A 10 kW solar system has been approved. The installer needs to confirm exactly how many 400W panels are required and the total roof area needed before the racking order is placed.
Solar Panel Count
Solar
Panels = System kW × 1000 ÷ Panel watts
Roof area includes 25% clearance around panels for access and cooling.
Reference: Clean Energy Council Australia
Results are estimates for planning purposes. Verify with current standards and a qualified professional.
1 What this calculator does
Calculates the number of solar panels required for a target system size. From panel wattage and system size in kW, gives panel count, actual system size (since panels come in whole numbers), and roof area needed including spacing clearance.
2 Formula & professional reasoning
Panels = Ceiling(System size kWp x 1000 / Panel watt rating)
Actual system size (kWp) = Panels x Panel watt rating / 1000
Roof area = Panels x Panel area per panel (m²) x 1.25 spacing clearance
Panels come in whole numbers, so the ceiling function is used to round up to the next full panel count. This means the actual installed system may be slightly larger than the target. For example, a 10 kW system using 400W panels: 10,000 / 400 = 25 panels exactly. If the panel is 390W: ceiling(10,000/390) = 26 panels = 10.14 kW actual. The 25% spacing allowance around panel dimensions accounts for racking rails, inter-panel gaps and roof edge clearances.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
10 kW system using 400W panels
Given: System: 10 kW | Panel: 400W | Panel size: 2.0 m²
Working:
Panels: ceiling(10000/400) = 25 panels | Actual: 25 x 400 = 10,000W = 10.0 kW | Roof area: 25 x 2.0 x 1.25 = 62.5 m²Answer: 25 panels | 10.0 kW system | 62.5 m² roof area
💡 25 panels is a clean calculation. Confirm the roof has at least 63 m² of usable north-facing area. Typical residential roof pitch gives 8-12 m of roof run with 6-8 m available -- most standard houses can fit 20-25 panels.
Standard
6.6 kW system using 415W panels
Given: System: 6.6 kW | Panel: 415W
Working:
Panels: ceiling(6600/415) = ceiling(15.9) = 16 panels | Actual: 16 x 415 = 6,640W = 6.64 kW | Area: 16 x 2.0 x 1.25 = 40.0 m²Answer: 16 panels | 6.64 kW actual | 40 m² roof area
💡 16 panels at 415W gives 6.64 kW -- slightly above the 6.6 kW target, which is standard practice. Check the inverter maximum input does not restrict this.
Advanced
Two-string system east-west split
Given: System: 10 kW | Panel: 400W | East string: 12 panels | West string: 13 panels | Total: 25 panels
Working:
East: 12 x 400 = 4.8 kW | West: 13 x 400 = 5.2 kW | Total: 10.0 kW | Each string needs separate MPPT input on the inverterAnswer: 25 panels total | East 12 | West 13 | Inverter must have dual MPPT
💡 East-west split is used where north-facing roof area is insufficient. Generation is more spread throughout the day but total peak output is lower. Confirm inverter supports dual MPPT inputs and the string sizing matches the inverter specifications.
4 Sanity check
Standard panel sizes
Typical residential panel: 1.7m x 1.1m = 1.87 m² | Large commercial: 2.0m x 1.0m = 2.0 m² | Confirm with specific product data sheet
Inverter to panel ratio (AS/NZS 4777.1)
Maximum panel DC capacity = 1.33 x inverter AC capacity | A 10 kW inverter can have up to 13.3 kWp of panels
Panel count impacts string design
String voltage must remain within inverter MPPT range | Number of panels per string depends on panel Voc and Vmp
Structural load
Each panel weighs 20-25 kg | Total system weight on 25 panels: 500-625 kg plus racking | Verify roof structure can take this load
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Not checking panel DC capacity against inverter AC rating | Selecting panel count based only on kWp without checking inverter ratio | DC input exceeds inverter rating -- inverter clips and refuses to operate above its AC rating | AS/NZS 4777.1 compliance issue | Maximum DC input = 1.33 x inverter AC rating (under AS/NZS 4777.1). A 10 kW inverter accepts up to 13.3 kWp of panels. Stay within this ratio. |
| Not verifying available roof area before confirming panel count | Calculating panel count without on-site measurement | Panel count exceeds available roof space -- system must be redesigned | Measure the available usable roof area during the site inspection. Account for setbacks from roof edges (300-600mm), obstructions (vents, skylights, air conditioning units) and shadowing from parapet walls. |
| Not accounting for string mismatch in east-west configurations | Treating east and west panels as a single string | Lower-producing string drags down higher-producing string -- significant generation loss | East and west-facing panels must be on separate MPPT inputs. Most modern string inverters have dual MPPT, but confirm the inverter specification matches the design. |
| Using roof pitch area instead of plan area for roof area calculation | Measuring along the slope rather than the plan view | Roof area appears larger than the horizontal footprint -- incorrect panel count for the available space | Solar generation calculations use horizontal (plan) area. A 10m roof run at 22-degree pitch has a plan area of 10 x cos(22°) = 9.27m. Use plan dimensions, not slope dimensions, for panel count feasibility checks. |
6 Reference & regulatory links
🇦🇺 Australia
7 Professional workflow
Common tools used alongside this one:
Solar System Size
Solar System Size Calculator determines the target kW before panel count is calculated here
→
Solar Inverter Sizing
Solar Inverter Sizing verifies the inverter capacity matches the panel DC capacity calculated here
→
Solar Savings & Payback
Solar Savings and Payback uses the confirmed panel count and system size to project annual savings
→