PID (Potential-Induced Degradation) in Solar Panels: Detection and Recovery

Potential-induced degradation (PID) is a performance loss mechanism that affects certain solar modules over time. Unlike immediate failures, PID develops gradually and can reduce array output by 10-30% if left unaddressed. Because the loss is progressive and distributed across many modules, it's often missed until a significant performance drop accumulates. This guide explains how to identify, confirm, and address PID.

What Causes PID?

PID occurs when leakage currents flow between the solar cells and the grounded module frame. These leakage currents are driven by the high voltage between the cells and the frame, which is highest for modules near the negative pole of the string. Over time, the leakage currents cause changes in the cell's semiconductor properties that reduce its ability to generate power. The effect is accelerated by high humidity, high temperature, and certain cell technologies (particularly older p-type crystalline silicon cells).

Signs of PID from Monitoring Data

PID has a characteristic signature that can often be identified from string-level monitoring data before any field testing is done. The key indicators are:

• Progressive, gradual performance decline across a section of the array over months or years — not a sudden drop
• The decline is worst on modules closest to the negative pole of the string and diminishes toward the positive pole (this voltage-dependent pattern is the hallmark of PID)
• Multiple strings affected in the same section of the array, with the worst-performing strings being those with the highest negative-pole voltage
• The affected modules show reduced current (not just voltage), as PID degrades the cell's ability to generate photocurrent
• The decline does not correlate with weather, soiling, or shading — it persists across all conditions

Confirming PID with Field Testing

While monitoring data can strongly suggest PID, field testing is needed to confirm the diagnosis and assess severity.

1. 1Perform an electroluminescence (EL) imaging test on suspect modules. PID-affected cells appear dark or partially dark in EL images, in contrast to the bright, uniform glow of healthy cells. The pattern is typically worst on cells near the module frame.
2. 2Compare the IV curve of a suspect module to a healthy module of the same type and age. PID reduces both the short-circuit current and the fill factor, creating a characteristic 'rounded' IV curve knee.
3. 3Measure the insulation resistance between the cell string and the module frame. PID-affected modules show lower insulation resistance than healthy modules.
4. 4If possible, compare modules from the negative end of the string to modules from the positive end. The voltage-dependent gradient is a strong confirming indicator.

PID Recovery Options

If PID is confirmed, several recovery approaches are available depending on the inverter and the severity of the degradation:

• Inverter-based PID recovery: Some modern inverters include a PID recovery function that applies a reverse voltage to the array at night, counteracting the leakage currents. This can partially or fully restore affected modules over weeks or months of nightly treatment.
• External PID recovery devices: For inverters without a built-in recovery function, external devices can be installed that apply the reverse voltage at night.
• Grounding configuration changes: In some cases, changing the array grounding configuration (if the inverter and system design allow it) can reduce or eliminate the voltage driving the PID. This must be done carefully and in compliance with electrical codes.
• Module replacement: For severely affected modules where recovery is not possible or not effective, replacement is the only option. Check the manufacturer's warranty — some manufacturers cover PID under extended warranties.

Preventing PID in New Installations

If you are designing or procuring modules for a new system, PID risk can be minimized by selecting modules that are certified as PID-resistant (tested per IEC 62804), using inverters with built-in PID recovery functions, and considering the system grounding configuration during design.

When to Call for Remote Support

If your monitoring data shows a progressive performance decline that you suspect may be PID, remote support can help by analyzing your string-level data to identify the voltage-dependent pattern characteristic of PID, estimating the severity and extent of the degradation, and recommending whether field testing or inverter-based recovery is the best next step.