Shading: obstacles, the 9 am–3 pm window, inter-row spacing

Shade is the most common cause of disappointing generation on Indian rooftops. Water tanks, staircase rooms (mumties), parapets, neighbours' buildings, trees, dish antennas and even the lightning arrester mast all cast shadows that move through the day and the year. A shadow covering 5% of the array can cost far more than 5% of the energy.

Why a small shadow costs so much

Modules in a string are connected in series, so the same current must flow through every cell. When one cell is shaded, it can pass only a little current, and it drags the whole string down with it.

Bypass diodes (three per module) limit the damage: the shaded third of the module is skipped, so the module loses about one-third of its voltage instead of nearly all its current. But the inverter's MPPT still has to find a new operating point for the string, and with several shaded modules it often settles on a compromise that wastes energy from the unshaded ones too.

Shade also creates hot spots. A shaded cell that is forced to carry current heats up, and over months this can brown the encapsulant, crack the cell or burn the backsheet.

Rule: design for zero shade in the main production hours, and treat any unavoidable shade as a design input, not an afterthought.

The 9 am–3 pm rule

Most of a day's energy arrives between 9 am and 3 pm (solar time). Shadows are longest on 21 December, when the sun is lowest in the southern sky. So the standard rooftop design rule is:

No shade on any module between 9:00 and 15:00 solar time on 21 December.

If the array is clear in that window on the worst day, it is clear in that window all year. Shadows before 9 am and after 3 pm cost a little energy, which is accepted.

Remember that solar time is not IST. In Pune, 9:00 solar time is about 9:35 IST; in Chennai about 9:10 IST.

Shadow lengths for Indian cities

For an obstacle of height h above the module's bottom edge, the shadow at 9 am (or 3 pm) on 21 December reaches:

• N–S distance = h × F (use for parapets and for rows of modules, which run east–west)
• Full shadow length = h × L (use for tanks, mumties and masts that sit to the south-east or south-west of the array)

Calculated from standard sun-position equations for 9:00 solar time on 21 December.

Notice how much more space northern cities need: a 1 m parapet in Delhi casts a 1.74 m shadow towards the north at 9 am, against 0.99 m in Chennai.

Inter-row spacing

On flat RCC roofs with tilted modules in several rows, each row shades the one behind it. The height that matters is the rise of the module from its low edge to its high edge:

h = module slope length × sin(tilt) Gap d = h × F Row pitch = module slope length × cos(tilt) + d

Worked example: Jaipur, portrait modules at 15°

• Module length in portrait: 2.279 m
• Rise: h = 2.279 × sin 15° = 2.279 × 0.259 = 0.59 m
• Horizontal footprint: 2.279 × cos 15° = 2.20 m
• Gap: d = 0.59 × 1.63 = 0.96 m
• Row pitch: 2.20 + 0.96 = 3.16 m

The same rows in Pune need a gap of 0.59 × 1.19 = 0.70 m, and in Chennai at 10° tilt only 0.40 × 0.99 = 0.40 m. In practice, keep a minimum of about 0.6 m anyway so the gap doubles as a cleaning walkway.

Parapets, tanks and mumties

Parapets

A parapet on the south side is the most common problem. Use the parapet's height above the module's low edge.

Example, Pune: parapet 1.0 m, front edge of modules 0.3 m above the slab. Effective h = 1.0 − 0.3 = 0.7 m. Keep the first row at least 0.7 × 1.19 = 0.83 m north of the parapet. Alternatively, raise the structure so the low edge sits at or above parapet height; elevated structures in Maharashtra often do this.

East and west parapets also shade at 9 am and 3 pm. Use the same method with the east-west component, or simply keep a setback of about 1–1.5 × parapet height from them.

Parapets on the north side do not shade the array in winter.

Tanks, mumties and masts

For an obstacle to the south-east or south-west, use the full shadow length.

Example, Jaipur: a 1,000 litre tank on a 0.3 m stand, total height 1.5 m above the module edge, sits south-east of the array. Shadow length at 9 am = 1.5 × 2.30 = 3.45 m towards the north-west. Either keep modules out of that zone or move the tank (often cheaper than it sounds; plumbers do it in a day).

Trees and neighbours

Trees grow. A mango tree that just clears the array today will shade it in five years. Ask the customer if the neighbour is planning another floor; in fast-growing areas of Pune, Bengaluru or Hyderabad, it is common.

Recording shade on the survey

1. Measure each obstacle's height and its distance and direction from the array area.
2. Photograph the obstacles from the array position.
3. Use a sun-path app, a shading tool such as a Solar Pathfinder, or 3D design software (Helioscope, PVsyst, SketchUp-based tools) for complex roofs.
4. Mark the shade-free zone on the roof sketch.

Key points

• In a series string, one shaded cell limits the whole string; bypass diodes reduce but do not remove the loss.
• Design for zero shade from 9:00 to 15:00 solar time on 21 December.
• Gap = obstacle height above the module edge × the city's N–S factor (about 1.0 in Chennai, 1.2 in Pune, 1.6 in Jaipur).
• Row pitch = slope length × cos(tilt) + rise × F; keep at least 0.6 m between rows for cleaning.
• Record every obstacle's height, distance and direction, and remember future growth.

Check yourself

1. Why is 21 December used for shading design in India?
2. Modules 2.279 m long are tilted at 20° in Ahmedabad (F = 1.40). What gap do the rows need?
3. A 1.2 m south parapet in Delhi; module front edge 0.2 m above slab. How far back should the first row start?