Potential-induced degradation is the failure mode that most closely resembles a bad batch of modules, and most often is not one. An array loses output in a pattern, the losses concentrate at one end of the string, and the first suspicion — faulty modules — is usually wrong. The modules are responding to the system they were installed in.
That distinction matters because it decides who pays. This guide covers what PID is, what produces it, how resistance is tested rather than claimed, and what to put in a purchase specification so that the question never has to be argued later.
What PID actually is
Potential-induced degradation is a voltage-driven, gradual loss of output in a crystalline module. In a string, the cells sit at a large potential difference relative to the earthed frame. That difference drives ion migration — sodium from the glass being the most commonly cited path — into the cell and its passivation layers. The result is increased recombination and, in the worst cases, shunting. Output falls.
Nothing about the module has to be defective for this to happen. It is an interaction between the module construction and the electrical environment it is placed in. Two identical modules can perform very differently depending on which position in the string they occupy.
Why it appears as a pattern, not as a few bad modules
This is the diagnostic signature, and it is what separates PID from manufacturing defects.
In a string, the potential relative to earth is not uniform: it is most negative at one end and least negative at the other. The modules at the most negative end experience the largest driving potential, and in a PID-affected string they show the greatest loss. Moving along the string towards the positive end, the loss reduces. The array does not fail randomly — it fails in a gradient.
A manufacturing defect does not do this. Cell cracks, solder joint failures and lamination faults appear scattered, and they usually correlate with specific modules rather than with string position. If the losses line up with position, the cause is electrical, not a bad batch.
The conditions that produce it
Four things have to be present, and the first one is a design choice rather than a property of the module.
- High system voltage with the cells at negative potential. This is the driving force. The move from 1000 V to 1500 V systems raised the stress on every module in the string; the same module in a 1500 V array is under more pressure than in a 1000 V one.
- Heat. Ion migration accelerates with temperature. Hot rooftops and hot climates see faster progression.
- Moisture. Humidity provides the conduction path along the glass surface. A damp morning on a coastal site is enough to matter over time.
- Module construction. The glass, encapsulant, anti-reflective coating and cell passivation all influence susceptibility. This is why two modules with the same datasheet headline can behave differently under the same electrical stress.
The practical consequence for a buyer: the first factor is fixed by your system design, and the fourth by your module selection. Temperature and humidity you inherit from the site. PID risk is therefore partly a specification decision, not purely a product quality question.
Why the highest-voltage end of the string is the one at risk
Installers and designers who have met PID before will recognise this immediately: on an affected array, the worst modules are at the same end every time. It is worth stating explicitly in the design review, because it changes two things.
- Where to look first. When an array underperforms, measuring from the negative end of the string saves a full-site survey. Our guide to diagnosing underperformance covers the sequence.
- How to interpret a warranty claim. A gradient pattern points at the system, not the module — and that determines whether the claim is a product claim at all.
"PID-resistant" is a test result, not a marketing word
Almost every commercial module datasheet says something about PID. Very few buyers ask what stands behind the phrase, and that is where the specification usually fails.
The useful question is not is this module PID resistant but resistant under what conditions, for how long, measured how, and to what limit. Those are the four things a test report answers and a datasheet adjective does not.
The standard test method for this is IEC 62804, which subjects the module to elevated voltage at elevated temperature and humidity for a defined period and measures the power before and after. The specific stress conditions, duration and pass threshold for a given model belong in that model’s test report and warranty documentation, and those are the documents to ask for. A claim without them is not a specification.
What the test actually measures
Two outputs, and you want both:
- Power before and after the stress period. The headline number. What matters is the measurement uncertainty and which lab did it, not the figure alone.
- Electroluminescence imaging before and after. EL images show the spatial pattern of the degradation. This is the evidence that distinguishes a uniform small loss from a severe loss concentrated at the cell edges, and it is what makes a before-and-after comparison meaningful.
If a supplier can quote a percentage but cannot produce the EL images, the percentage is not worth much. Ask for both at the time of specification, not at the time of the claim.
Recovery: some of it reverses, and some does not
PID is not automatically permanent. Part of the loss can be recovered — by applying a reverse potential overnight, or in some cases by the array simply sitting idle through a dry period, depending on the mechanism and on how far it has progressed. Specialised recovery devices exist and are used on affected sites.
Two cautions. First, recovery depends on the mechanism: the reversible component recovers, the irreversible structural damage does not, and you cannot tell which you have from the power reading alone. Second, a site that needs repeated recovery treatment has an unresolved design problem, and treating the symptom year after year is more expensive than fixing the cause once.
Where PID lands in a warranty claim
This is where it becomes a commercial issue rather than a technical one. Whether PID is covered depends on two conditions being true at the same time: the module is certified PID-resistant, and the system was designed within the voltage limits stated for it.
If both hold, the degradation is a product performance matter. If the system voltage exceeded what the module was rated for, or the array configuration put the module outside its stated conditions, the claim is not a product claim — and no amount of testing documentation will convert it into one. The evidence that settles it is the same in both directions: measured string data and EL images, taken at commissioning and repeated when the loss is identified. Our module warranty claims guide walks through what to assemble before filing.
What to put in the purchase specification
Six lines, and they cost nothing to add at the point of order. They are expensive to reconstruct after the array is energised.
Reference table
Swipe horizontally to view all columns →
| Item | What to specify | Why it matters later |
|---|---|---|
| System voltage | The array’s maximum system voltage, stated | Determines which modules are rated for the installation |
| String position | Whether modules will sit at the negative end | The highest-stress position; worth confirming rating for it |
| Test method | PID testing to IEC 62804, with the report supplied | Turns an adjective into evidence |
| Test evidence | Power before/after plus EL images before/after | The only comparison that shows the pattern |
| Pass limit | Degradation limit from the model’s warranty document | Your reference if a claim is ever needed |
| Commissioning record | String-level measurement and EL at handover | Without a baseline, later loss cannot be attributed |
The last line is the one that is most often skipped and most often regretted. A PID claim without a commissioning baseline rests on an argument about what the array used to produce.
How this fits the rest of the specification
PID susceptibility is one of the properties that differ between cell technologies, alongside temperature coefficient, bifaciality and degradation rate. It belongs in the same comparison as those, not in a separate conversation — our module technology comparison sets them side by side, and the broader selection criteria are in the module buying guide.
We supply photovoltaic modules with the manufacturer’s datasheet and certification package, and the available PID test report and EL inspection records for the selected model can be confirmed in the order documentation schedule. What we will not do is quote you a single degradation number as though it applied to every installation: what a module loses depends on your system voltage, your climate and your string configuration, and a figure given without those is an assumption.
Send the system voltage and the string layout with the enquiry, and the module recommendation comes back against those numbers — including which positions in the string need the rating checked. Supplier documentation is one of the items covered in our supplier qualification checklist.
BUYER CHECKLIST
Buyer considerations for this scope
Ask for the test report, not the adjective
Nearly every datasheet claims PID resistance; very few buyers ask what stands behind the claim. The useful questions are under what conditions, for how long, measured how, and to what limit. Those are answered by a test report to IEC 62804, not by a datasheet phrase.
State system voltage and string position at enquiry
PID risk is partly a design decision, not purely a product quality question. The move to 1500 V arrays raised the stress on every module, and the modules at the most negative end of a string carry the largest driving potential — confirm the rating covers that position.
Get a commissioning baseline
Without string-level measurements and EL images at handover, a later loss cannot be attributed to anything. It is the line most often skipped at specification and most often regretted when a claim is filed.
PRACTICAL ANSWERS
Questions buyers ask about this
Is PID a manufacturing defect?
Usually not. It is an interaction between the module construction and the electrical environment it is placed in — high system voltage at negative potential, heat and moisture together. A manufacturing defect appears scattered across modules; PID appears as a gradient along the string, worst at the most negative end. That pattern is what distinguishes the two.
How do I tell whether an underperforming array has PID?
Measure at string level and compare position by position. Losses that increase towards the negative end of the string are the signature. Electroluminescence imaging confirms it — the images show the spatial pattern of degradation rather than just a power number. Both are easier to interpret if you have a commissioning baseline to compare against.
Can PID be reversed?
Partly. Some of the loss recovers, through applied reverse potential or in some cases through the array sitting idle, but the recovery depends on the mechanism and on how far it has progressed — irreversible structural damage does not recover. Repeated recovery treatment on the same site usually means the underlying design issue has not been addressed.
Is PID covered by the module warranty?
It depends on two conditions holding at the same time: the module is certified PID-resistant, and the system was designed within the voltage limits stated for it. If the system voltage exceeded the module's rating, the claim is not a product claim. String-level measurements and EL images taken at commissioning are what settle which case applies.
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