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PV I-V curve: measured vs expected, translation to STC and diagnosis

Open a measurement from an I-V curve tracer (a CSV file) or use an example, and compare it with the curve the module would give, using its datasheet values, under the test conditions. It calculates the maximum power, the fill factor, the apparent resistances and a diagnosis from the shape of the curve. The file never leaves your device.

IEC 62446-1commissioning tests of photovoltaic systemsIEC 60904-1current-voltage characteristic measurementIEC 60891translation to STC: reference; its procedures are not reproduced here

Example cases
PV I-V curve: measured vs expected, translation to STC and diagnosis

Only the basic data; the rest is taken with typical values.

ALearn

E1Module (nameplate, at STC)
A

From the datasheet, at 1000 W/m² and 25 °C.

V
A
V

1 = a single module; the number of modules in a string if you measure the string.

E2Test conditions
W/m²

With the reference cell or the pyranometer mounted in the plane of the modules.

°C

Not the air temperature: the cell temperature (back-surface sensor or calculated). A 1 °C error shifts Voc by 0.27 %.

E3Measurement

The examples are our own curves, measured at 870 W/m² and 47 °C ("healthy b", at 650 W/m² and 41 °C). "My measurement" uses the table below.

Your file never leaves your device. It is read in the browser and is neither uploaded nor stored. The table is not included in the copied link.

Curve points

One row per point: string voltage in volts and current in amperes. Paste the two columns of the tracer's CSV, with or without a header; semicolon, comma and tab are recognized.

RowVoltageVCurrentARemove

E4Model and criterion
%/°C
%/°C
%

Use a decimal point. The calculation runs in your browser and updates as you type.

Measured maximum power

— kW

Fill factor

—

Measured short-circuit current

— A

Measured open-circuit voltage

— V

Pmax against expected

— %

Isc against expected

— %

Voc against expected

— %

Fill factor against expected

— %

Power at STC

— kW

Against the nameplate power

— %

Isc at STC

— A

Voc at STC

— V

Diagnosis

—

Apparent series resistance

— Ω

Apparent shunt resistance

— Ω

Power peaks

—

Current against voltage
  • Measured
  • Expected
  • Measured Pmax
The thick line is your measurement and the thin one is the expected curve from the nameplate data at the irradiance and temperature of the test. The dot is the maximum power point.
Power against voltage
  • Measured
  • Expected
A healthy curve has a single peak. Two peaks indicate a step: shading, localized soiling or a conducting bypass diode.
Current difference: measured minus expected
  • Measured − expected
  • Zero
A flat line near zero is a string that follows the model. A uniform offset points to irradiance; a gap only near the knee points to the resistances; spikes point to steps.
Deviations from expected
  • Deviation
  • Pmax band
Each bar is measured against expected under the same conditions. The dashed line is the Pmax band you accepted; the others have no normative threshold.
Measured curve translated to STC
  • Measured during the test
  • Translated to STC
  • Nameplate at STC
The translated curve is obtained by fitting the module model to your measurement and bringing it to 1000 W/m² and 25 °C. If it lies above the nameplate curve, the string delivers what is guaranteed.
Power at STC
  • Measured during the test
  • Translated to STC
  • Nameplate at STC
The peak of the translated curve is the power at STC, which is compared with the nameplate.
Normalized slope of the curve
  • Measured
  • Expected
The −dI/dV slope of a healthy curve only increases. A dip followed by a new rise is a step: a bypass diode has started to conduct.
Power and steps
  • Measured
  • Expected
Power peaks are counted on this curve: a single one is normal.

Step-by-step calculation

What it means for your plant

    What it is for

    The I-V curve of a string is the DC-side test used in the field to check that the modules deliver what their datasheet says. This tool reads the measurement from your tracer, compares it with the expected curve, brings it to standard conditions and tells you which pattern it shows: steps, low current, low voltage, high series resistance or shunt leakage.

    If the string belongs to a plant you own, use the strings tool to find out how many modules the inverter accepts, and the losses and performance ratio tool to see how much it produces. What a curve does not show, such as a module with cracked cells, thermography does.

    How it is calculated

    IEC 60904-1current-voltage characteristic

    Module model (one diode, five parameters)

    I = Iph − I0 · (exp((V + I · Rs) / a) − 1) − (V + I · Rs) / Rsh · a = n · Ns · k · T / q

    From the four nameplate points (Isc, Voc, Imp, Vmp), Rs and Rsh are found so that the curve passes through the maximum power point and peaks there; Iph and I0 follow from Isc and Voc. This is the fit from the open literature (Villalva, Gazoli and Filho, 2009; De Soto, Klein and Beckman, 2006).

    Test conditions

    Isc(G, T) = Isc · G/1000 · (1 + α · (T − 25)) · Voc(T) = Voc · (1 + β · (T − 25)) · Rsh(G) = Rsh · 1000/G

    Current is proportional to irradiance, open-circuit voltage falls with cell temperature according to the datasheet's β coefficient, and the shunt resistance grows as irradiance drops. Series resistance is taken as constant. The string adds the voltage of its modules at the same current.

    Measurement indicators

    FF = Pmax / (Isc · Voc) · Rs,ap = −dV/dI near Voc · Rsh,ap = −dV/dI near Isc

    The apparent resistances are the slopes of the curve at the two ends (least-squares fit in the low-current region and in the low-voltage region). They include the diode's dynamic resistance, so they are not the model's Rs: compare them with those of the expected curve, calculated the same way. Steps are counted on the normalized slope: when it drops and rises again, a bypass diode is conducting.

    Translation to STC (using the model fitted to your measurement)

    Isc,STC = Isc · (1000/G) / (1 + α · (T − 25)) · Voc,STC = (Voc + a · ln(1000/G)) / (1 + β · (T − 25))

    The model is fitted to the four key points of the measurement (at the test temperature), its Rs is kept, Rsh is brought to 1000 W/m² and the curve is rebuilt with Isc,STC and Voc,STC. It is not the IEC 60891 procedure: its equations are in the standard, which you should consult for a formal report. A curve with steps is not translated.

    Your CSV file

    • Choose "My measurement (CSV)" in "Measured curve", tap "Open file" and select the CSV from your tracer, or paste the columns into the table. The file is read in your browser.
    • Voltage and current are accepted in two columns, separated by semicolon, comma or tab, with a decimal comma or point, with or without a header. If there are more columns, those named voltage/V and current/I are used; otherwise, the first two numeric ones.
    • The voltage must be that of the whole string, in volts, and the number of modules in series must match the measurement.
    • The irradiance and temperature of the test are not read from the file: enter them in "Test conditions".

    Limits of the calculation

    • An average module: the modules in the string are treated as identical. The real spread between modules, the datasheet power tolerance and the uncertainty of the tracer and the pyranometer are not included; the Pmax band is yours to set.
    • The diagnosis classifies the curve pattern with this tool's starting thresholds, not those of a standard. A pattern is not a cause: confirm in the field with thermography and by checking the connections.
    • The translation to STC uses the one-diode model with the datasheet's linear coefficients and applies the irradiance to both resistances; for an IEC 60891 report, use the standard's procedure or the tracer's software. Measurements taken between 600 and 1000 W/m² change little; below 400 W/m² the uncertainty grows.
    • The examples are curves generated with the same model, not measurements from real equipment. Effects the model does not reproduce, such as potential-induced degradation (PID) in detail, mismatch between modules from different batches or irradiance drift during the sweep, are not simulated.
    • The apparent resistance includes the diode's dynamics; compare it only with this tool's expected value, not with values from other sources.

    Frequently asked questions

    How do you compare a measured I-V curve with the expected one?

    The curve the module would give with the datasheet values (Isc, Voc, Imp, Vmp and temperature coefficients) at the irradiance and cell temperature of the test is calculated and overlaid on the measurement. Maximum power, short-circuit current, open-circuit voltage and the shape of the curve are compared, not just a single number.

    What does a step in a string's I-V curve mean?

    It means that one part of the string delivers less current than the rest. A bypass diode bridges that part and the curve shows a step and two power maxima. Common causes are shading, localized soiling, cracked cells or a module with different degradation from the others.

    Why does the measurement have to be brought to standard conditions?

    Because the module nameplate is given at 1000 W/m² and 25 °C and the test rarely takes place under those conditions. To know whether the power is the guaranteed one, the measurement is translated to those conditions. This tool does it with the module model fitted to the measurement itself; the standardized procedure is the one in IEC 60891, which it does not reproduce.

    We test your solar plant

    We measure the I-V curves of the strings, carry out thermography, drones included, and run the other DC-side tests of your plant, with a report for each test.

    See solar plant testingSee thermography

    Related guides

    PV String I-V Curve: What It Measures, How to Read ItWhat a PV string I-V curve measures, why it is corrected to STC with IEC 60891, and what steps, slopes and low Voc or Isc reveal.PVPM1500X: I-V Curve Tracer for Strings up to 1500 VWhat the PV-Engineering PVPM1500X I-V curve tracer is: 1500 V and 20 A ranges, Rs and Rp, sensors, limits and how to interpret a string curve.IEC 62446-1: PV System Testing and DocumentationWhat IEC 62446-1 requires for a grid-connected PV system: documentation, inspection, category 1 and 2 tests, additional tests and reports.Solar Panel Thermography: IEC TS 62446-3 InspectionPV module thermography per IEC TS 62446-3: simplified or detailed inspection, from the ground or by drone, required conditions and thermal anomalies.PV Array Faults: Ground Faults, Arcs and ConnectorsTypical PV array faults: DC ground fault, series arc, connectors, PID, bypass diodes and hot spots. What gives them away and how to confirm each one.

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