A little under bright overcast, and very little under a hard shadow. The loss is worse than the shaded area suggests, because the cells are wired in series and the weakest one limits the rest — which is what bypass diodes exist to contain.
Shade is the difference between a panel’s rating and what it delivers on a real roof, and it does not behave proportionally.
Why series wiring punishes shade
Cells in series carry the same current. A cell in shadow can only pass what its own light allows, so it limits the string rather than simply contributing less. That is why a shadow across a corner costs far more than its share of the area.
What the diodes change
Bypass diodes divide the panel into sections and let current route around a limited one. The shaded section stops contributing, and the rest continues. It is containment rather than a cure, and the number of sections is a design choice the maker documents.
What happens across panels
At the array level the same logic repeats: panels in series share a current, panels in parallel do not. Which arrangement is used decides whether one shaded module drags its neighbours down, and per-panel electronics remove the question altogether.
The practical consequence
A shading survey across a day and across the seasons is worth more than a comparison of panel efficiencies. A slightly less efficient panel in full sun beats a better one behind a chimney, every day of the year.
Where this page and the manufacturer’s documentation differ, the documentation is the one to follow. Every capacity, output, input window, current limit, cycle count and temperature threshold on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any panel rating: peak watts are measured under standard test conditions — a defined irradiance, a defined cell temperature and a defined spectrum. Those conditions exist so that two panels can be compared, not so that a yield can be predicted. This page reports the maker’s figure as a test result and derives no daily energy from it.
What people ask before they order
What does a bypass diode do?
It lets current route around a shaded section of a panel instead of being throttled by it. It limits the damage rather than preventing it, and it is why a panel is divided into sections rather than being one long string.
Does diffuse light count?
Yes — a panel under bright cloud produces a real if much reduced output, because diffuse light still reaches it. A hard shadow from a chimney or a branch is a different situation entirely.
Last reviewed 16 September 2026