Solar Lattice

Symptom

Low module open-circuit voltage

ID
symptom:low-module-voc
IRI
https://solarlattice.org/kb/symptom/low-module-voc
Status
active
Also known as
Voc

Low module or string open-circuit voltage means the measured Voc is below what the datasheet, flash report, an earlier IV curve or a similar string would predict. Find it with an IV trace or a string Voc measurement, and compare against a model corrected for module temperature or against identical strings under stable irradiance. Possible causes include a shorted bypass diode, failed cell interconnections, cell-to-cell shorts, light induced degradation and potential induced degradation. A shorted bypass diode lowers module voltage all the time and can heat cells unevenly, while an open diode usually shows no loss until the module is mismatched or shaded. PID typically affects modules unevenly across the field and shows a lower Voc together with a steeper IV slope near short-circuit current; EL, thermography and Voc checks help confirm it. LID appears early in the system's life as lower power, current and voltage with no visible defect, sometimes with a patchwork EL pattern. At the string level, a voltage well below expected can also mean a module wired with reversed polarity, poor insulation or water in conduits or junction boxes, so check the wiring and the number of modules in series first.

<!-- review notes

Claims

observed_as

c1 drafted · lead Low open-circuit voltage

Measured Voc falls below the value expected from the datasheet, flash report, an earlier IV curve, or a comparable string. Causes include failed cell interconnections, cell-to-cell shorts, a shorted bypass diode, reversed module polarity, poor insulation, LID or PID.

c2 drafted · lead IV curve step

Steps in the IV curve can point to a bypass diode defect, damaged cells or heavy cell mismatch, and can appear together with reduced voltage.

review note: support check: partial - Steps are linked to bypass diode defect, damaged cells, or heavy mismatch, but the passages do not say they appear together with reduced voltage.

c3 drafted · lead Sloped IV curve near Isc

In PID, lower Voc comes with a steeper slope at the current axis because shunt resistance has dropped.

c4 verified · fact Patchwork EL pattern

With LID, an EL image that grows more non-uniform (patchwork) can sometimes reveal ongoing degradation.

verified by: joshuahamsaverified on: 2026-10-20

typical_power_loss

c5 disputed 1–3 PERCENT

Conditions: Boron-oxygen LID in c-Si modules, within the first days of sun exposure

conflicts with: Low module open-circuit voltage (c9)

c6 disputed ≤ 10 PERCENT

Conditions: LeTID, mainly in PERC modules running above 50 °C; this loss adds to the LID loss and develops over years

conflicts with: Low module open-circuit voltage (c9)

c7 disputed 2–4 PERCENT

Conditions: Initial light-induced degradation, as limited loss reported in the IV-curve failure review

conflicts with: Low module open-circuit voltage (c9)

c8 verified · fact ≤ 30 PERCENT

Conditions: Light soaking effect (LID) in general, as potential loss

verified by: joshuahamsaverified on: 2026-10-20review note: support check: partial - LID losses up to 30% are stated, but the passage does not link this to Voc and it is an upper potential bound, not a typical value.; reviewer: the compiler-flagged conflict is a different scenario or technology, not disagreement

c9 verified · fact nominal 41 PERCENT

Conditions: Average performance loss from PID at one 10.7 MW plant in Spain

verified by: joshuahamsaverified on: 2026-10-20review note: support check: partial - The 41% average performance loss at the 10.7 MW plant in Spain is stated, but it is not tied to Voc.; reviewer: the compiler-flagged conflict is a different scenario or technology, not disagreement

Referenced by

Data