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TL;DR
Partial shading (chimney, tree, antenna) can reduce solar production by 5–30% depending on its severity. A single shaded panel can penalize its entire string (string inverter). Solutions: microinverters, optimizers, or careful design that avoids mixed strings.
Definition
Partial shading refers to the situation where part of the solar installation is in the shade for a fraction of the day or year, reducing the overall production of the installation.
Production reduction
5–30% depending on severity
PV string effect
The entire string is affected
Optimal solution
Microinverters
Snow shading CH
Often overrated
In a series string, the least productive panel limits the entire string ('weak link' effect). Bypass diodes make it possible to short-circuit a shaded panel per third of a module, but at the cost of a significant loss. Solutions: (1) microinverters: each panel is independent, shading does not propagate, (2) DC optimizers: individual MPPT, loss limited to the shaded panel, (3) careful design: never put panels with different shading profiles in the same string.
In Switzerland, the main sources of residential shade: chimneys and skylights (morning/evening shade), satellite or TV antennas, trees (summer foliage, less problematic in winter when production is low). Snow shading in winter (Valais, Jura) is often overestimated — it melts quickly on inclined panels.
Related guides
With a classic string inverter and without optimizers: in the event of severe shading on 1 panel out of 10, the string production loss can be 30–50% (10x the affected panel). With microinverters or optimizers: the loss is limited to 1/10 of the string (~10%).
Depends on position and season. An eastern deciduous tree that shades the corner of the roof in the morning in summer can cost 3–8% of annual production. In winter (without leaves), the impact is less. A PVsyst simulation with 3D shading measurement gives the exact answer.
Sources & method
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