Warranty curves are minimum performance commitments
Many module warranties allow a larger output reduction in the first year and then specify a smaller annual degradation allowance for later years. The resulting percentage is usually a warranted minimum under the warranty terms, not a prediction that every panel will degrade exactly along that line. Some modules will perform above the warranty floor throughout their life.
First-year and later-year rates should be separated
A statement such as a first-year maximum degradation followed by a linear annual rate should be calculated in the order the manufacturer specifies. Do not multiply one annual figure by the full warranty term if the first-year allowance is different. Also check whether the warranty describes remaining power relative to original nameplate, a stepwise floor or another defined basis.
Real energy yield includes effects outside module aging
Soiling, shading growth, inverter availability, clipping, temperature, snow, wiring faults and downtime can change annual production without being module degradation. Conversely, weather variation can hide gradual module aging in year-to-year energy totals. A performance investigation therefore needs normalized data rather than simply comparing two utility bills.
Use degradation consistently in financial models
For long-term project modelling, choose a documented degradation assumption and keep it separate from equipment downtime and other losses. When comparing products, record the exact warranty document and revision. A stronger warranted curve can be valuable, but it should be considered alongside product warranty, manufacturer support, module construction, installation quality and verified availability.
Keep the original commissioning baseline
Preserve module model numbers, array layout, initial performance test results and monitoring configuration. Years later, those records make it easier to investigate an apparent production decline without guessing whether the cause is module aging, changed shading, inverter downtime or an altered monitoring system.