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.

Partial shading is problematic for solar panels because:

1. partial shade causes voltage differences between cells
2. voltage differences cause power to rush into the lower-voltage (shaded) cell
3. overheating the cell and reducing overall voltage

When a panel is partially shaded the individual cells are running at differing voltages. If the cells were bare then the unshaded (normal voltage) cells would backfeed power into the shaded (lower voltage) cells, overheating them. To prevent this, panel manufacturer's insert diodes. Partial shading trips the diodes and effectively takes the shaded cells/strings offline.

The takeaway:

• 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) it's close but parallel will probably still yield more.
• With MPPT controllers and higher voltage serial configs (say Vmp is >=3x bank voltage) we see an increasing serial advantage over parallel in partial shade. This occurs because the MPPT has a broader range of voltages to sweep and can find power points (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.