Shading can have surprisingly dramatic effects on panel output.

Even shading (as with overcast skies) will reduce output but not as critically as partial shading.

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. With the narrower 2x voltage overhead MPPT might be able to bring the cells back online but the panel voltage would be below battery charging voltage.