Home / Asset care guides / Degradation vs underperformance
When a solar system produces less than it used to, the easy explanation is “the panels are getting old.” Sometimes that is true. But normal degradation is slow and fairly predictable — and it is often used to explain away losses that are much larger and entirely fixable.
This guide explains what normal degradation looks like, what underperformance looks like, and how facility managers and finance teams can tell the two apart before accepting lost savings as inevitable.
Crystalline silicon modules typically lose around 0.5% of their output per year, with a slightly larger drop sometimes seen in the first year. Exact figures depend on the product and are usually shown in the manufacturer's performance warranty curve.
Normal degradation has a recognisable shape:
After six years at around 0.5% per year, a system would typically be producing roughly 3% less than when new, all else being equal.
Underperformance is any loss beyond what normal ageing explains. It tends to show up as:
1. Normalise for weather. A cloudy year produces less than a sunny one. Compare output against irradiance using performance ratio or specific yield (kWh per kWp installed). IEC 61724 describes common monitoring metrics for this.
2. Compare like with like. Inverters and strings of the same type, orientation and age should perform similarly. Large differences between peers are rarely caused by degradation.
3. Look at the shape over time. Degradation is a gentle slope. Steps, dips and seasonal saw-tooth patterns suggest a specific cause.
4. Check against the expected curve. Apply typical degradation to the original design estimate and see how big the remaining gap is.
5. Confirm in the field. Thermal imaging, IV curve tracing and visual inspection by qualified technicians can confirm what the data suggests.
These are illustrative numbers, not a client result. Imagine a rooftop system designed to produce 100 units of energy a year (weather-adjusted). In year 7 it produces 87. Typical degradation of around 0.5% per year explains roughly 3–4 units of that loss. The remaining 9–10 units need another explanation — and that is where soiling, shading, inverter derating, failed strings or monitoring errors usually show up. Recovering even part of that gap can matter far more than the degradation itself.
For a high-consumption site such as a hospital running around the clock, that difference can be significant over the remaining life of the system.
Consider an independent review if:
SolarMD analyses your production, weather and billing data to separate normal ageing from recoverable losses, and presents it in a plain-English report — see the sample Solar Health Score report. Where fixes are needed, the work is done by licensed contractors, and we can help you find a qualified EPC.
Crystalline silicon modules typically degrade around 0.5% per year, sometimes with a slightly larger drop in the first year. The exact rate depends on the product and is shown in the manufacturer's performance warranty.
Compare weather-normalised output, such as performance ratio or specific yield, against the design estimate adjusted for typical degradation. Also compare inverters and strings against each other. Gaps much larger than normal ageing usually have a specific, often fixable, cause.
Often, yes. Common causes such as soiling, new shading, tripped protection, failed strings, inverter faults and monitoring errors can usually be addressed by a licensed contractor once identified. Some module defects may be covered by warranty.
It can be. If modules are declining faster than the manufacturer's performance warranty allows, you may have grounds for a claim. You will need normalised data and usually field test evidence to support it.
SolarMD is an independent solar intelligence and lifecycle coordination service for commercial, industrial and hospital sites using around 30,000 kWh per month or more. We diagnose performance, organise the evidence and, where needed, help you find qualified EPC contractors or explore partner financing options. Repairs and engineering sign-off stay with licensed contractors.
Or email hello@solarmd.net · See a sample Solar Health Score report (PDF)
