Solar Panel Degradation: What's Normal and What Isn't
Every solar panel degrades over time, but 'fast degradation' usually means something else is wrong.
Manufacturers spec roughly 0.5% annual degradation. What you actually see in production is often much worse, and the difference is almost never the panels themselves.
Spec degradation rates
Tier-1 panel manufacturers typically guarantee 90% of nameplate output at 10 years and 80–85% at 25 years. That's roughly 0.4–0.6% annual degradation, small enough that you shouldn't notice it year-to-year.
Real-world degradation rates
Real-world San Diego data on properly maintained systems matches spec closely, typically 0.4–0.7% per year. Systems that look like they're degrading at 3–5% per year almost always have a different problem (soiling, a failing optimizer, partial shading from new tree growth).
Why soiling is usually misdiagnosed as degradation
If production drops smoothly and steadily year over year, it looks like degradation, but it's usually accumulated soiling. A single cleaning that recovers 15–20% confirms the diagnosis. True degradation is invisible to the naked eye and shows up only in long-baseline data.