Potential Induced Degradation in Solar Modules

A solar module that loses a noticeable share of its output in its first few years is often assumed to be defective, or to be degrading faster than the datasheet promised. Sometimes it is. But there is a distinct failure mode, potential induced degradation, that produces a step-like loss rather than a slow slide, and it is concentrated at one end of a string rather than spread evenly across the array. Recognising the pattern matters, because PID is partly reversible and partly preventable, and because it is routinely confused with ordinary ageing.

Potential induced degradation is an electrochemical effect. A module’s cells sit at some voltage relative to the module frame and to ground. In a long series string, the cumulative voltage can reach several hundred volts, and the modules at the negative end of the string sit furthest below ground potential. That sustained voltage difference, combined with heat and moisture, drives a small leakage current through the encapsulant and the glass. The current carries ions, and the resulting charge accumulation and migration degrades the cell’s ability to collect current. The module is not physically broken; its internal electric field has been disturbed.

Why the negative end of a string goes first

A string is a series circuit, so the voltage rises along it. The module nearest the negative terminal sees the largest negative potential relative to ground, and the module nearest the positive terminal sees the largest positive potential. PID is strongly associated with the negative end. This is why a monitoring graph can show one string producing less than its neighbours while the rest of the array looks healthy, and why the loss often appears at the start of the string rather than uniformly.

The magnitude of the stress depends on system voltage, module position, temperature, and humidity. Higher system voltages, hotter modules, and damp conditions all increase the leakage current. That is why PID is more common in hot, humid climates and in arrays operating at high DC voltage, and why it can appear in a system that has performed well for a few years.

How it shows up in monitoring and in images

In monitoring, PID usually appears as a gradual or stepwise decline in a string’s current or power relative to other strings, often with the affected string’s voltage looking normal. Because the loss is in current collection rather than in open-circuit voltage, a string can look electrically present but weak. The decline may be more pronounced in the afternoon or in hot weather.

Electroluminescence imaging makes the mechanism visible. Under forward bias in a dark room, a healthy cell emits uniformly. A PID-affected cell shows dark regions, often starting near the frame or at cell edges, and the pattern can be patchy across the module. The images are a diagnostic tool, not something a system owner produces; they are taken by inspectors and module testers.

PID compared with normal degradation and LID

Normal annual degradation is a slow, roughly linear loss that affects every module in the array and is anticipated in the performance model. Light-induced degradation (LID) is a separate, early loss that occurs in the first hours and days of exposure and then stabilises; it is a property of the cell material and is not voltage-driven. PID is different again: it is driven by system voltage and environmental conditions, it is concentrated at one end of a string, and it can be partly reversed.

Countermeasures

PID-resistant modules are the first line of defence. Manufacturers vary the encapsulant, the glass, and the cell passivation to reduce leakage current, and some publish a PID test result. The property to compare is the module’s resistance to the effect under high voltage, heat, and humidity, not a brand name.

System grounding and array configuration also matter. The voltage stress on a module depends on where it sits relative to ground. In some designs, the array is configured so that the negative end is not held far below ground, or the string voltage is kept lower. These are design decisions made at the proposal stage, and they are difficult to change later.

Night-time recovery devices apply a reverse bias to the array when it is not producing, which can restore some of the lost performance. The effect is partial and depends on the module and the severity of the degradation. It is a mitigation, not a repair, and it adds equipment to the system.

What people get wrong

The most common misdiagnosis is to treat a weak string as a failed module or a bad connector, and to replace hardware before the pattern is understood. That is a natural mistake: a single string underperforming looks like a fault in that string, and the usual troubleshooting steps point at connections and modules. PID is a system-level effect, so replacing one module may restore output briefly and then see the loss return as the new module is stressed in the same position.

The second mistake is to assume any early loss is PID. Light-induced degradation and ordinary mismatch also produce early losses, and they call for different responses. The distinguishing features are the concentration at the negative end of a string and the sensitivity to heat and humidity.

The third mistake is to expect a night-time recovery device to restore a module fully. Recovery is partial, and the underlying susceptibility remains.

What to check, and what not to touch

A PV array produces dangerous DC voltage whenever there is light on it, and it cannot be switched off at the panel. Do not open a combiner, probe a live string, or disconnect an energised connector. Describing what happens on the DC side is not permission to work on it. Roof work, mains connection, and meter changes are licensed work.

What a system owner can do is read the monitoring data and compare strings over time, note whether the loss is concentrated at one end of a string, and note whether it worsens in hot or humid weather. Those observations are what an inspector needs.

The jurisdictional caveat

Interconnection rules, export compensation, and incentive programmes are set by state and by utility, and they change. Nothing on this page should be read as a statement of the rule where you live. Before acting on a performance concern, check the current interconnection agreement, the utility’s metering rules, and any applicable state programme, and confirm with the installer or a licensed professional what work is permitted and by whom.

The pattern to look for is a string that is weaker than its neighbours in current while its voltage tracks the others, with the loss concentrated at the negative end and worse in heat and humidity. That pattern points toward PID rather than toward a failed module.