About the Shading Loss
Shading loss is the disproportionate reduction in solar output caused by partial shadows on the array. Because panels in a string are connected in series, a shadow on even one cell can reduce the entire string's output — the 'weak link' effect. Shading is one of the leading causes of underperformance in rooftop systems.
Formula
String shading loss ≈ Fraction of shaded string × (1 − bypass diode factor)
- Fraction of shaded string
- Proportion of modules in the string that are partially shaded
- bypass diode factor
- Typically ~0.33 — each module has 3 bypass diodes dividing it into 3 sub-strings; a diode bypasses the shaded sub-string, losing 1/3 of module output
How to calculate it
- 1Map all shading objects: water tanks, parapets, staircase rooms, adjacent buildings, trees, poles.
- 2Calculate shading angles and times using a solar path diagram or simulation software (PVsyst, Helioscope).
- 3Identify which panels are shaded in which hours; determine which strings they belong to.
- 4Estimate string output loss using bypass diode model for partial shading.
- 5Consider restringing or microinverter/power optimizer placement to isolate shaded panels from the string.
Worked example
A 10-panel string where 2 panels are shaded by a water tank for 2 hours each morning. Without power optimisers, shading those 2 panels halves the output of all 10 panels during those 2 hours. With panel-level MPPTs or power optimisers, only the 2 shaded panels lose output while the other 8 operate at full capacity — recovering 80% of the shading loss.
What you can calculate
- % shading input
- Revenue impact
- Effective PR
- Optimiser guidance
Frequently asked questions
How much shading is acceptable for a rooftop solar system?
Shade affecting peak generation hours (10am–2pm) is most damaging. As a rule, keep critical solar panels completely free of hard shadows during the 4-hour peak window. A shading analysis showing less than 3% annual shading loss is acceptable; above 5% warrants redesign or panel-level power electronics.
Do microinverters eliminate shading losses?
Microinverters and DC power optimisers dramatically reduce shading losses by giving each panel its own maximum power point tracking. A partially shaded panel no longer drags down the rest of the string. However, they add cost (₹5,000–15,000/panel). They are most cost-effective when shading is unavoidable — for example, on complex rooftops with multiple obstructions.
Does the orientation of shadows matter for shading loss?
Yes. East-west shadows (from a parapet running north-south) shade entire rows simultaneously and are most damaging. North-south shadows shade individual panels within a row — with bypass diodes, the impact is localised. Row-to-row self-shading from the array itself depends on row pitch and tilt, and is minimised by correct shadow spacing design.
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