Rooftop Calculator

Calculate gross roof area for gable, hip, shed, and flat roofs, then apply setback, obstruction, and walkway deductions. Typical usable area runs 75–85% of the gross surface once chimneys, vents, dormers, skylights, and the 36" fire-code setback at ridges are accounted for.

Work with common modules from 250 W to 500 W. A 400 W panel typically measures about 1.72 m² (18.5 sq ft), so one kilowatt of capacity needs roughly 4.6 m² (50 sq ft) of usable roof, including walkways and spacing between rows.

Size the array from panel count and wattage, then estimate annual production with: Annual kWh = System kW × Peak Sun Hours × 365 × Performance Ratio. A performance ratio of 0.75–0.85 covers inverter losses, soiling, temperature derating, and wiring losses.

Enter the local electricity tariff and self-consumption share to compute annual bill savings, then divide net system cost by annual savings for the simple payback. Apply national or regional incentives such as the 30% US federal solar ITC, feed-in tariffs, VAT exemptions, or net-metering credits.

Get a full budget covering panels, inverter, mounting, racking, cabling, labour, permits, and grid connection. In 2026, installed residential solar runs roughly $2.50–$3.50 per watt before incentives, so a 6 kW system lands near $15,000–$21,000 gross.

Generate branded PDF proposals showing roof layout assumptions, panel count, system capacity, annual generation, savings, payback, and net cost after incentives. Export to CSV for financial models. All calculations run on-device and work fully offline.