Clear answer, explained.
Most reputable commercial solar panel manufacturers offer a linear power warranty guaranteeing that panels will produce at least 80% of their rated capacity at 25 years — consistent with a degradation rate of 0.5–0.8% per year. This rate is built into the original project financial model, which accounts for declining output over the system's 25–30 year life.
A system losing output at a rate significantly above 0.8% per year — or showing a sudden sharp decline rather than a gradual trend — indicates a fault condition rather than normal degradation. Common causes include cell delamination, moisture ingress, inverter degradation, or persistent soiling. Thermographic inspection is the most effective diagnostic tool because these faults are often invisible in production data alone until they affect entire strings.
Monitoring tools that track actual generation against the degradation curve specified in the original project model provide an ongoing benchmark. A production shortfall of more than 5–10% against the model value for the same weather period, sustained over several months, is the threshold for professional investigation. Systems that appear to be meeting absolute production targets but are below the degradation-adjusted model may still be underperforming relative to expectations.
What this means in practice.
- Normal degradation: 0.5–0.8% of output per year
- 25-year panel warranties typically guarantee 80% of rated capacity
- Losses above 1% per year indicate a fault, not normal aging
- Sudden declines suggest inverter or wiring fault
- Thermographic inspection identifies cell-level faults not visible in data
- Production vs degradation curve is the correct comparison benchmark
Best-fit environments.
- Systems more than 5 years old where performance trends need review
- Facilities where production appears stable but may be below the model
- Any system where output has declined noticeably year-over-year
- Installations approaching or within a manufacturer warranty claim window
