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What Is an I-V Curve?
An I-V curve (Current-Voltage curve) is a graph of the electrical output of a solar panel or string across its full range of operating voltages — from short-circuit (maximum current, zero voltage) to open-circuit (maximum voltage, zero current). The shape of this curve uniquely characterises the panel's health. Deviations from the manufacturer's specified curve reveal specific fault types.
Normal vs Faulty I-V Curve Characteristics
Deviation from Normal Curve | Likely Fault |
|---|---|
Reduced ISC (lower than STC-corrected value) | Soiling, partial shading, or cell crack reducing current generation |
Reduced VOC (lower than STC-corrected value) | Open-circuit bypass diode, cell internal short, or series string mismatch |
Stepped shape (multiple peaks) | Two or more groups of cells at different temperatures — partial shading or hotspot |
Reduced fill factor (curve less "square") | Increased series resistance — corroded contacts, degraded cell interconnects, poor connector termination |
Flat low-power region at maximum power | Bypass diode conducting — check for shading on cell group covered by that diode |
Correction to STC
Field measurements must be corrected to STC (1000 W/m² irradiance, 25°C cell temperature) before comparison to the datasheet. Correction requires measuring irradiance with a reference cell or irradiance sensor and cell temperature with a contact thermocouple:
ISTC = Imeasured × (1000 / G)
VSTC = Vmeasured - β × (Tcell - 25°C)
Most modern I-V curve tracers perform these corrections automatically when a reference cell and temperature probe are connected.
Engr. Jason Morales — Founder, SolarEnergyPH
