Partial shading can have surprisingly dramatic effects on panel output. The weirdest part is that partial shading can have more devastating effects on output than full shading like heavy overcast or evenly shaded forest canopies.

Before we begin: solar panels are “current sources”; their voltage pops up into the normal range in any kind of meaningful light(>= 20% insolation) but current will suffer. Partial shading in this context means:

• light is falling on the panel
• but not evenly on the panel – it is different on some cells

To prevent power from rushing into the shaded string and overheating them, panels have bypass diodes between the strings. Basically the shaded strings get cut off, electrically speaking, to protect them. In a perfect world each cell would be protect by a lossless, costless diode but that's not possible yet.

The way to avoid the problems associated with partial shade is to avoid partial shade. Failing that, there are steps one can take to minimize the losses:

• avoiding shade is best
• a dedicated controller for each panel is the next best thing
• amorphous/thin-film panels are less affected by shade
• With PWM controllers, parallel panel configurations typically yield more power in partial shade than serial.
• With MPPT controllers and low-ish voltage serial panel configurations (where the total Vmp is ⇐2x battery bank voltage) parallel will probably still yield more.

There is another approach, which is to bring the shaded cells/strings back online by bringing the rest of the panel down to their level. It sounds counterproductive, but with MPPT controllers and in some higher-voltage series configurations (say Vmp is >=3x bank voltage) it works.

This occurs because the MPPT has a broader range of voltages to sweep and can find other power peaks (panel voltages) that are low enough to bring the shaded cells back online but still high enough to charge the battery bank. It's not reality, but we can think of it as MPPT evenly “shading” the entire panel voltage-wise in order to get max juice from it in partial shade conditions.²⁾

• altE video on partial shading
• wiring shaded panels