In one line
The plot of current against voltage for a cell, module or string — the diagnostic fingerprint that reveals shading, mismatch, degradation and wiring faults.
What it means
An I-V curve traces every operating point from short circuit (maximum current, zero voltage) to open circuit (maximum voltage, zero current). The knee between them is the maximum power point, and its position defines Vmp, Imp and Pmax. Because different faults distort the curve in characteristic ways, I-V tracing is the standard field diagnostic: a stepped curve indicates bypass diodes conducting under partial shade or cell damage; a shallow slope near short circuit suggests shunt paths; a rounded knee with reduced fill factor points to series resistance from corroded connections or poor solder bonds. Commissioning specifications increasingly require I-V traces on a sample of strings, referenced back to STC using measured irradiance and temperature.
Worth knowing
- The curve's knee is the maximum power point that MPPT algorithms hunt for.
- Curve shape diagnoses faults that a simple voltage or current reading cannot.
- Field traces must be translated to STC before comparing against the datasheet.
- Steps in the curve almost always mean shading, mismatch or an active bypass diode.
At a glance
Reading common curve distortions
| Curve symptom | Likely cause | First check |
|---|---|---|
| Distinct steps | Partial shading or active bypass diode | Visual shade survey, thermal scan |
| Low Isc, normal Voc | Soiling, uniform light loss | Module cleanliness, irradiance sensor |
| Low Voc, normal Isc | Cells or a substring shorted | Electroluminescence imaging |
| Rounded knee, low fill factor | Series resistance — connectors, solder | Connector torque, IR scan of joints |
| Sloped near short circuit | Shunt paths, cell cracks, PID | PID test, EL imaging |
Field notes
- 1Trace on a clear, stable-irradiance day; passing clouds invalidate the translation to STC.
- 2Record cell temperature and POA irradiance at the moment of each trace, or the result cannot be compared to nameplate.
- 3Keep commissioning traces on file — they are the baseline against which year-five degradation claims are argued.
Common mistakes
- Comparing a raw field curve directly to the datasheet without STC translation, then declaring the modules underrated.
- Tracing during variable cloud and interpreting irradiance transients as module faults.
Frequently asked
What does an I-V curve tell you about a solar panel?
It reveals the module's full operating behaviour — maximum power point, open-circuit voltage, short-circuit current and fill factor — and its shape identifies specific faults such as shading, cell cracks, series resistance or potential induced degradation.
What causes steps in an I-V curve?
Steps almost always indicate that one or more bypass diodes are conducting, which happens when a cell group is shaded, cracked or otherwise mismatched with the rest of the string.
Sources
Related terms
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