Two questions come up before any solar installation. How much roof do I need, and will that tree be a problem? The first has a simple formula. The second decides whether the whole thing is worth doing.
Roof space by system size
| System | Polycrystalline | Mono PERC | High efficiency |
|---|---|---|---|
| 1 kW | 110 – 120 sq ft | 90 – 100 sq ft | 75 – 85 sq ft |
| 2 kW | 220 – 240 | 180 – 200 | 150 – 170 |
| 3 kW | 330 – 360 | 270 – 300 | 225 – 255 |
| 5 kW | 550 – 600 | 450 – 500 | 375 – 425 |
Mono PERC is the common choice now. Higher efficiency panels only matter when roof space is your binding constraint — if you have plenty of roof, paying extra for compact panels buys you nothing.
Only about 80% of your roof is usable
This is the part people miss. A 400 sq ft roof is not 400 sq ft of panels. Subtract:
- Water tank and its shadow
- Staircase head room
- Parapet wall shadow, especially in winter when the sun is low
- Walking space between rows for cleaning and maintenance
- Any existing structure — dish antenna, vents, clothes lines
Should you use a raised structure?
A raised or elevated mounting puts the panels on legs at about 8 to 10 feet, leaving the roof below usable.
| Normal structure | Raised structure | |
|---|---|---|
| Extra cost | — | Rs 15,000 – 40,000 |
| Roof below | Unusable | Still usable |
| Cleaning access | Easy | Slightly harder |
| Wind load | Lower | Higher — needs proper design |
If your terrace is used — drying clothes, sitting in the evening, kids playing — the raised structure is usually worth it. People who skipped it to save Rs 25,000 are the ones who complain later that they lost their terrace.
Now the part that actually matters: shading
Shade does far more damage than most people expect, and the reason is how panels are wired.
Panels are connected in strings, like old Christmas lights. Current flows through the whole string. When one panel is shaded, it restricts the current for every panel in that string — not just itself.
Where shade comes from
- Trees. Remember they grow. A branch that clears the roof today may not in three years.
- Neighbouring buildings. The serious one — and the one you cannot control.
- Your own water tank, parapet or stair room. Fixable by repositioning the array.
- Dish antennas and poles. A thin pole casts a moving shadow across several panels through the day.
Check it properly before you install
Go to your roof three times in one day — around 9 am, noon, and 3 pm — and photograph where the shadows fall. That is your generating window, and anything shaded during it is a problem.
Also check in winter. The sun sits lower, shadows run longer, and a roof that is clear in June can be half-shaded in December.
What to do if you cannot avoid shade
| Option | What it does | Cost |
|---|---|---|
| Move the array | Best answer if there is a clear spot | Free at design stage |
| Panel-level optimisers | Stops one shaded panel dragging the string down | Rs 3,000 – 5,000 per panel |
| Micro-inverters | Each panel works independently | Higher still |
| Restring the layout | Group shaded panels together on their own string | Free at design stage |
The last one costs nothing and is often enough. Ask your installer how the strings are arranged and whether shaded panels can be kept on a separate string.
Direction and tilt
South-facing is best in India. But this matters less than people think, and far less than shading:
| Direction | Output vs south |
|---|---|
| South | 100% |
| South-east / south-west | 95 – 98% |
| East / west | 85 – 90% |
| North | Avoid |
A clear east-facing roof beats a shaded south-facing one every time. If you have to choose, choose the clear one.
The short version
- Measure your roof, then multiply by 0.8
- Check shadows at 9 am, noon and 3 pm — and think about winter
- Ask whether anyone nearby can build higher
- If your terrace is used, price the raised structure
- A clear roof facing east beats a shaded roof facing south
Measuring your roof properly
Take a tape and do this yourself before any vendor visits. It takes twenty minutes and it stops you being sold a system your roof cannot hold.
- Measure the full length and breadth. Multiply for gross area.
- Subtract permanent obstructions — water tank, stair room, lift room, vents.
- Subtract a walking margin — about 2 feet along the edges and between panel rows.
- Subtract the parapet shadow — roughly the parapet height in feet, along the north edge.
- What is left is your real usable area.
A 20 x 30 roof is 600 sq ft gross. After the tank, the stair head, walking space and parapet shadow, you may be down to 420 to 450 sq ft — around 4 kW, not the 6 kW the gross figure suggests.
Roof type matters too
| Roof | Mounting | Watch out for |
|---|---|---|
| RCC flat | Ballasted or bolted frame | Waterproofing at bolt points |
| Sloped concrete | Frame following the slope | Only useful if the slope faces south, east or west |
| Metal sheet | Clamps on the ribs | Sheet condition and the structure below |
| Asbestos sheet | Usually needs replacing first | Old sheets will not take the load |
| Tiled | Hooks under the tiles | Leaks if done badly. Needs an experienced installer. |
How much weight does it add?
A normal rooftop system adds roughly 12 to 20 kg per square metre including the structure. A sound RCC roof handles this comfortably.
Get it checked if your roof is over about 30 years old, was built without a structural design, or already carries a heavy overhead tank. A structural engineer's opinion costs a few thousand rupees and is worth it on an old building.
Questions people ask
My roof is 300 sq ft. What can I fit?
About 240 sq ft usable, which is roughly 2.5 kW on mono PERC. If you need 3 kW, look at higher efficiency panels or a raised structure that lets you use a bit more of the footprint.
Can I put panels on a sloped roof facing north?
Avoid it. North-facing slopes in India get very poor output. If that is your only surface, a raised frame on the flat portion, tilted correctly, is a better answer.
One tree shades the corner for two hours. Is that a problem?
Possibly a significant one, depending on how the panels are strung. Ask your installer to keep the affected panels on their own string, or price optimisers for those panels. Do not accept "it will be fine" without a plan.
Will panels damage my terrace?
Properly installed, no. Badly sealed bolt penetrations cause leaks. Insist on how the waterproofing will be handled, and inspect after the first heavy rain.
Do panels need to face exactly south?
No. Anywhere between south-east and south-west loses very little. Shading matters far more than a few degrees of orientation.