Guides

How partial shading affects solar panels

Published 8 October 2026. About 6 minutes to read.

Shading losses are not proportional to the shaded area. A shadow covering a few percent of a panel can take a third of its output, or all of it, depending on where the shadow falls and how the panel is wired.

Cells in series are only as strong as the weakest one

A panel is a chain of cells connected in series, and the same current has to flow through every cell. A shaded cell can only pass a small current, so it holds back all the cells in series with it. Worse, the other cells push current through it in reverse, and it heats up. Left unchecked, this creates hot spots that damage the panel.

What bypass diodes do

To prevent this, most panels have three bypass diodes in the junction box. Each one protects a group of cells, called a substring. When a cell in a substring is shaded enough, its diode switches on and current flows around the whole substring instead of through it. The panel keeps working, but without that third of its cells.

So a single shaded cell typically costs about a third of the panel's output. Shading cells in all three substrings costs nearly all of it.

Portrait or landscape matters

In a standard full-cell panel the three substrings run along the long side. That decides what a shadow rising from the bottom edge does, for example the shadow of a parapet, the row in front, or a roof ridge late in the day.

• Landscape: the bottom edge belongs to one substring. The shadow costs about a third of the panel until it climbs into the next substring.
• Portrait: the bottom edge crosses all three substrings. A shadow on just the bottom row of cells can take almost the whole panel.

For rows on a flat roof, where inter-row shading comes from below, landscape is often the better choice for this reason.

Half-cut cell panels

Most panels sold today use half-cut cells. The panel is split into a top half and a bottom half that are connected in parallel. In portrait, a shadow along the bottom edge now affects only the lower half, and the upper half keeps producing. That makes half-cut panels noticeably more tolerant of bottom-edge shading in portrait than older full-cell panels.

Shading across a string

Panels in a string are also in series, so the same logic applies one level up. When one panel is partly bypassed, the string's power curve gets more than one peak. A good MPPT scans for the highest peak, but a string with shaded panels still loses more than the shaded area suggests.

Some ways to limit the damage:

• Put panels with similar shading on the same string, and strings with different shading on different MPPT inputs.
• Use module-level power electronics (optimisers or microinverters) where shading is unavoidable, so each panel works at its own best point.
• Move the array. Often the cheapest fix is leaving out the two panels that sit in the shadow of a chimney all winter morning.

Seeing it before you install

Lekker Sun traces rays to the sun from six points on every panel, every half hour of a typical day in each month, and shows the share of each panel in shade on a month-by-hour grid. That tells you which panels are shaded and when, so you can decide on orientation, stringing and placement before anything goes on the roof. It reports the share of the panel area in shade; the extra loss from bypassed substrings and string mismatch depends on your stringing, so treat shaded panels with caution.

This guide is general information to help you plan and compare options. It is not engineering, electrical or financial advice. Always follow the equipment manufacturer's instructions and your local wiring rules, and have a qualified installer check the design.