How Many Solar Panels Do I Need?
Free Australian calculator to size your solar system. Uses CEC-aligned irradiance, average household consumption (6,500 kWh/yr), and 440 W panel specs typical of 2026 installs.
How Many Solar Panels Do I Need?
How to use this calculator
Enter five figures and the tool returns the system size in kW, panel count, required roof area, and estimated annual production:
- Monthly electricity use (kWh) — pull this from your last 12 retailer bills. AER’s 2026 benchmark for a 4-person Sydney household sits at 540 kWh/month; Melbourne 510; Perth 600; Brisbane 580.
- Peak sun hours per day — Bureau of Meteorology and CEC publish regional figures: Brisbane 4.8, Perth 5.4, Sydney 4.5, Melbourne 4.0, Hobart 3.8.
- Panel wattage (W) — leave at 440 W. Trina Vertex S+ and Jinko Tiger Neo are the dominant CEC-approved choices in 2026.
- System efficiency (%) — 78% for a typical north-facing pitched-roof install with a string inverter. Drop to 75% for hot inland sites (Mildura, Alice Springs) where ambient temperature pushes panel temperatures over 65°C in summer.
- Offset target (%) — 100% covers your annual use. Going beyond rarely pays under two-way tariffs.
The formula
required_kW = (annual_kWh × offset / 100) ÷ (peak_sun_hours × 365 × derate)
panel_count = ceil(required_W ÷ panel_wattage)
roof_area_m² = panel_count × panel_area × 1.08 (mounting clearance)Worked example for a Brisbane household:
- Monthly use: 720 kWh → annual 8,640 kWh
- Peak sun hours: 4.8 (BOM data for Brisbane)
- Derate: 78%
- Required: 8,640 ÷ (4.8 × 365 × 0.78) = 6.32 kW
- Rounded to standard 6.6 kW: 15 panels at 440 W
- Roof area: 15 × 2.16 m² = 32 m²
- Annual production: 6.6 × 4.8 × 365 × 0.78 = 9,015 kWh (104% offset)
Reference table by Australian capital
Using 78% derate, 440 W panels, 100% offset, average household 540 kWh/month:
| City | PSH | System size | Panels | Roof area |
|---|---|---|---|---|
| Brisbane | 4.8 | 4.7 kW | 11 | 24 m² |
| Sydney | 4.5 | 5.1 kW | 12 | 26 m² |
| Adelaide | 4.6 | 5.0 kW | 12 | 26 m² |
| Perth | 5.4 | 4.2 kW | 10 | 22 m² |
| Melbourne | 4.0 | 5.7 kW | 13 | 28 m² |
| Hobart | 3.8 | 6.0 kW | 14 | 30 m² |
| Darwin | 5.7 | 4.0 kW | 10 | 22 m² |
Note: most Australians install the next-larger 6.6 kW system regardless of the calculation, to maximise the inverter export ratio and STC rebate per dollar invested.
What changes the panel count
Orientation in the southern hemisphere
In Australia the optimum is north-facing. CEC orientation multipliers for a 25–35° pitch:
- North: 1.00 (reference)
- North-east or north-west: 0.96
- East or west: 0.85
- South: 0.65 (rarely worth installing)
A north-east plus north-west split (common on hipped roofs) typically loses about 5% versus a single north-facing array but doubles the available area, which usually allows a 50%+ larger system.
Climate zone and temperature derating
Australian Standard AS/NZS 4509.2 publishes temperature derating factors. In Cairns or Darwin, panel cell temperatures routinely exceed 65°C in summer, knocking output down 15%+. Sydney’s coastal cooling brings effective derate closer to 80%; inland Mildura and Alice Springs sit around 73–75%. The calculator’s 78% default is a national average.
STC rebate and system pricing
STCs in 2026 are valued at $36–39 per certificate. A 6.6 kW system in Zone 3 (Sydney) earns 76 STCs — about $2,800 off the upfront price. Pricing through hipages and Service.com.au averages $5,200–6,800 fully installed for 6.6 kW after STC. Going to 10 kW (typical for split-system air conditioning + EV households) lifts price to $8,500–11,000 after STC.
Battery storage and the federal Cheaper Home Batteries Program
The 2026 federal program (replacing several state schemes) offers up to $2,300 per kWh of installed battery, up to 50 kWh per household. With a battery, oversize the array by 15–20% so the battery can charge from solar in winter rather than from grid. The solar battery bank sizing calculator walks through the matching logic.
Common Australian mistakes
- Sizing off a summer bill. Air-con loads inflate January and February consumption 50%+. Always average 12 months from your retailer’s online portal.
- Ignoring the inverter export cap. Sizing 10 kW DC behind a 5 kW inverter without applying for higher export through the DNSP wastes 30%+ of generation in summer.
- Cheap panels without 25-year output warranty. CEC’s approved-product list still includes Tier 3 panels with 80% year-25 warranties (vs. Tier 1 at 88–92%). Cost difference at install is often only $300–500 — but lifetime kWh difference is 8,000–12,000.
- Skipping the STC paperwork window. STCs must be claimed within 12 months of install. Reputable CEC installers handle this; backyard installs often forfeit the rebate.
How this differs from a CEC installer’s quote
A CEC-accredited designer uses Aurora, PVsyst, or Helioscope tied to your exact roof — measured azimuth, pitch, BOM TMY irradiance for your postcode, and 8,760-hour shading simulation. This calculator uses CEC regional averages and AS/NZS standard losses, so expect ±10% variance. Use it for budgeting and as a fit check before booking a CEC installer via Service.com.au, hipages, or SolarQuotes.
Sources
- Clean Energy Council CEC Approved Products — panel and inverter approved lists
- Australian Energy Regulator AER 2026 Benchmarks — household consumption by state
- Bureau of Meteorology Solar Exposure Data — peak sun hours by postcode
- Clean Energy Regulator STC Calculator — federal rebate certificate counts
- SolarQuotes Industry Reports 2026 — installer pricing and panel-mix data
- AS/NZS 4777.2:2020 — inverter grid-connect requirements