Solar Panel Quantity and Roof-Area Calculator
How many panels, how much roof, how much energy — the three questions every solar project starts with. Begin from any of them: a target capacity in kWp, a target monthly generation from your bill, or simply the roof area you have. The calculator resolves the panel quantity, installed capacity, roof area (with realistic packing for pitched or flat roofs) and the approximate annual generation — plus the array weight for the structural conversation and a suggested inverter size.
Then take it further: system design with the Solar PV Calculator, the investment case with the Solar Payback and ROI Calculator, and export economics with the Net-Metering Savings Calculator.
Array summary CALC-13 · REV 2026-07
| Panel quantity | – |
|---|---|
| Installed capacity | – |
| Mounting area occupied | – |
| Total roof area required | – |
| Approximate annual generation | – |
| Approximate monthly generation | – |
| Array weight (incl. mounting) | – |
| Suggested inverter | – |
Guidance tool only. A real layout must account for the actual roof geometry and orientation, shading analysis, wind uplift and structural capacity (especially ballasted flat-roof systems), string design and KEBS/EPRA-compliant equipment — all part of a professional solar design.
How this calculator works
- Panel geometry. Presets cover today’s mainstream modules (450–620 Wp, roughly 2.2–2.7 m² each), with custom wattage and area accepted. Panel quantity is rounded up from the target capacity (or down to what the roof holds).
- Packing. A flush-mounted pitched roof fits panels at about 85% of its usable area; a flat roof with tilted rows only manages about 60%, because each row must clear the shadow of the one in front. On top of that, a usable-roof fraction (default 80%) allows for edges, walkways, vents and tanks.
- Generation. Annual energy is capacity × specific yield (default 1,600 kWh/kWp/year for Kenya, consistent with our ROI calculator), with the monthly figure alongside for comparison with your bill.
- The practical checks. Array weight at about 32 kg per mounted module is reported in kg/m² and flagged above 15 — the trigger for a structural review. The suggested inverter targets a DC/AC ratio near 1.2, flagging ratios above 1.35 that would clip midday output. Capacities above 10 kWp trigger a note on the domestic net-metering cap.
Assumptions and limits
- Packing factors are planning figures — the real count depends on the roof outline, orientation and shading, which a layout drawing resolves.
- Wind uplift, waterproofing penetrations and ballast design on flat roofs need engineering input.
Want the full layout drawing, string design and structural check? See our consultancy services, or learn the method in a training program.