How shading cuts solar output

Why does a small shadow cost so much?

A panel isn't one big sheet. It's a row of small cells joined one after another, which engineers call a series string. Current has to pass through every cell in turn.

When one cell is shaded, the current through the good cells drops.¹ The string can't carry more than the shaded cell lets through.² That's why a leaf, a vent pipe or a power line shadow can cost much more than the patch of glass it covers.

Solar writers even have a name for it, the "Christmas light effect". Output from a cell is extremely sensitive to shading.²

What happens to the shaded cell itself?

A shaded cell can stop adding power and start absorbing it, turning it into heat. That happens when the load lets enough voltage from the other cells in the string force current through the shaded cell by breaking down its junction.²

The source gives an example. At a high current level, a shaded cell may drop 8 volts instead of adding 0.5 volts, which absorbs the power produced by 16 other cells.² One shaded cell can absorb the power of many others in the string.² Bypass diodes limit this, as the next section explains.²

This is called hot-spot heating. Essentially the entire generating capacity of the good cells is dissipated in the one poor cell. The result is local overheating, with effects such as cell or glass cracking, melted solder or damage to the solar cell.¹

What does a bypass diode do?

A diode is a one-way valve for electricity. A bypass diode is wired alongside a group of cells, facing the opposite way. In normal sunshine it does nothing. When a cell is shaded and starts to hold the string back, the diode switches on and lets the current from the good cells flow around the problem.³

That limits the current through the shaded cell and prevents hot-spot heating.³ Its main job is to stop hot spots, which can damage the array and cause fires.²

One diode for every cell would cost too much, so diodes are placed across groups of cells.³ A typical module has 36 cells, and a normal 36-cell module uses 2 bypass diodes.³ Most modules include bypass diodes between each cell or string of cells.² Check the spec sheet for the panel you plan to buy.

So does one shadow switch the whole panel off?

Not usually. A shadow on one section drops the output of one or more strings of cells to near zero, but doesn't take the whole module to zero.⁴

With bypass diodes, a panel mainly loses the power that the shaded portion would have supplied, plus a little that the diodes themselves use.² The same logic applies to a whole array. Panels in a string should be alike and face the same way, and bypass diodes help by letting current go around broken or shaded panels.⁴

Is there any fix other than moving the panels?

There are add-ons. Power optimizers are devices that try to get the most power from each panel despite shade.⁴ Even so, techniques that use diodes to cut shading losses bring losses of their own.²

So the plain answer stays the same. It's important that an installation isn't shaded by trees or other obstructions.² Find the shade before you fix anything down. The page on checking a site for shade shows how.

Who fits optimizers or changes the wiring?

Not you, unless you're licensed. Fixed mains wiring in Australia is licensed electrical work. If an add-on means moving a cable on the battery, fuse or inverter side, switch the battery off from the system first and keep to the maker's manual. A licensed electrician is the safe choice when you aren't sure.