Calculator Inputs
Example Data Table
Illustrative sample values in metric units.
| Roof Length | Roof Width | Faces | Slope | Obstacle Area | Coverage | Panel Size | Panel Count | Estimated DC Size | Estimated Annual Energy |
|---|---|---|---|---|---|---|---|---|---|
| 12 m | 8 m | 2 | 22° | 6 m² | 85% | 1.722 × 1.134 m | 69 | 37.95 kW | 54,660 kWh/year |
Formula Used
1) Gross Plan Roof Area
Gross Plan Area = Roof Length × Roof Width × Roof Faces
This is the flat projected roof footprint before slope correction.
2) Gross Sloped Roof Area
Gross Surface Area = Gross Plan Area ÷ cos(Roof Slope)
Slope increases the real roof surface available compared with plan view.
3) Setback-Adjusted Area
Effective Length = Roof Length − Left Setback − Right Setback
Effective Width = Roof Width − Top Setback − Bottom Setback
Setback Surface Area = (Effective Length × Effective Width × Roof Faces) ÷ cos(Roof Slope)
4) Usable Installation Area
Obstacle-Free Area = Setback Surface Area − Obstruction Area
Usable Installation Area = Obstacle-Free Area × Coverage Factor
Coverage factor accounts for design margins, access, and spacing losses.
5) Panel Count and DC Size
Panel Area = Panel Length × Panel Width
Panel Count = floor(Usable Installation Area ÷ Panel Area)
DC System Size = Panel Count × Panel Power ÷ 1000
6) Energy Estimate
Annual Energy = DC System Size × Peak Sun Hours × 365 × Performance Ratio × Inverter Efficiency × (1 − Shading Loss)
This is a planning estimate, not a final engineering or utility production guarantee.
7) Theoretical Module Power
Theoretical Module Power = Panel Area × 1000 × Module Efficiency
This compares entered efficiency against rated panel wattage under standard test conditions.
How to Use This Calculator
- Choose metric or imperial units before entering dimensions.
- Enter roof length, width, slope, and the number of similar roof faces.
- Fill in all fire, maintenance, and installation setback distances.
- Enter total obstruction area for vents, skylights, chimneys, or blocked regions.
- Set the usable coverage factor to reflect spacing, layout losses, and access paths.
- Enter panel dimensions and panel wattage. You may also rely on efficiency instead.
- Enter sun hours, performance ratio, inverter efficiency, and expected shading loss.
- Click the calculate button to display area metrics, panel count, DC size, and estimated annual energy above the form.
- Use the CSV or PDF buttons to export your results for reporting or comparison.
FAQs
1) What does this calculator estimate?
It estimates gross sloped roof area, setback-adjusted area, usable installation area, module count, DC system size, and annual energy output for a proposed solar layout.
2) Why is roof slope included?
Plan dimensions show the roof footprint, but panels sit on the sloped surface. Dividing by cosine of slope converts plan area into actual roof surface area.
3) What is the coverage factor?
Coverage factor reduces theoretical area to a practical design area. It captures spacing, mounting clearances, edge rules, and conservative planning assumptions.
4) Can I use feet instead of meters?
Yes. Switch the unit system to imperial. The calculator converts inputs internally and returns areas in square feet and lengths in feet.
5) What if I only know panel efficiency?
Enter panel dimensions and efficiency. If wattage is zero, the calculator derives approximate module power from area and efficiency under standard test assumptions.
6) Is annual energy guaranteed?
No. It is a planning estimate. Real production depends on irradiance, orientation, weather, temperature, soiling, inverter behavior, and site-specific losses.
7) Why can installed panel area be lower than usable area?
Modules are counted as whole units. After flooring the panel count, a small leftover area usually remains and may be unusable for another full module.
8) Should I use this for final permit drawings?
Use it for early sizing and comparison. Final design should still be checked against local fire code, structural limits, electrical standards, and installer layout rules.