Assessment: PID Degradation — 31% Power Loss at Negative String Ends of a 280 kWp System

Assessment type: Damage assessment / Warranty System size: 280 kWp Polycrystalline Region: Schleswig-Holstein (coastal area) Period: Sep – Dec 2024

Assessment summary:

280 kWp commercial roof system (installed 2018) with progressive yield decline: −31% at the negative string pole
EL testing confirms PID-typical pattern: edge cell inactivation on ground-side modules
Contributing factors: coastal climate (high humidity), missing anti-PID potential at the inverter
PID regeneration attempt (night voltage +1,000 V): partial recovery of 18 percentage points
Remaining irreversible damage: 13% — manufacturer recourse recommended (performance warranty breached)

How did the PID issue manifest?

The operator of a 280 kWp rooftop system on a cold storage facility in Schleswig-Holstein (20 km from the North Sea coast) noticed a gradual yield decline from the 4th year of operation. The monitoring data revealed a conspicuous pattern: strings with negative grounding polarity (minus pole grounded at the inverter) lost disproportionate power, while positive strings were barely affected.

Performance comparison by polarity (measurement September 2024)

String group	Strings	Pmax (STC, nominal)	Pmax (STC, actual)	Deviation
Positive polarity (inverter+ grounded)	8	140 kWp	131.6 kWp	−6.0% (age-related)
Negative polarity (inverter− grounded)	8	140 kWp	96.6 kWp	−31.0%

The 25 percentage point difference between the positive and negative side is the classic PID indicator.

Which measurements confirmed PID?

Electroluminescence testing (sample of 48 modules)

The EL images of the modules at the negative string end showed the PID-typical pattern: edge cells (particularly the outer 2–3 cell rows) were partially to fully inactive (dark in EL). The degree of inactivation decreased with increasing distance from the grounding point — a clear PID profile.

Shunt resistance measurement

The shunt resistances (Rsh) of the affected modules ranged from 8–45 ohm. Unaffected modules showed Rsh values of 200–800 ohm. The low shunt resistance confirms the leakage-current-induced degradation mechanism (Na+ ion migration in the anti-reflective coating).

Contributing factors for PID at this location:

Coastal proximity: average relative humidity 82% (national average: 76%)
Salt aerosol exposure: increased conductivity on module surface
Inverter without anti-PID function (no nighttime voltage reversal)
System voltage 1,000 V (maximum PID risk at the negative pole)

Was regeneration possible?

In November 2024, a PID regeneration attempt was conducted: application of positive voltage (+1,000 V DC, module frame against cells) during nighttime hours over 21 days. Result:

Time point	Pmax neg. strings	Regeneration
Before regeneration (Sep 2024)	96.6 kWp (−31%)	—
After 7 days	108.2 kWp (−22.7%)	+8.3 pp
After 14 days	114.8 kWp (−18.0%)	+13.0 pp
After 21 days (completion)	121.8 kWp (−13.0%)	+18.0 pp

The regeneration plateaued after 21 days — the remaining 13% loss is to be considered irreversible (permanently damaged cell structures).

What are the economic consequences?

Damage calculation + warranty claim:

Item	Amount
Cumulative yield loss (years 4–6, progressive PID)	EUR 52,800
Irreversible permanent damage (−13% on 140 kWp, NPV 15 years)	EUR 68,400
PID regeneration system (retrofit anti-PID box)	EUR 4,200
Assessment + EL + IV measurement	EUR 12,600
Total damage	EUR 138,000

The module manufacturer's performance warranty (90% after 10 years, 80% after 25 years) is clearly breached at −31% after 6 years. A warranty claim for module replacement is justified.

Expert assessment: The cause is a combination of a PID-susceptible module type (without anti-PID certification per IEC TS 62804) and unfavorable site conditions. The installer should have mandated modules with PID resistance certification or an inverter with anti-PID function for coastal locations. Co-responsibility of the system designer is established in the assessment.

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Last updated: 2026-06-16 | Author: Christoph S. Prestele, TUV-certified expert assessor | PV-BESS-Assessor.com