The short answer: it depends on your monthly electricity bill, not the size of your roof. A rough rule of thumb — 1 kW of solar needs about 100 sq. ft of shadow-free roof and produces roughly 120–150 units a month in most parts of India.
What actually decides the number
- Average monthly bill — higher usage needs a bigger system to offset it.
- Panel wattage — a 540W panel needs less roof space than an older 330W panel for the same output.
- Roof orientation and tilt — south-facing roofs in India generally generate the most.
- Shading — trees, tanks and neighbouring walls cut usable roof area. More on that in Rooftop Shading: The Silent Killer of Solar Output.
A 3kW system, for instance, usually needs 8–9 panels and around 300 sq. ft. But two identical-looking roofs can need a different panel count once shading and orientation are accounted for — which is why an eyeballed estimate is rarely accurate.
Get an exact number, not a guess
The only reliable way to size a system is to map the actual roof in 3D — real dimensions, real shadows, real panel layout. See how EPCs do this with Solar 3D Design, or read why 3D design beats guesswork when it comes to proposals.