The I-V curve of a string records the current it delivers at each voltage, from short circuit to open circuit, and with it the maximum power point. It is measured on site with a curve tracer, together with irradiance and cell temperature, corrected to reference conditions with IEC 60891, and compared with the expected curve and with the curves of identical strings. Its shape exposes shading, conducting bypass diodes, high series resistance or leakage that a simple Voc and Isc reading does not show.
What the I-V curve measures
A PV string does not deliver a fixed current: depending on the voltage the load forces on it, it delivers more or less current. The I-V curve is the record of that relationship, from short circuit (zero voltage, maximum current, Isc) to open circuit (zero current, maximum voltage, Voc). At the knee of the curve is the maximum power point (Vmp, Imp), which is where the inverter’s MPPT tracker tries to operate.
The curve yields the parameters that define the condition of the string:
- Isc and Voc, the end points.
- Pmax, the maximum product of voltage and current.
- Fill factor, the ratio between Pmax and the product Voc × Isc. The more “square” the curve, the higher the fill factor and the healthier the string.
- Slopes near Isc and near Voc, which give an idea of the shunt resistance and the series resistance.
IEC 62446-1 includes the I-V curve in the category 2 test regime, for larger or more complex systems, because it shows whether performance matches what is expected and helps identify modules or strings with defects. The framework of the standard is described in IEC 62446-1: testing and documentation of PV systems.
How it is obtained
The tracer is connected to the string, disconnected from the inverter, and sweeps the load from short circuit to open circuit in a fraction of a second while it records voltage and current. At the same time, the irradiance in the plane of the modules is measured with a reference cell or an equivalent sensor, along with the cell temperature.
IEC 61829:2015 defines the procedures for measuring on site the characteristics of a flat-plate PV array, the weather conditions that apply to that measurement, and how to translate the results to standard test conditions or to other chosen conditions. Its 2015 revision accepted the use of the commercial tracers in common use and refers to the IEC 60904 series and to IEC 60891 for several of its procedures.
Instruments vary between manufacturers; what matters is that the instrument is suitable for the maximum voltage and current of the string and that the irradiance sensor lies in the same plane as the modules.
Correction to reference conditions
A curve measured mid-morning with passing clouds cannot be compared directly with the module data sheet. Current changes almost in proportion to irradiance, and voltage drops when cell temperature rises. That is why the curve is corrected.
IEC 60891:2021, third edition, defines the procedures for correcting to other temperature and irradiance conditions the I-V curves measured on cells, cell assemblies and modules. Each type of device uses its own set of parameters. The 2021 edition gives guidance on which procedure to use for each application and added one for crystalline silicon with unknown temperature coefficients. The temperature coefficients of a module can be calculated from single-cell measurements, but the series resistance and the curve correction factor must be measured on the module itself.
In practice, the correction loses accuracy the further the measurement is from the reference conditions. That is why the measurement is taken at high, stable irradiance, and why comparing identical strings measured at the same moment is often more revealing than comparing with the data sheet.
How to read it: typical shapes
| Curve shape | What it usually indicates | How it is confirmed |
|---|---|---|
| Step or intermediate “knee” | Part of the string produces less current and its bypass diode conducts: shading, localized soiling, cracked cell, faulty diode | Visual inspection and module thermography of the string |
| Steep slope near Voc | High series resistance: loose or oxidized connectors, damaged cable, internal solder joints | Thermography of connectors and boxes, continuity measurement |
| Downward slope near Isc | Low shunt resistance: internal cell leakage | Module-by-module comparison, electroluminescence in the laboratory |
| Low Isc with normal shape | Uniform soiling, misaligned irradiance sensor, degradation | Sensor check, cleaning and a new measurement |
| Low Voc with normal shape | Fewer modules than expected, shorted bypass diode, uncorrected high temperature | Module count, Voc measurement per module |
| Normal curve but low Pmax compared with identical strings | Module mismatch, potential-induced degradation | Module-by-module curves, specific laboratory test |
- Does the curve have a step?
- Yes: Is there visible shading or soiling over the string at the time of the measurement?
- Yes: Repeat the measurement without the shade or after cleaning
- No: Look for the cell or the diode with thermography of the string
- No: Is the slope near Voc steeper than in identical strings?
- Yes: Check connectors, boxes and cables: high series resistance
- No: Compare the corrected Isc and Voc with the expected values and with those of neighboring strings
- Yes: Is there visible shading or soiling over the string at the time of the measurement?
Errors that distort the reading
- Unstable irradiance during the sweep or between the sensor reading and the string reading.
- Sensor in another plane or with a different orientation than the modules, typical on solar trackers.
- Temperature measured at a single point of a long string, with modules at different temperatures.
- Bifacial modules measured without accounting for the rear-side irradiance, which also contributes current.
- Midday or afternoon shadows from neighboring structures that did not exist at the design hour.
The tracer software corrects none of these errors. A curve with “defects” measured under poor conditions can lead to replacing healthy modules.
Where it applies
| System | Is the I-V curve traced? |
|---|---|
| Low-voltage residential | Uncommon; category 1 of IEC 62446-1 is enough unless performance is low |
| Commercial and industrial | Often at acceptance or when performance is low |
| Solar park | Usual at acceptance as category 2 and in maintenance by sampling |
The curve is repeated during maintenance to compare against the baseline taken at acceptance. A drop in corrected Pmax on the same string, year after year, measures the real degradation of the array.
Relationship with the other tests
The I-V curve does not replace the category 1 tests: before tracing it, the polarity, Voc and current of each string and the insulation resistance of the array are confirmed. Nor does it replace monitoring of the inverter: when the inverter disconnects because of low insulation or residual current, the diagnosis starts with the typical faults of the PV array, not with the curve. When the curve suggests damaged cells or PID, electroluminescence in the field shows the module from the inside.
Before measuring, it helps to have the expected values of the string clear. You can calculate the Voc at the temperature of the moment with our free string sizing tool; the calculation criteria are in the guide on PV string sizing.
To obtain the expected curve of your string and compare it with the measured one, calculate it here.
Equipment for this test
This test is performed with equipment from recognized manufacturers; these guides explain what each one measures. In the quotation we recommend the test and the equipment your case needs.
- PVPM1500X: I-V curve tracer for strings up to 1500 V
- Metrel MI 3105 and MI 3115: LV installations and PV systems
We leave your plant ready to operate
Our team performs the commissioning of solar plants at medium, high and extra-high voltage: transformer stations, collector network, step-up substation and interconnection protection. We also trace I-V curves and run the other DC array tests (insulation resistance, polarity, Voc and Isc, and module thermography, also by drone), in Colombia or elsewhere on request, and the results go into the energization documentation delivered to the utility. Write to us through contact.
Related guides
- Solar panel thermography: IEC TS 62446-3
- PV array faults: ground, arc and connectors
- Solar plant commissioning tests
- Protections of a solar plant: inverter and interconnection
References
- IEC 61829:2015, Photovoltaic (PV) array – On-site measurement of current-voltage characteristics.
- IEC 60891:2021, Photovoltaic devices – Procedures for temperature and irradiance corrections to measured I-V characteristics.
- IEC 62446-1:2016+AMD1:2018, Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1: Grid connected systems – Documentation, commissioning tests and inspection.
- IEC TS 62804-1:2025, Photovoltaic (PV) modules – Test methods for the detection of potential-induced degradation – Part 1: Crystalline silicon.
Frequently asked questions
What is the difference between measuring Voc and Isc and tracing the I-V curve?
Voc and Isc are two points at the ends of the curve. A string can have both within range and still underperform because of high series resistance or module mismatch, which only show in the shape of the curve and in the position of the maximum power point.
Why does the curve have to be corrected to STC?
Because current is almost proportional to irradiance and voltage depends on cell temperature, and in the field neither is ever what it is in the laboratory. IEC 60891 defines the procedures to translate the measured curve to reference conditions, and only then can it be compared with the data sheet or with a measurement taken on another day.
Does a step in the curve always mean a damaged module?
No. The step appears when part of the string produces less current than the rest and its bypass diode starts to conduct. The cause can be shading, localized soiling, a cracked cell or a faulty diode. Before replacing a module, rule out shading and confirm with thermography or a module inspection.