A gradual performance decline is a slow fall in irradiance-normalized production while the plant stays online. Track performance ratio trends and normalized output against rain and cleaning records, soiling station data and model residuals. If the decline builds up during dry periods and recovers partly or fully after real rain or washing, dust soiling is the likely cause. Do not judge soiling from one dirty irradiance sensor, because sensor drift or local obstruction can look like soiling. A decline that lasts for years and runs faster than the module warranty rate can point to system-level degradation: modules age at different rates, and the weaker ones limit the current of their strings. To separate this from average module ageing, compare string or inverter trends, IV curve data and module test results. Broken glass may not cut power at first, but moisture getting in later speeds up corrosion and degradation, so check the rear glass visually and watch for insulation faults. Potential induced degradation usually shows up after commissioning and can strongly reduce string performance.
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observed_asc1 verified · fact Declining performance ratio
For dust soiling, irradiance-normalized production falls slowly through dry periods; compare performance ratio trends and model residuals for dry accumulation periods.
sec=h4 h=dea316ba61e1sec=front h=931a735f2be9sec=h2 h=5d4993c87d37verified by: joshuahamsaverified on: 2026-10-20
c2 verified · fact Production recovery after rain
Soiling-driven decline is often partly or fully recovered after meaningful rain or washing.
sec=h4 h=dea316ba61e1verified by: joshuahamsaverified on: 2026-10-20
c3 drafted · lead Long-term normalized output decline
For system vs module degradation, long-term normalized plant output and string or inverter trends decline faster than the module nameplate degradation rate.
sec=front h=ff0c8c5f07ccsec=h2 h=14c4627b4866review note: support check: partial - Passages support comparing long-term normalized output and string/inverter trends and system degradation exceeding nameplate, but not the specific yearly energy or string current spread signals.
degradation_ratec4 verified · fact nominal 0.1 PERCENT-PER-YR
Conditions: Allowance some independent engineers add to the module degradation rate to cover mismatch from modules degrading at different rates; a modeling allowance, not a universal rule.
sec=h6 h=03fbe99d08d3verified by: joshuahamsaverified on: 2026-10-20
typical_power_lossc5 verified · fact 1–2 PERCENT
Conditions: Loss from glass breakage events in temperate climates, per the Herz et al. study.
sec=h5 h=086a578b6c8averified by: joshuahamsaverified on: 2026-10-20review note: reviewer: the compiler-flagged conflict is a different scenario or technology, not disagreement
c6 verified · fact 20–80 PERCENT
Conditions: Reduction of PV string performance from potential induced degradation after commissioning, per Libby et al.; source states it as up to 20-80%.
sec=h3 h=0587f1d564aeverified by: joshuahamsaverified on: 2026-10-20review note: reviewer: the compiler-flagged conflict is a different scenario or technology, not disagreement
presents_as c5presents_as c23presents_as c8presents_as c9presents_as c2presents_as c3