Degradation warranty claims are usually decided on documentation rather than on physics. A manufacturer is being asked to accept that specific modules have fallen below a guaranteed output, and the burden of demonstrating that falls on the owner. Without a baseline, weather-corrected records and identifiable serial numbers, a claim that is entirely valid in reality can be impossible to prove years after commissioning.
What a performance warranty actually promises
A module performance warranty guarantees that output will not fall below a stated fraction of the rated value at a stated point in time, measured under standard test conditions. That last phrase is important. Standard test conditions are a laboratory reference, not a sunny afternoon on your roof, so demonstrating a shortfall means either measuring the module under controlled conditions or performing a defensible correction from field measurements to the reference. Manufacturers commonly require flash testing at an accredited laboratory, which means removing modules from the array and shipping them. Some will accept properly documented field measurements taken with calibrated equipment. Which route applies is in your warranty document, and it is worth reading before you have a problem. Note also that the guarantee is generally per module rather than per system. A system producing less than expected overall does not by itself demonstrate that any particular module has fallen below its guaranteed output, which is why identifying and testing specific modules matters more than presenting a whole-system shortfall.
The baseline you cannot recreate later
The single most valuable record is what the system did when it was new. That means commissioning test results, string open-circuit voltages and operating currents with the irradiance and temperature at which they were taken, the module serial numbers and where each one sits in the array, the as-built string diagram, and at least a year of production data compared against a weather-corrected model. None of this can be reconstructed after the fact. Installers should provide it at handover, and owners should ask if it does not appear. Store it somewhere that survives a change of email address, a failed hard drive and a house sale.
Ruling out everything that is not the module
Before a manufacturer will look at a degradation claim, they will want the ordinary causes excluded. Soiling, shading that has grown since installation, a failed module-level device, a derating inverter, a degraded connector, a blown string fuse, a failed bypass diode and a monitoring sensor error all reduce apparent output without any module having degraded. A qualified technician’s report that documents the elimination of these, ideally with an infrared scan and an IV curve trace of the affected string compared against a healthy one, turns a complaint into a claim. Submitting a claim without that work usually produces a request to do it, months later.
Correcting for weather is the technical core
Comparing this year’s output to last year’s proves nothing, because the weather differed. A defensible claim compares measured production against modeled production for the same period using actual local irradiance and temperature data, and shows a shortfall that persists across seasons and is not explained by the causes above. Free modeling tools give a reasonable expectation for a given location and configuration, and more rigorous analysis uses on-site sensors. The stronger your weather correction, the harder the claim is to dismiss. This is the work that distinguishes an owner who gets modules replaced from one who gets a polite letter.
The commercial realities to plan around
Even a successful claim may only cover the modules themselves. Labor to remove and reinstall, scaffolding or lifts, shipping, replacement wiring and the cost of testing may all fall to the owner, and on a rooftop array those can exceed the value of the modules. Replacement modules may not match the originals electrically, which creates a mismatch problem in the string and may require rearranging the array. Manufacturers merge, exit markets and restructure, and a warranty from an entity that no longer exists is difficult to enforce. None of this argues against making a claim; it argues for reading the terms and choosing suppliers with those realities in mind at purchase time. Ask at the point of sale whether the warranty is backed by an insurance arrangement independent of the manufacturer, whether it transfers automatically on sale of the property or requires a registration step, and who administers claims in your country. Those three answers tell you more about the practical value of a long warranty than the number of years printed on it.
What to check before you act
- Collect commissioning data, serial numbers and the string map at handover.
- Keep at least a year of weather-corrected production data as a baseline.
- Have a technician rule out soiling, shading, electronics and wiring first.
- Read what the warranty pays for beyond the module itself.
Common questions
Can I prove degradation with my monitoring app alone?
Rarely. Monitoring data without weather correction and without the other causes excluded is not usually persuasive. It is a starting point for a technician’s investigation, not a claim.
How long do I have to file a claim?
That depends on the warranty document, which may impose notice periods and procedures. Read the claims section before you need it, because missing a procedural step can end an otherwise valid claim.
The practical takeaway
Build the evidence at commissioning, not at the point of dispute. Keep serial numbers, test data and a weather-corrected production baseline, have a qualified technician exclude the ordinary causes before filing, and know in advance what the warranty pays for.
References and further reading
- NREL: photovoltaics research including module reliability and degradation studies
- NREL PVWatts: modeled production for weather-corrected comparison
- NREL: a technical report from the laboratory’s photovoltaics publication library
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
