Roof area · India
Plan on roughly 100 sq ft per kW — about 300 sq ft for 3 kW. The catch is that only shadow-free, reachable area counts, and on a typical Indian terrace that is a good deal less than the area you measured.
Usable area, not gross area
| Capacity | Usable area to plan for | Note |
|---|---|---|
| 1 kW | ~100 sq ft | Worst economics — see 1 kW price |
| 2 kW | ~200 sq ft | Common on small urban terraces |
| 3 kW | ~300 sq ft | Subsidy sweet spot |
| 5 kW | ~500 sq ft | Subsidy no longer increases |
| 10 kW | ~1,000 sq ft | Usually needs three-phase |
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The modules themselves occupy noticeably less — somewhere around 60 to 70 square feet per kilowatt with current mainstream panels. So why plan for a hundred?
Because a working installation is not just glass. It needs walkways so every panel can be reached and cleaned, and a plant you cannot clean quietly loses output forever. It needs clearance from the parapet, both for access and because the parapet casts its own shadow. Where panels are tilted in rows it needs spacing between rows, otherwise the front row shades the one behind it in winter when the sun sits low. And it needs somewhere sensible for the inverter, ideally shaded and ventilated.
If a quote fits your target capacity into markedly less area than this, that is worth questioning rather than celebrating. The usual outcome is panels butted together against a wall where nobody can reach them.
Measure your roof, then take away everything in this list. What remains is what you can actually size against.
This is why a 600 sq ft terrace often supports 3 kW rather than 6 kW. The arithmetic is not wrong; the gross measurement was never the right input.
Area you can sometimes work around. Shade you largely cannot, and it is the constraint most often assessed too casually.
The test is not whether the roof is sunlit during the site visit. It is whether it stays clear through the productive middle hours, across seasons. The sun sits lower in winter, so shadows reach further precisely when generation is already weaker. A neighbouring block, a growing tree, or a floor being added next door can each remove usable area that looked perfectly fine one bright afternoon.
Partial shade also does disproportionate damage depending on how modules are wired together, which is part of why panel-level considerations get discussed in mono vs poly. In dense areas — Mumbai, Delhi, older parts of Chennai — treat a serious shading assessment as non-negotiable.
Three levers, in order of how often they help.
Elevated mounting. Raising the array on a structure above the terrace, or above a shed or parking area, recovers rather than consumes space: the ground beneath stays usable for drying or parking. It costs more in structure and needs a genuine load assessment, but on a cramped roof it is frequently the difference between a viable plant and none.
Higher-efficiency modules. A real, if modest, gain — more capacity in the same footprint. Worth paying for specifically when area is the binding constraint, and not otherwise.
Accepting less capacity. Genuinely fine. A well-built 2 kW plant that fits, with clear access for cleaning, beats 3 kW crammed into a space that guarantees neglect. Note the financial consequence though: since central assistance peaks at 3 kW, dropping from 3 kW to 2 kW forfeits part of the ceiling, which is set out in 2 kW price and how the subsidy is calculated.
On a flat RCC terrace, the standard case, panels are tilted on a mounted frame and the considerations above apply directly. On a sloped metal sheet roof, common on independent houses and small workshops, the slope may already supply useful tilt and clamps can avoid drilling the sheet — the real questions are structural load and how penetrations are sealed against the monsoon. Asbestos sheeting is the awkward case: brittle, sometimes needing replacement rather than mounting, and with handling implications worth taking seriously.
If you do not control the roof at all, the constraint is legal rather than physical — covered in apartment rooftop solar, society NOC and solar for tenants.
Tell us your usable area and monthly units in the free home check and we will say what capacity is realistic — including when the honest answer is less than you hoped. Related: units per kW · how many panels · 3 kW price · choosing an installer.
Plan on roughly 300 square feet of genuinely usable, shadow-free terrace for a 3 kW rooftop system, working from a planning figure of about 100 square feet per kilowatt. The glass itself occupies rather less than that, closer to 60 to 70 square feet per kilowatt, but a real installation also needs walkways to reach panels for cleaning, clearance from the parapet, spacing between tilted rows so one row does not shade the next, and a location for the inverter. The larger figure is the one to size against.
About 100 square feet per kilowatt is the sensible planning number for a residential rooftop in India. If a vendor quotes a much tighter figure they are probably measuring only the module area and ignoring access and row spacing, which produces a layout you cannot clean properly. Higher-efficiency modules do genuinely reduce the module footprint, so on an area-constrained roof they can be the difference between fitting 2 kW and fitting 3 kW, but they do not remove the need for walkways.
Everything that is occupied, shaded or needed for something else. In practice that means the overhead water tank and its shadow, the stair room or head room and its shadow, the parapet shadow along the relevant edges, any lift machine room, dish antennas, vent pipes, existing solar water heater, and the space the household actually uses for drying clothes. On a typical Indian urban terrace these deductions remove a substantial share of gross area, which is why a roof that measures 600 square feet may only support 3 kW rather than 6 kW.
Yes, and it is one of the most commonly underestimated constraints in dense urban areas. What matters is not whether the roof is sunlit at the moment of the site visit but whether it stays clear through the productive middle hours of the day across seasons, because the sun sits lower in winter and shadows reach further. A neighbouring block, a tall tree or an under-construction floor next door can remove usable area that looked fine in a single afternoon inspection. It is fair to ask an installer how they assessed shading across the year.
Often yes, with adjustments. Three levers help. Higher-efficiency modules pack more capacity into the same footprint. An elevated mounting structure raises the array above obstructions so that space beneath remains usable for drying or parking, which effectively recovers area rather than sacrificing it. And accepting a smaller capacity is a legitimate outcome, since a well-built 2 kW plant that fits is better than a 3 kW layout crammed into a space where panels cannot be reached for cleaning. What does not work is ignoring the constraint and hoping.
A metal sheet roof is frequently workable and can be straightforward, because the sheet slope may already provide useful tilt and specialised clamps avoid drilling through the sheet. The questions are whether the underlying structure carries the additional load and how penetrations will be sealed against monsoon leaks. Asbestos sheeting is more difficult: it is brittle, it may need replacement rather than mounting, and handling it has safety implications. In both cases ask specifically how the structure was assessed and how waterproofing will be handled.
Scaling the same planning figure gives roughly 500 square feet of usable area for 5 kW and about 1,000 square feet for 10 kW. Larger systems tend to be harder to fit than the arithmetic suggests, because bigger arrays are more likely to run into an obstruction or a shaded corner somewhere across that span, and row spacing requirements accumulate. At 10 kW there are also electrical considerations beyond area, including the likely need for a three-phase connection.
