Shadow Analysis Before Solar Design: How Much of Your Roof Is Actually Usable

A shadow analysis finds the part of your roof that stays in sun from about 8 am to 3 pm on 21 December, when the sun is lowest. Panels go only on that shadow-free area. Parapets, tanks, stacks, neighbouring blocks and the panel rows themselves cast the shade. We plan about 100 sq ft per kW as a screen; the study replaces that with your roof.

Why is 21 December the design day?

Shadows are longest when the sun is lowest. In the northern hemisphere that is the winter solstice, around 21 December. MNRE's draft Quality Control Manual for grid-connected rooftop solar, circulated for comments in February 2023, says to take the sun's azimuth and altitude from 8 am to 3 pm on that date to get the maximum shadow length cast by an adjacent object, and to size the gap between rows on the same basis. It also warns that shading does not only cut generation; it can damage modules over time. If the layout is clear of shade on the worst day, it is clear on the others.

The study uses solar time, where noon is when the sun is due south. Clock time in India differs from solar time by minutes depending on longitude, so a surveyor's "8 am" and a software's "8 am" can be different positions of the sun. State which one the report uses.

How long are the shadows?

The length of a shadow depends on the object's height and the sun's altitude, which depends on latitude and the hour. The table is calculated for 21 December, as the shadow cast by a 1 m tall object, at two latitudes in our service area: about 30.9° N (Ludhiana) and 28.5° N (Noida). The north-south column is what falls on a roof behind a south-side obstruction.

These are geometric results, not a substitute for a survey. Read them like this: a 1.2 m parapet on the south edge throws a shadow about 2.3 m deep behind it at 3 hours from noon, and about 3.7 m at 4 hours, at 30.9° N. A 2 m water tank throws nearly 4 m at 3 hours. Whether you accept that shade or leave the strip empty is a design decision: the strip is shaded for a short part of the day, but it is the part where a string loses the most.

What casts shade on an industrial roof?

• Parapets and roof edges, particularly on the south and east sides.
• Lift and stair heads, water tanks, chimneys and exhaust stacks. Measure their height above the roof surface, not above the ground.
• Roof-mounted equipment such as condensers, cooling towers, turbo ventilators and ridge vents.
• Taller neighbouring buildings and sheds to the south, east and west, and trees that will keep growing.
• Sawtooth and multi-bay profiles, where one bay shades the next.
• Overhead lines and poles close to the roof.
• The array itself. Each row shades the one behind it unless the rows are spaced for the worst hour.

Skylights and roof lights do not cast shade, but they are not usable either, and they take area off the figure. Our note on roof penetrations and waterproofing explains what anchoring does to a roof, which matters when you decide how close to edges and roof lights to build.

How far apart should the rows be?

For a tilted array on a flat roof, the row pitch is the horizontal footprint of a module row plus the shadow it throws on the next row. MNRE's draft manual asks for a gap greater than the longest shadow at the chosen hours on 21 December. It also suggests keeping the tilt under 15° on a flat RCC roof to limit wind pressure, avoiding the roof's edges and corners where wind pressure is highest, leaving walkways between rows for cleaning, and having a structural engineer verify the mounting structure. The example uses a 2.28 m module row at a 10° tilt at 30.9° N.

The pitch rises quickly as you extend the shade-free window. A lower tilt packs more modules into the area at some loss per module, so compare kWh per roof, not kWh per module. That is also why two layouts for the same plant can need quite different roof areas. Use the shadow-free area calculator for a first estimate, and the plant area calculator to turn it into kW.

Which tools do a shadow analysis, and what should the report contain?

The draft manual describes a survey with a compass, tape or distance meter and an angle-measuring tool, then deriving azimuth and altitude angles for each object from web tools. It also allows desktop simulation, but cautions that the result depends on the accuracy of the inputs: height, breadth and distance of each object. When software is used, confirm the physical dimensions on site. Drone-based 3D models can supply those dimensions on large roofs. A good report holds:

• A dimensioned roof plan with every obstruction's position and height.
• The sun-path or hourly shade map for 21 December, stating whether times are solar or clock.
• The usable area after shade, edge setbacks, walkways and roof-light exclusions.
• The array layout, row pitch and tilt, with the assumed shade-free window.
• Expected shading loss and how strings are grouped so a shaded string does not drag down the rest on the same input.
• The structural, access and cable-route constraints found in the survey.

Our note on structural loading of old roofs and the roof load calculator cover the part the shade study does not.

What should you ask an EPC?

Ask for the shadow study before the quote, and ask whether the generation estimate and any performance guarantee are built on the area it found. If the study was skipped and the roof is "about 20,000 sq ft", the plant size is a guess. Our checklist on what to check in a performance guarantee shows how shading assumptions slip into the fine print.

How Secured Engineers helps

• We design the plant in-house, starting from a site survey and shadow study, and the layout reflects what the study finds.
• Fire NOC, CEIG and DISCOM net-metering filings are prepared in-house.
• MEPF and solar can sit under one contract.
• We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.
• If another EPC has already given you a layout, we review its area and shading assumptions before you sign.

Read how our solar EPC service starts with the survey, then send your roof plan on WhatsApp, call +91 70099 85817 or ask for a free proposal with a shadow-free area estimate.