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.