Most 25-year panel warranties still guarantee output based on degradation assumptions written a decade ago — roughly 0.5% annual loss. Independent field data we reviewed from installations five to twelve years old shows top-tier monocrystalline panels degrading closer to 0.3% annually.
This matters for payback calculations more than it sounds: over 25 years, that difference compounds to roughly 4-5 percentage points of additional lifetime output that most solar calculators don’t credit you for, because they use the conservative warranty number by default.
