Modules produce the most when they face the sun. Because the sun moves across the sky every day and shifts north and south through the year, we fix modules at a compromise direction (azimuth) and angle (tilt) that collects the most energy over the whole year. In India, the answer is simple to state and easy to get slightly wrong.
The sun's path over India
India lies between about 8°N (Kanyakumari) and 37°N. For most of the year the sun travels through the southern part of the sky.
Sun paths drawn for Pune (18.5°N). The centre is the point overhead; the circle is the horizon.
- December: the sun rises in the south-east, stays low in the southern sky, and sets in the south-west. Noon altitude in Pune is about 48°.
- March and September (equinoxes): it rises due east, sets due west, and at noon stands about 71° high, in the south.
- June: it rises north of east and sets north of west. South of the Tropic of Cancer (23.4°N), the noon sun is actually slightly north of overhead for some weeks around June. In Chennai (13°N) that period runs from about late April to late August.
Averaged over the year, the southern sky delivers the most energy everywhere in India. Hence the rule: face modules true south.
How much do direction and tilt matter?
The table shows annual energy relative to the best possible orientation, from Sunlectric's orientation model (pvlib, clear-sky and monsoon cloud profile; indicative values).
Pune, Mumbai, Hyderabad (about 18°N)
| Tilt | South | South-east or south-west | East or west | North |
|---|---|---|---|---|
| 0° (flat) | 94% | 94% | 94% | 94% |
| 10° | 98% | 97% | 94% | 89% |
| 15° | 99% | 97% | 93% | 86% |
| 20° | 100% | 97% | 91% | 82% |
Jaipur, Lucknow (about 26°N)
| Tilt | South | South-east or south-west | East or west | North |
|---|---|---|---|---|
| 10° | 97% | 95% | 91% | 86% |
| 20° | 99.5% | 96% | 89% | 78% |
| 25° | 100% | 96% | 87% | 73% |
Three lessons hide in these numbers:
- The optimum is broad. In Jaipur, 20° gives 99.5% of the best; 25° adds half a percent but raises wind load and rear-leg height.
- East and west lose about 6–13%, more as tilt increases. A low-tilt east-west layout is a perfectly reasonable choice on a tin shed.
- North-facing is the real loser, and gets worse as tilt increases. Avoid north-facing slopes above 10°.
Choosing tilt on a flat RCC roof
The textbook says "tilt = latitude". In practice, Indian rooftop designers go a little lower:
- The optimum for the whole year is close to latitude, but very flat around it.
- Monsoon months add diffuse light, which favours lower tilts.
- Higher tilt means taller rear legs, more wind load, more steel and wider row spacing (lesson 4).
Practical guide for flat roofs:
| Region | Example cities | Suggested tilt |
|---|---|---|
| South, below 15°N | Chennai, Bengaluru, Kochi | 10–15° |
| Deccan and central, 15–24°N | Pune, Hyderabad, Nagpur, Indore | 15–20° |
| North, above 24°N | Jaipur, Lucknow, Delhi NCR | 20° (up to 25° on ground mounts) |
Do not go below about 10°. Flatter modules collect dust and bird droppings that the rain does not wash off, and water ponds along the bottom frame.
Worked example: a south-west shed in Pune
A 5 kWp system will lie flush on a 10° tin shed facing south-west (azimuth 225°).
- Best-case Pune yield: 1,513 kWh per kWp per year
- Orientation factor (10°, south-west): about 0.97
- Generation: 5 × 1,513 × 0.97 ≈ 7,340 kWh/yr, about 3% less than ideal
If the shed had a north-facing slope instead (factor 0.89), generation would be 6,730 kWh/yr. That is 600 units a year lost by using the wrong slope. Use the south-facing side, or split between east and west.
Finding true south on site
Compass method
A compass points to magnetic north, which in India is within about 2° of true north (check an online declination calculator for the exact figure). The bigger risk is local interference from rebar, steel tanks and railings. Stand away from steel, take three readings, and compare with the satellite map.
Shadow method at solar noon
At solar noon, a vertical stick's shadow points to true north (in the months when the sun is south of overhead). Solar noon is not 12:00 IST. IST is set at 82.5°E, and the sun reaches its highest point about 4 minutes later for every degree you are west of that:
- Pune (73.9°E): about 12:35 IST
- Jaipur (75.8°E): about 12:27 IST
- Chennai (80.3°E): about 12:09 IST
- Kolkata (88.4°E): about 11:37 IST
Add or subtract up to about 16 minutes depending on the date (the "equation of time"). A sun-position app gives the exact time.
Key points
- Face modules true south anywhere in India; the southern sky delivers most of the year's energy.
- The optimum tilt is broad: 10–15° in the south, 15–20° in central India, about 20° in the north.
- East and west faces lose about 6–13%; north-facing slopes lose much more and get worse with tilt.
- Keep tilt at least 10° for self-cleaning; on sheet roofs, mount flush at the roof slope.
- Find true south with a compass away from steel, or with a shadow at solar noon, which is not 12:00 IST.
Check yourself
- Why do designers in Pune seldom tilt modules at 25° on RCC roofs even though that is near the optimum?
- A 3 kWp array in Jaipur faces east at 10°. Using the table, roughly what yearly generation do you expect (best case 1,546 kWh/kWp)?
- At about what time IST is solar noon in Pune?
Answers
- 20° gives almost the same energy, while 25° increases wind load, steel, rear-leg height and row spacing.
- 3 × 1,546 × 0.91 ≈ 4,220 kWh a year.
- Around 12:35 IST, varying by up to about 16 minutes through the year.