That water tank looks harmless. Your solar panels may disagree.

Roof area is not the same as usable roof area.

Most Bengaluru terraces are crowded. There is an overhead water tank, a staircase headroom, a satellite dish, a parapet wall on every side, and often a neighbour's taller building or a mature tree nearby. An installer's quote usually starts from how many panels fit on the roof. The better question is how many panels sit in sun for most of the day, because that is what generates units.

Why a small shadow can cost more than its size.

A solar panel is made of cells wired in series. When one part of the panel is shaded, the current through that whole series string is limited by the weakest cells. Panels have bypass diodes that let current skip shaded sections, which limits the damage, but the shaded section still stops contributing. On a conventional string inverter, panels are also wired in series with each other, so a shadow falling on one panel can pull down the output of the others on the same string. The exact loss depends on the panel layout, the wiring and the inverter, so treat any single percentage you hear as a rough guide, not a promise.

This is why a thin shadow from a pole, a railing or a tank leg can hurt output more than you would expect from the area it covers.

The usual shading sources on Bengaluru roofs.

Overhead water tanks. Common and large. Their shadow moves across the roof as the sun moves, so placement relative to the tank matters more than distance alone.

Parapet walls. They shade the panels closest to them, mostly early and late in the day. Installers often keep panels a set distance back from the wall for this reason.

Staircase headroom and lift rooms. Tall, solid, and permanent. Anything on the roof to your south or east or west can cast a long shadow.

Trees and neighbouring buildings. Trees grow. A coconut palm or a jackfruit tree that is harmless today can shade the array in five years. A new floor going up next door can do the same overnight.

Rows of panels shading each other. On flat roofs with tilted panels, rows placed too close will shade the row behind, especially in the mornings and evenings. Spacing is a design decision, not an afterthought.

What Bengaluru's sun angle changes.

Bengaluru sits at roughly 13 degrees north, so the sun is high in the sky for much of the year and shadows are shorter than in northern India. That helps. But the sun is lowest, and shadows longest, in the early morning and late afternoon, and the monsoon months already cut generation through cloud cover. Shading is an extra loss on top of that, not a substitute for it.

How to check before you sign.

First, visit the roof at different times, ideally around mid-morning, noon and mid-afternoon, and look at where shadows fall. Photos on your phone at each time are enough to compare against the proposed panel layout.

Second, ask the installer for a shadow analysis or a layout drawing showing where each panel goes relative to the tank, parapets and nearby obstructions. Many installers use simple sun-path software or a handheld tool for this. If your installer has not looked at shading at all, that tells you something.

Third, ask whether the design uses a string inverter, microinverters or power optimizers. Microinverters and optimizers handle partial shading better because panels operate more independently, but they cost more and add components on the roof. It is a trade-off, not an automatic upgrade, so ask for the price difference in rupees and judge it against how much shade you really have.

What you can do about it.

Move the array. Panels on the shadow-free part of the roof beat more panels in the shade. Trim or remove a tree you own if it is going to matter. Raise the panels on a taller structure if the shading is low and close. Or, if the shade is severe and cannot be fixed, take a smaller system and skip the panels that will only produce in bursts. A smaller, unshaded system often pays back better than a larger, partly shaded one.

Finally, ask for the shading assumption to be written into the generation estimate. A quote that promises the same units per kW regardless of your roof has not looked at your roof.