Understanding your string mismatch loss
Why one weak panel affects the whole string
Panels wired in series share the same current path, and that current is limited to whatever the weakest panel in the string can produce. This means a single underperforming panel can cap the output of every other panel in that string, not just its own share. This "weakest link" behavior is a fundamental property of series electrical circuits, not a flaw in the equipment. A NLR study on partially shaded series strings found that current in a single string is limited by its weakest panel, the underlying mechanism behind string mismatch loss.
Why this isn't a simple averaging problem
No, and this is a common misconception. Because of how series circuits work, the loss reflects the weakest panel's shortfall applied across the entire string's output, which is often far larger than a simple average of the panels' individual differences would suggest. A single panel running 8% weaker than its neighbors can cost a whole string 8% of its output, not a diluted fraction of that number. The inverter efficiency loss calculator works out what the inverter itself takes from that loss.
What typically causes mismatch within a string
Common causes include partial shading on just one panel, manufacturing tolerance variance, uneven soiling or debris, a developing hotspot or microcrack, or simply mixing panels from different batches or manufacturers within the same string. Any of these can create the kind of gap that drags down an entire string's output. Mixing panels from different brands or production runs is one of the more avoidable causes, which the degradation rate comparison by brand tool can help you think through before combining equipment from different sources.
How different equipment avoids this problem entirely
Yes, largely. Power optimizers and microinverters manage each panel's output independently rather than relying on a single series current shared across the whole string, which removes the weakest-link effect that causes mismatch loss in a standard string inverter setup. See the power optimizer output calculator or microinverter output calculator for how these setups compare. For a central inverter, that loss is small but always present: NLR's PVWatts documentation treats module mismatch as one of its standard system loss categories, applied as a small default percentage on top of every other loss factor.
What to do with this number next
If mismatch loss is significant, identifying and replacing or repositioning the underperforming panel often recovers most of the lost output, since the issue is isolated to a single weak link rather than a systemic problem. Check the wiring loss output calculator too for another electrical-side factor worth reviewing.