How to analyze an I-V curve
Analyzing an I-V curve is one of the most reliable ways to diagnose performance issues in a photovoltaic (PV) array. By comparing a measured curve against a predicted or standard curve, technicians can quickly determine whether a PV source circuit is performing normally or whether shading, soiling, manufacturer defects, wiring faults, or component degradation are affecting system output.
Modern I-V curve tracers, such as the Fluke PVA-1500 PV Analyzer™, provide high-resolution curve data and software that automatically adjusts predicted performance for field conditions. This makes it easier to isolate abnormal behavior, benchmark circuits, and troubleshoot efficiently.
If you’re ready to learn more about these instruments, read the article on Best I-V Curve Tracer for Solar Panel Efficiency Testing.
The flowchart referenced in this article (often called the “PV Array Troubleshooting Flowchart”) reflects extensive field research and expert collaboration, including studies of PV module reliability and guidance from the National Renewable Energy Laboratory (NREL).
Below is a simplified, two-step method for analyzing I-V curves using a structured, deviation-based approach.
Step 1: Capture a Useful, Valid I-V Curve
Before analysis is possible, confirm that the test produced a valid, high-quality curve.
Verify test connections
If the curve is incomplete or irregular:
- Confirm all test leads are fully seated and polarity is correct.
- Ensure the PV source circuit is closed (fuse installed and intact).
- If a fuse is present, test it for continuity.
- Inspect wiring for open module interconnections or visible damage (burn marks, charring, loose connectors).
Address intermittent electrical connections
Rarely, a curve may show vertical dropouts or downward spikes. These typically indicate:
- A jostled or loose test lead
- An intermittent series splice or loose terminal
- A failing module interconnection
- If identified within the source circuit, isolate and repair the connection before retesting.
Confirm normal baseline performance
A healthy I-V curve should:
- Match the predicted curve shape
- Have a performance factor between ~90% and 100%
- Exhibit expected open-circuit voltage (Voc), short-circuit current (Isc), maximum power voltage (Vmp), and maximum power current (Imp) characteristics, adjusted for temperature and irradiance.
- The Fluke PVA-1500 software uses manufacturers' curves under Standard Test Conditions (STC) and adapts them to field conditions to generate a predicted curve that can be compared to the measured curve, making it easier to confirm “normal” performance.
Step 2: Identify Which I-V Curve Deviation You Are Seeing
Once you have captured a valid curve, analyze it for one of six common deviations. These deviations correlate with specific faults, environmental conditions, or measurement errors. (The numbers in the Flowchart correspond to the numbered deviations below.)
1. Stepped I-V Curve
Appearance: Steps or notches caused by bypass diode activation.
Cause: Current mismatch in the string.
Likely sources:
- Partial shading
- Nonuniform soiling
- Damaged cells or cell strings
- Shorted bypass diodes
Interpretation: Steps indicate bypass diodes are protecting weak cell groups from reducing total output.
2. Low Short-Circuit Current (Isc)
Appearance: Short circuit current is lower than expected.
Cause: Lower-than-expected light capture or measurement error.
Likely sources:
- Shading or soiling
- Incorrect irradiance measurement
- Operator shading the array during measurement
- Degraded modules
This deviation appears early in the troubleshooting flowchart because many causes are easily corrected.
3. Low Open-Circuit Voltage (Voc)
Appearance: Open circuit voltage is lower than expected
Cause: Field temperature or measurement inaccuracies.
Likely sources:
- Incorrect cell temperature measurement
- Shading during testing (can temporarily reduce Voc)
- Hardware issues (rare — Voc changes very slowly as modules age)
Rule out environmental and measurement errors before concluding module degradation.
4. Rounder-Than-Expected Knee
Appearance: Maximum power point is lower than expected
Cause: Potential module aging or wear.
However:
- This can be subtle and difficult to differentiate from normal curve slope variations.
- Retesting and trending over time is the best approach.
- Knee rounding alone is not a definitive diagnosis. Cause is usually related to deviations 5 and 6.
5. Low Voltage Ratio (Vmp ÷ Voc)
Appearance: Reduced slope on the vertical leg of the curve.
Cause: Increased resistance in the circuit
Likely source:
- Thinner-than-planned conductors used in the home runs
- Broken bonds between cells
- Corroded cell and ribbon bus conductors
- Corrosion or degraded connections in the module J-box
- Potential Induced Degradation (PID)
Interpretation: A low voltage ratio indicates the curve exhibits an earlier power drop-off than predicted.
Action: Compare other strings under similar conditions using the voltage ratio as the metric — ensure mismatch-related steps are absent before comparing.
6. Low Current Ratio (Imp ÷ Isc)
Appearance: Increased slope on the horizontal leg of the curve.
Cause: Current at the maximum power point is lower than expected.
Likely source:
- Tapered sliver of shade or soiling across all cell groups
- Reduced shunt resistance in cells, which is a degradation effect
- Potential Induced Degradation, PID
- Typically reduces Voc and changes in the slopes of both legs of the I-V curve
Interpretation: A low current ratio suggests abnormal current behavior even under expected irradiance.
Before suspecting module hardware:
- Rule out shading
- Rule out soiling
- Verify correct irradiance measurement
- Once environmental factors are eliminated, investigate for module or string degradation.
Using I-V Curve Tracers in PV Troubleshooting
Tools like the Fluke PVA-1500 PV Analyzer™ are essential for diagnosing PV system performance because they provide:
- High-resolution I-V curves
- Automated STC-corrected predicted curves
- String-to-string comparison metrics
- Built-in documentation for commissioning or Operations and Maintenance (O&M) records
- By combining systematic capture techniques with deviation-based analysis, technicians can quickly isolate performance issues and ensure reliable long-term operation of PV assets.
Learn about the value of these tools in our article I-V Curve Tracing Delivers ROI in Solar Power System Applications.