Potential induced degradation (PID) is a loss of module output caused by the high voltage between a module’s cells and its grounded frame. It is a multi-level problem, with causes and solutions at cell, module and system level.
It is one of the failure mechanisms that operators look for under solar O&M services, and it shows up in the data as an accelerated version of the normal solar panel degradation rate. This page explains the cause, the exposure and the usual responses.
| Item | What it means |
|---|---|
| Voltage between cell and frame | Up to 1000 V, or at some places 1500 V |
| Leakage current and ion diffusion | The electrical field causes both |
| Sodium ions | Sodium in soda lime glass combines with leakage current pathways |
| Recovery | Lost power may be recovered by applying reverse potentials |
| Stress test result | Only 22 % of tested modules passed the 5 % loss criteria |
Figures come from a PID test campaign paper; this information can change, so check the source for the latest details.
How potential induced degradation happens
Modules in a string are connected in series, so the voltage builds up along the string while the metal frames are grounded. A conference paper from Hasselt University and imec explains that when multiple PV modules are connected in series, a potential difference up to 1000 V or at some places even 1500 V is created between the cell and the grounded frame, and that this electrical field causes a leakage current and ion diffusion, in the PID test campaign paper.
The same paper describes the mechanism in the common crystalline silicon module with p-type cells. According to the authors, PID occurs at module level when the combination of soda lime glass, EVA encapsulant and the anti-reflective coating on the cell promotes it, with sodium ions in the glass combined with leakage current pathways. Studies cited in the paper found that sodium sticks in stacking faults in the silicon lattice and thereby creates a direct shunt path across the p-n junction. A shunt lets current bypass the junction, so the cell produces less power.
The effect depends on polarity and voltage. The paper notes that PID depends on polarity and the level of the potential between the cell and ground. Cells at the negative end of a string are usually the most exposed, which is why a plant affected by PID often shows the lowest output in the strings’ most negative modules. Humidity and temperature speed up the process, so it tends to be worse in warm, damp climates and in the early morning when dew is present.
Why modules differ in susceptibility
PID is not a universal fate. It depends on the combination of cell design, glass, encapsulant and the system voltage and grounding. The Hasselt paper describes PID as a multi-level degradation with causes and solutions at cell, module and system level. Two modules of different designs under the same string voltage may behave very differently, and so may two modules from the same factory with different bills of materials.
Test standards exist to screen for the problem. In a campaign by the same authors, modules were stressed according to the foil method of IEC 62804 in a climate chamber. After the stress test, only 22 % of the tested modules passed the 5 % loss criteria as described by IEC 62804. The campaign used a set of commonly installed module types in Flanders, so the share is not a world statistic, but it shows that a standard test can reveal wide differences between products. Buyers can ask for PID test reports as part of procurement.
Recognising PID on a plant
PID rarely announces itself with an alarm. Operators can look for these signs:
- Strings or inverter inputs whose current is steadily lower than that of comparable neighbours.
- Low-output modules concentrated at the negative end of strings.
- Thermal images that show warmer cells or whole modules in a pattern linked to the string position.
- Current-voltage curves with a reduced shape, indicating shunting, measured on a sample of suspect modules.
Good monitoring data makes this analysis possible. If string-level currents are logged and compared, the pattern can be found long before the energy loss is large. Without that data, PID can be mistaken for general ageing and be left uncorrected for years.
Prevention and recovery
Prevention starts with module selection and system design. Buyers can select modules with a PID-resistant bill of materials and a passed standard test, and designers can choose the grounding and string voltage configuration with PID in mind. The Hasselt paper cites work that showed ways to stop or minimise PID on panel and system level.
For modules already affected, recovery is sometimes possible. The paper notes that it is possible to recover the lost power by applying reverse potentials. That recovery is not guaranteed: in the authors’ test campaign, modules which lost more than 85 % due to PID showed a lower recovery rate or in worst case did not recover at all. Severe cases may therefore need module replacement, and warranty claims should be documented before any treatment changes the evidence.
The inverter side is part of the solution as well, because grounding and voltage settings are determined there. The routine checks that belong with that equipment are covered in the page on solar inverter maintenance.
Deciding what to do when PID is suspected
A suspected case calls for evidence before action. The first step is to confirm that the pattern really follows the electrical layout, using string data and, where possible, measurements on a handful of modules from different positions in the string. The second step is to estimate the energy lost so far and the loss expected in future years, because the economic case for a remedy depends on that number. A small, stable loss may be tolerated, while a growing one justifies treatment or replacement.
The third step is contractual. Module warranties and performance guarantees may cover PID, and some product certificates include a PID test. The owner or asset manager should collect the purchase documents, the test reports and the monitoring history, notify the manufacturer or installer within the notice period and keep samples in their untreated state until the supplier has had the chance to inspect them. Treating the modules first can weaken a claim.
The last step is to check the cause at system level. If one inverter or one grounding arrangement concentrates the affected strings, the remedy may be to change the configuration and not the modules. Because the mechanism is multi-level, the best result usually combines a cell and module fix with a sensible system design, and a plan that addresses all three levels is more reliable than one that addresses a single level.
Frequently asked questions
What causes potential induced degradation?
Show answer
High voltage between the cells and the grounded frame produces a leakage current and ion diffusion, which in common p-type modules lets sodium create shunt paths in the cell.
Which modules are most affected?
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Those at the negative end of a string are usually the most exposed, and susceptibility varies widely between module designs and materials.
Can PID be reversed?
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Often partly. Applying a reverse potential can recover lost power, but heavily degraded modules may recover slowly or not at all.
How can a plant owner check for PID?
Show answer
By comparing string currents over time, using thermal imaging and measuring the current-voltage curves of sample modules.
