Quick answer: It depends on your electricity usage, not the panel count. As a worked example, a Sydney household using 20kWh a day needs roughly a 6.6kW system — about 15–16 panels at 415–440W — but your own bill is what settles it.
The honest answer is: it depends on your electricity usage, not on what size system your neighbour bought or what a doorknocker quoted you on the spot. As a rough anchor, a Sydney household using 20kWh a day lands on about a 6.6kW system — roughly 15–16 panels — but the whole point of this guide is to walk through the actual calculation so you can sanity-check any quote against your real numbers.
The single most useful number you have is sitting on your electricity bill already: your kWh usage, usually shown as a daily average or a quarterly total. This is the starting point for any proper sizing calculation — everything else (panel count, system size, battery size) flows from it.
Pull your last 12 months of bills if you can, or at minimum your most recent quarterly bill. Look for:
| Household type | Typical daily usage |
|---|---|
| 1–2 people, no pool, gas cooking/hot water | 8–14 kWh/day |
| 3–4 people, electric hot water | 16–22 kWh/day |
| 4–5 people, pool pump, ducted air conditioning | 25–35 kWh/day |
| Large household with EV charging at home | 30–45+ kWh/day |
These are ballpark figures — your actual usage could sit outside these ranges, which is exactly why checking your own bill matters more than any table.
A simplified version of what a proper design tool does:
Daily usage (kWh) ÷ average daily sun hours for your area ÷ system derating factor ≈ required system size (kW)
For Sydney, a north-facing, unshaded roof typically gets the equivalent of around 4–4.5 peak sun hours per day on average across the year (more in summer, less in winter). Real-world systems also produce somewhat less than their rated capacity due to inverter losses, wiring losses, temperature effects and panel orientation — a reasonable derating factor is roughly 0.78–0.85.
Worked example: a household using 20kWh/day → 20 ÷ 4.2 ÷ 0.80 ≈ 5.95kW, which rounds to a 6.6kW system in practice (6.6kW is the most common "step" size in Australia, largely because it sits at the boundary of typical single-phase inverter export limits).
Once you know your target kW, divide by the wattage of the specific panel being quoted:
| Target system size | Panels at 415W | Panels at 440W | Panels at 470W |
|---|---|---|---|
| 6.6kW | 16 panels | 15 panels | 14 panels |
| 8.8kW | 21 panels | 20 panels | 19 panels |
| 10kW | 24 panels | 23 panels | 21 panels |
| 13.2kW | 32 panels | 30 panels | 28 panels |
This is why panel count alone is a meaningless number to compare between quotes — 20 panels from one installer could be a smaller system than 16 panels from another if the wattage differs. Always compare system size in kW, not panel count.
A north-facing roof is the benchmark. East or west-facing roofs typically produce roughly 15–20% less annual output than true north for the same panel count; south-facing is materially worse again and rarely recommended as a primary array in the southern hemisphere. Shading from trees, neighbouring buildings or your own roof features (chimneys, vents) can cut output further and may steer you toward microinverters or panel-level optimisers (see our inverter guide).
A standard panel is roughly 1.9m x 1.1m (around 2.1m²). A 6.6kW system (roughly 15–16 panels) needs approximately 32–35m² of usable, unshaded roof area. Hip roofs, dormers, and multiple small roof planes can make fitting a larger system trickier than the raw area suggests — this is where higher-efficiency monocrystalline panels (see our mono vs poly guide) earn their keep.
A pattern we'd flag: many households size solar purely against their current bill, then a year or two later add a pool, an EV, or ducted air conditioning, and find their "perfectly sized" system now only covers 60% of usage. Because the incremental cost of a few extra panels at installation time is much lower than adding a second array later, it's worth sizing with a 3–5 year view of your household, not just where you are today.
Ask for these three numbers on any quote you receive, and compare them against your own bill:
If an installer can't answer question 3 with a number specific to your roof — not a generic "6.6kW systems produce about 26kWh/day" — they likely haven't done a proper design. Any installer you go with should also be CEC-accredited, which is a legal requirement for STC eligibility.
It depends on your daily kWh usage. As a worked example, a Sydney home using 20kWh/day needs about a 6.6kW system — roughly 15–16 panels at 415–440W. Use the formula (daily usage ÷ ~4.2 sun hours ÷ ~0.80 derating) with your own bill figure to get your number.
No. Compare system size in kW, never panel count. Because panel wattages differ (415W, 440W, 470W), 20 panels from one installer can be a smaller system than 16 panels from another.
Roughly 32–35m² of usable, unshaded roof area for about 15–16 standard panels (each around 1.9m x 1.1m). Hip roofs, dormers and multiple small roof planes can make fitting a larger system trickier than the raw area suggests.
Often, yes. Many households later add a pool, EV or ducted air conditioning and find a system sized only to today's bill now covers 60% of usage. Extra panels are cheap at install time compared with adding a second array later, so sizing with a 3–5 year view usually pays off.
Get an accurate, roof-specific sizing recommendation rather than a rough estimate — try Blue Energy Solar's free Solar & Battery Calculator, built around your actual usage and address, or call 0421 458 217 or email sales@blueenergysolar.com.au to talk to a Sydney-based installer.
