Understanding your partial shading loss
Why a thin shadow costs more than its size suggests
Solar panels are built with bypass diodes that protect groups of cells in blocks, usually thirds of the panel. A shadow covering even a small width of that block can trigger the diode to shut down the entire third, not just the shaded sliver. This threshold effect is why a shadow from something as thin as a TV antenna or a power line can cost far more output than its physical size would suggest. A NLR field study of shaded PV strings found shading over just 40% of a single cell can trigger complete power loss for that cell's entire bypass diode group, the threshold effect behind disproportionate shading losses.
How this differs from general array-wide shading
General shading loss covers shadows across whole panels or large sections of the array, while partial shading specifically covers a thin shadow crossing part of individual panels, which trips bypass diode protection rather than simply blocking a proportional amount of light. If your shading situation involves whole panels going dark, the general shading loss estimator is the better tool.
Why even a brief daily shadow adds up
It adds up more than people expect, since the substring shutdown applies for the full duration the shadow crosses that section, not just proportional to the shadow's size. A short daily shadow across many panels can still create a measurable annual loss, especially if it happens during otherwise strong production hours. A DOE assessment of 75 federal PV systems found real-world performance ratios varying by more than 14 percentage points across its sample, a spread wide enough to reflect exactly this kind of site-specific shading loss that a simple nameplate calculation would miss entirely.
How equipment choice changes the outcome
Yes. Power optimizers manage each panel's output independently and often include finer-grained maximum power point tracking that handles partial cell shading more gracefully than a standard string inverter relying only on the panel's built-in bypass diodes. Microinverters go a step further by isolating each panel completely. See the tree shading calculator if trees are your primary shade source rather than a thin fixed object. The power optimizer vs microinverter comparison can help decide between the two if you're weighing equipment upgrades specifically to address this issue.
What to do with this number next
If a specific object like a vent pipe or antenna is the culprit, weigh the cost of relocating it against the recovered output, or check the string mismatch loss calculator to see how this loss compounds at the string level. If trees are the shade source instead of a fixed object, distance is the fix, and the tree line setback sizing tool works out how far back the array needs to sit from the tree line to stay clear.