.9 × 365 = about 6,890 kWh
Melbourne, 25 kWh a day, only 20 m² of usable roof.
- Your use wants 25 ÷ 3.6 = 6.94 kW, which is 16 panels
- Panels that fit 20 ÷ 2 = 10 panels
- System 10 × 440 = 4.4 kW, the roof decided, not the bill
- That covers 4.4 × 3.6 = 15.8 kWh of your 25, about 63%
- The fix is more roof faces, higher-wattage panels, or accepting the shortfall, not a bigger inverter
How many solar panels do I need for 6.6, 10 or 15 kW?
Divide the system by the panel. At 440 W a panel, a 10 kW system is 23 panels and wants about 46 m² of one unshaded roof face. The rest of the table is the same sum, and the daily figures below use the city table above.
| System | 440 W panels | Roof |
|---|---|---|
| 6.6 kW | 15 | 30 m² |
| 10 kW | 23 | 46 m² |
| 13.2 kW | 30 | 60 m² |
| 15 kW | 35 | 70 m² |
What each one makes on an average day, in kWh.
| System | Syd | Melb | Bris | Perth |
|---|---|---|---|---|
| 6.6 kW | ~26 | ~24 | ~28 | ~29 |
| 10 kW | ~39 | ~36 | ~42 | ~44 |
| 13.2 kW | ~52 | ~48 | ~55 | ~58 |
| 15 kW | ~59 | ~54 | ~63 | ~66 |
Going the other way, a house using 20 kWh a day wants about 5.1 kW in Sydney, 5.6 kW in Melbourne and 4.8 kW in Brisbane, and one using 40 kWh a day wants about 10 kW. Whether a 10 or 15 kW system can connect at all is your distributor’s call, and a single phase home is often held to less.
Four things that change the answer
- Shade is not proportional. One shaded panel can drag a whole string down. Optimisers or microinverters exist for that reason, and a designer will tell you whether you need them.
- Orientation. North gives the most over a day. West gives less overall but more in the late afternoon, which matters if that is when you are home and paying peak rates.
- What you can export. Distributors set limits on what a home may connect and push back into the grid, they differ by network, and single-phase homes are usually capped lower than three-phase. Approval comes before installation, not after.
- Self-consumption beats system size. Power you use while the sun is up is worth far more than power you export. Shifting the dishwasher, the pool pump and the washing to the middle of the day changes the economics more than another two panels.
Who is allowed to install it
Grid-connected solar is licensed electrical work, and there is a second layer on top of it: the Clean Energy Council accreditation scheme that certificate entitlements depend on. An install signed off by someone without Clean Energy Council accreditation can leave you unable to claim, and unable to get the distributor to connect it.
Our solar install requirements checker walks through who may do the work, what approvals are needed and who signs off, state by state. Do that before you get quotes, not after.
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General information for Australian homeowners and trades, and an estimate only, not legal advice, and not electrical or engineering advice, and not a system design. Only a Clean Energy Council accredited designer can size a system for your roof, and your electricity distributor decides what may be connected and exported. The output-per-kW figures are rules of thumb, not the Clean Energy Regulator’s zone ratings, we could not read those from the Regulator’s site on 3 September 2026 and do not present these as theirs. Grid-connected solar is licensed electrical work and Clean Energy Council accreditation affects certificate entitlements. No regulator has reviewed this page.