A homeowner uses 28 kWh per day and gets 5.2 peak sun hours in their area. Before the site inspection, you need an estimated system size and roof area requirement to see if it is even feasible.
Solar System Size
Solar
Check your power bill
AU avg: 4–5hrs · Darwin: 5.5 · Melbourne: 4
System kW = Daily kWh ÷ (Sun hours × Efficiency)
Reference: Clean Energy Council Australia
Results are estimates for planning purposes. Verify with current standards and a qualified professional.
1 What this calculator does
Estimates the required solar PV system size in kW from daily energy consumption, peak sun hours and system efficiency. Shows approximate panel count and roof area needed. Guides selection of the next standard system size.
2 Formula & professional reasoning
System size (kWp) = Daily energy use (kWh) / (Peak sun hours x System efficiency)
System efficiency: 0.75-0.85 (accounts for inverter, cable and temperature losses)
Panels = Ceiling(System kWp x 1000 / Panel watt rating)
Roof area = Panels x Panel area (m²) x 1.25 (25% spacing clearance)
A solar panel rated at 400W generates that power under Standard Test Conditions (STC). In real conditions, efficiency losses from heat (panels are less efficient when hot), inverter conversion and cable resistance reduce actual output to about 75-85% of the rated figure. Dividing the daily energy requirement by the effective daily generation (peak hours x efficiency) gives the required system size. Peak sun hours are location-specific -- Bureau of Meteorology provides data for Australian locations.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
Average household system size
Given: Daily use: 28 kWh | Peak sun hours: 5.2 | Efficiency: 0.80
Working:
System size: 28 / (5.2 x 0.80) = 28 / 4.16Answer: System size: 6.73 kWp -- round up to 6.6 kW or 10 kW standard system
💡 A 6.6 kW system (the most common residential size in Australia) uses approximately 16-17 x 400W panels and requires 30-35 m² of north-facing roof area.
Standard
Commercial building with high daytime consumption
Given: Daily use: 120 kWh | Peak sun hours: 4.8 | Efficiency: 0.78
Working:
System size: 120 / (4.8 x 0.78) = 120 / 3.744Answer: System size: 32.1 kWp -- commercial-scale system
💡 32 kW exceeds standard single-phase residential inverter limits (10 kW AU). Requires three-phase connection and multiple inverters. Site structural and electrical assessment required.
Advanced
Battery-backed system sizing with storage target
Given: Daily use: 18 kWh | Peak sun hours: 4.5 | Target: 80% self-sufficiency | Battery storage: 10 kWh
Working:
Solar covers daytime use + charges battery | System needs to generate approximately 18 x 1.4 to account for export and battery inefficiency = 25 kWh equivalent | System: 25/(4.5x0.80)Answer: System size: 6.94 kWp -- suggest 10 kW system with 10 kWh battery
💡 Battery sizing adds complexity. A detailed energy audit matching generation to consumption profile is recommended for battery-backed system design.
4 Sanity check
Peak sun hours by location (AU)
Darwin: 5.5-6.0 | Brisbane: 5.2-5.5 | Sydney: 4.5-5.0 | Melbourne: 4.0-4.5 | Hobart: 3.5-4.2
Use Bureau of Meteorology solar radiation data for precise location values.
Standard residential system sizes (AU)
Common: 6.6 kW | Large: 10 kW | Single-phase inverter limit: 10 kW | Three-phase inverter: up to 30 kW
Panel area allowance
Standard 400W panel: approximately 2.0 m² | Allow 25% extra spacing for racking and airflow = 2.5 m² effective per panel
Grid connection limits
Many networks limit single-phase export to 5 kW or 10 kW | Check with the network before specifying system size
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Using energy consumption from the electricity bill without adjusting for the bill period | Bill is in kWh for 90 days -- dividing by 90 gives daily average but seasonal variation is ignored | Summer or winter bias in the estimate | Use the lowest seasonal quarter for system sizing to avoid over-investing for self-sufficiency during low-production periods. Summer sizing will result in export in summer and shortfall in winter. |
| Not checking network export limits before specifying system size | Sizing on consumption without checking grid rules | Larger system installed but export is curtailed -- system never reaches payback | Check the network connection application requirements (DNSP rules) before committing to a system size. Many networks limit single-phase export to 5 kW even on a 10 kW system. |
| Using panel efficiency instead of system efficiency for the calculation | Confusing panel STC efficiency with overall system efficiency | System significantly undersized -- does not meet consumption targets | System efficiency (0.75-0.85) accounts for inverter conversion losses, cable resistance, temperature derating and soiling. Panel efficiency (typically 20-22%) is a separate specification used to calculate panel physical size, not system output. |
| Not checking roof orientation and tilt | Assuming all roof space is equally productive | North-facing assessment used for east-west orientation -- system produces significantly less | North-facing at the local latitude tilt is optimal in Australia. East-west split systems can be effective for self-consumption. South-facing panels in AU produce 30-40% less than north-facing -- rarely worthwhile. |
6 Reference & regulatory links
🇦🇺 Australia
7 Professional workflow
Common tools used alongside this one:
Solar Panel Count
Solar Panel Count Calculator determines the exact number of panels for the system size calculated here
→
Solar Inverter Sizing
Solar Inverter Sizing ensures the inverter is correctly sized for the panel capacity
→
Solar Savings & Payback
Solar Savings and Payback Calculator estimates annual savings and payback period for the sized system
→