Partial shading can have surprisingly dramatic effects on panel output. Perhaps counterintuivitely, partial shading can have more devastating effects on output than full shading like heavy overcast or evenly shaded forest canopies.

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⁾

[draft section]

This section will assume an array of 2x generic 100w 18Vmp panels

• the panel is made up of 36 0.5Vmp cells in series for 18v; 36 x 0.5v = 18v.
• since rated wattage is 100w, the cells must be 5.55Isc (amps at max power); 100w / 18v = 5.55A.

For reasons discussed below the string of 36 cells will be subdivided into substrings of cells in series. 3 strings of 12 cells is common. 2 strings of 18 cells or 4 strings of 9 cells are also possible, etc. So we can think of this panel, electrically

  ############
  ############
  ############
  

as

  ############ ############ ############   
  

Under lab conditions the cells will each make 5.55A at 0.5v, delivering the rated 100w at 18v. In the real world the Vmp will likely be lower due to cell temperature derating and current will likely be much lower due to imperfect sun. But to keep things simple and numbers even we will assume lab conditions.

• power moves in the direction of higher voltage to lower voltage.
• when shaded sufficiently any given cell's voltage will drop off sharply³⁾
• so power from neighboring cells moves into the cell instead of out of it
• the cell converts this power into heat which can damage or destroy the cell, or catch the panel on fire

When the entire panel is lit or shaded evenly the harvest is reduced but no individual cell is running at a lower voltage than its neighbor. So overcast days or dense tree canopies are not a problem. And when people talk about “shade” problems it usually means “partial shade”.

We can't stop the cell voltage from collapsing in shade so we need a way to remove it, electrically, from the circuit. This is typically done with diodes. They are, in effect, electrical valves that allow power to flow in only the intended direction.

Right now we are concerned with the interatctions of panel internals so we are talking about bypass diodes. They can bypass a voltage-collapsed cell or substring and avoids the overheating problem.

Simplest example: we have a string of three cells in series. In full sun all three contribute 0.5v and the total string voltage is 1.5v. In partial shade one of them drops top 0.2v. It gets bypassed so this string is now 1.0v. Without the diode the string would be 1.2v (0.5v + 0.5v + 0.2v) until the shaded cell failed from overheating.

But diodes for each cell would be more expensive in components, assembly, and design. The compromise most manufacturers make is to have a bypass diode at the substring level. Our panel is now like this:

Real-world example: Simplest example: we have a string of three cells in series. In full sun all three contribute 0.5v and the total string voltage is 1.5v. In partial shade one of them drops top 0.2v. It gets bypassed so this string is now 1.0v. Without the diode the string would be 1.2v (0.5v + 0.5v + 0.2v) until the shaded cell failed from overheating.

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.

• altE video on partial shading
• wiring shaded panels
• Solar Panel Shadows and Bypass Diodes