According to the data sheet, a PV string has an open-circuit voltage of approximately 850 V. During testing, 842 V is measured – at first, everything appears plausible. Nevertheless, the string delivers significantly less power than neighboring strings in the same system.
A simple voltage measurement provides only limited help when troubleshooting this type of problem.
The short-circuit current alone does not reveal every deviation either.
Significantly more information is provided by the complete:
I-V curve
of the PV module or string.
It shows how the generated current behaves over the entire voltage range from short circuit to open circuit. This makes faults visible that may remain hidden during an individual voltage, current or power measurement.
Typical examples include:
- partial shading,
- uneven soiling,
- mismatch between modules or cell groups,
- activated or damaged bypass diodes,
- increased series resistance,
- damaged cells,
- different irradiance levels within a string,
- degraded modules.
However, the shape of the curve alone is not sufficient for every definitive diagnosis. Irradiance, module temperature, measurement setup and stored module data also influence the result.
Reliable PV I-V curve measurement therefore always consists of two steps: First, the actual measurement conditions must be recorded correctly. Then it must be assessed whether the shape and characteristic values of the measured I-V curve correspond to the expected behavior of the module or string.
What Does the I-V Curve of a PV Module Show?
A photovoltaic module does not deliver a constant current at a constant voltage.
Its operating point depends on the connected electrical load as well as on:
- irradiance,
- cell temperature,
- module condition,
- shading,
- soiling.
The I-V curve therefore represents the relationship between:
current I
and:
voltage V
over the entire possible operating range.
At one end is the condition:
V = 0 V
with the short-circuit current:
I = ISC
.
At the other end is:
I = 0 A
with the open-circuit voltage:
V = VOC
.
Between these points lies the operating point that is decisive for energy generation:
Maximum Power Point
or:
MPP
.
Electrical power is calculated as:
P = V × I
.
The point at which this product is at its maximum has:
VMPP
and:
IMPP
.
Understanding Voc, Isc, MPP and Fill Factor
Several important values can be determined from an I-V curve measurement.
| Parameter | Meaning | Typical Significance |
|---|---|---|
| VOC | Open-circuit voltage | Voltage of the module or string without load |
| ISC | Short-circuit current | Maximum current at approximately 0 V |
| VMPP | MPP voltage | Voltage at the maximum power point |
| IMPP | MPP current | Current at the maximum power point |
| PMPP | Maximum power | Product of VMPP and IMPP |
| FF | Fill factor | Measure of the shape or “squareness” of the I-V curve |
The fill factor can be calculated in simplified form as:
FF = PMPP / (VOC × ISC)
.
A module can therefore have an almost normal open-circuit voltage and a plausible short-circuit current while still delivering reduced maximum power.
The cause must then be investigated in the shape of the curve or in the region around the MPP.
Why Open-Circuit Voltage and Short-Circuit Current Alone Are Not Enough
A measurement of:
VOC
and:
ISC
checks only two points on the complete curve.
Many relevant faults lie between these points.
A string may, for example, produce a plausible voltage under open-circuit conditions. Under load, however, a weaker cell group may activate its bypass diode.
The result may be a clearly visible step within the curve.
Increased contact or series resistance may also become apparent primarily around the knee of the I-V curve.
Voc and Isc may remain comparatively normal while the actually usable peak power has already been reduced.
I-V curve measurement therefore does not merely examine the two end points, but the electrical behavior of the PV generator over its complete operating range.
How Irradiance Changes the I-V Curve
The current generated by a PV module depends strongly on the solar irradiance incident on the module.
If irradiance decreases, this particularly reduces the:
short-circuit current ISC
and therefore also the available operating current.
The open-circuit voltage also changes, but significantly less than the current.
Example
An I-V curve measured at:
950 W/m²
cannot be compared directly with a curve recorded at:
600 W/m²
.
Even a completely intact module would show different currents and power levels.
Irradiance measurement is therefore an integral part of I-V curve measurement.
How Module Temperature Influences the I-V Curve
Cell or module temperature also changes the electrical behavior.
As temperature increases, the voltage of typical crystalline silicon modules decreases in particular.
This reduces, among other things:
- VOC,
- VMPP,
- and generally also the maximum available power.
Current changes much less with temperature and may increase slightly in crystalline silicon cells.
A hot module on a roof can therefore deliver a significantly lower voltage than the STC voltage specified on the nameplate without being defective.
For a reliable assessment, the module temperature must therefore be recorded together with the I-V curve.
Why Measurements Are Converted to Standard Test Conditions
The performance data of a PV module are normally specified for defined reference conditions.
These include:
Standard Test Conditions
or:
STC
.
The main reference values include:
- 1,000 W/m² irradiance,
- 25 °C cell temperature.
These conditions are rarely present exactly in the field.
A professional I-V curve measurement therefore records:
- the current I-V curve,
- irradiance,
- module temperature
and can convert the measured values to reference conditions using suitable module parameters.
The methods used for this purpose are described, among other places, in IEC 60891.
This allows a curve measured at a module temperature of 43 °C, for example, to be compared more meaningfully with the manufacturer data.
Without temperature and irradiance correction, a completely intact module may incorrectly be assessed as underperforming.
Detecting Shading in the I-V Curve
Partial shading is one of the most characteristic causes of distorted I-V curves.
If only part of a module or string receives less light, the affected cells can generate less current.
Because the cells within a cell string are electrically connected in series, a current mismatch occurs.
The affected cell group can no longer fully carry the current of the more strongly illuminated cell groups.
Depending on the severity, the module’s bypass diodes become active.
Typical I-V curve characteristics:
- steps,
- notches,
- abrupt changes in slope,
- multiple possible power maxima.
The exact shape depends on:
- which cells are shaded,
- how many cell groups are affected,
- how strongly irradiance is reduced,
- how the bypass diodes are connected within the module.
A single small shadow edge can therefore have a significantly greater effect on electrical power than the visually shaded area might initially suggest.
How Bypass Diodes Cause Steps in the I-V Curve
Bypass diodes protect cell groups against excessive reverse bias and limit the effect of weak cell groups on the total module current.
If a cell group is heavily shaded, its bypass diode can become conductive.
The current then flows around the affected part of the module.
Electrically, this means:
part of the module voltage is lost
but:
the string current can continue to flow
.
In the I-V curve, this transition often appears as a step or notch.
Such a shape therefore indicates that different current conditions exist within the module or string.
The underlying cause must then be determined.
Possible causes include:
- partial shading,
- local soiling,
- damaged cells,
- electrical interruptions within a cell group,
- a faulty bypass diode.
A step in the I-V curve therefore does not automatically mean “bypass diode defective”. It first indicates that a mismatch or bypass event is present. Further tests are required to determine the specific cause.
Detecting Mismatch Between Modules and Cell Groups
Modules within a string are electrically connected in series.
The same current therefore flows through all modules.
If one module has a lower current capability, it affects the entire string.
Such mismatch can be caused by:
- different module types,
- different power classes,
- ageing,
- local soiling,
- different orientation,
- shading,
- cell damage,
- different temperatures.
For strings connected in parallel, current differences between individual strings may also become visible.
For troubleshooting, comparison between:
String 1
String 2
String 3
…
under environmental conditions that are as identical as possible is therefore particularly informative.
If only one string differs significantly from several identical neighboring strings, the fault can be narrowed down much faster.
Distinguishing Uniform and Uneven Soiling
Soiling can affect the I-V curve in different ways.
Uniform soiling
Relatively uniform soiling primarily reduces the irradiance reaching the cells.
The typical result is reduced current or a reduced overall power level.
The behavior can resemble generally lower irradiance.
Local soiling
Local soiling, on the other hand, can affect individual cells or cell groups much more strongly.
Typical examples include:
- bird droppings,
- leaves,
- remaining snow,
- local deposits,
- dirt accumulation at edges.
This creates mismatch within the module.
The I-V curve may then show steps or notches similar to partial shading.
A repeat measurement after cleaning can therefore quickly show whether soiling was actually responsible for the abnormal behavior.
Detecting Increased Series Resistance
A PV module and its connection network have electrical series resistance.
Contributors include:
- cell interconnects,
- busbars,
- cables,
- connectors,
- terminals,
- contact points.
If this resistance increases, additional voltage losses occur under load.
Possible causes include:
- corroded contacts,
- poorly installed connectors,
- damaged cell interconnects,
- increased contact resistance,
- ageing or cell damage.
In the I-V curve, increased series resistance typically appears as an unfavorable change or rounding in the region between the MPP and the open-circuit voltage.
The maximum available power decreases.
A simple Voc measurement can still appear largely plausible because practically no load current flows under open-circuit conditions.
Assessing Abnormalities in the Short-Circuit Region
The shape of the I-V curve near the short-circuit current also contains useful information.
An unusual slope may indicate additional internal current paths or reduced parallel or shunt resistance.
Possible causes include:
- cell damage,
- moisture ingress,
- internal leakage paths,
- local defects.
Measurement conditions and uneven irradiance can also influence the curve shape.
The I-V curve should therefore never be diagnosed without checking the environmental conditions and instrument settings.
Systematically Investigating Low Open-Circuit Voltage
If VOC is significantly lower than expected, several causes are possible.
Check, among other things:
- Is the actual number of connected modules correct?
- Was the correct module type selected in the measuring instrument?
- Is the module temperature significantly higher than under the reference conditions?
- Is a bypass diode short-circuited or permanently conducting?
- Is a module or part of a string electrically bypassed?
- Are connections or modules interrupted or wired incorrectly?
In a string, the module voltages are added together.
A clearly missing voltage step can therefore indicate that a module or cell group is not contributing to the total voltage as expected.
The specific cause should then be investigated further in the affected section.
Systematically Investigating Low Short-Circuit Current
A low short-circuit current does not automatically mean that the module is damaged.
The optical and measurement conditions must be checked first.
Possible causes include:
- insufficient irradiance,
- uniform soiling,
- shading,
- incorrectly positioned irradiance sensor,
- rapidly changing cloud cover,
- incorrect stored module or string data,
- degradation or an actual module fault.
Especially under changing cloud conditions, irradiance can change significantly within just a few seconds.
If irradiance is not recorded at the same time as the I-V curve, the expected-versus-measured comparison may be incorrect.
Correctly Comparing Expected and Measured I-V Curves
A measured I-V curve becomes particularly informative when it can be compared with an expected curve.
For this purpose, the correct module data must be stored.
Depending on the measuring system, this includes:
- module manufacturer,
- module type,
- rated power,
- VOC,
- ISC,
- VMPP,
- IMPP,
- temperature coefficients,
- number of modules in the string,
- number of parallel strings.
An input error can create an apparent deviation even though the PV system is technically operating correctly.
Before diagnosing a fault, the following should therefore first be checked:
measurement data
and:
configuration data
.
Correctly Positioning Irradiance and Temperature Sensors
The quality of an STC evaluation depends directly on the recorded environmental data.
The irradiance sensor should measure irradiance as closely as possible to the conditions experienced by the PV array being tested.
Particularly important are:
- the same orientation,
- the same tilt angle,
- no local shading of the sensor,
- a clean sensor surface,
- a suitable reference cell or pyranometer.
If the irradiance sensor is placed horizontally on the roof, for example, while the modules are installed at a 35° inclination, the measured irradiance may differ significantly from the irradiance actually acting in the module plane.
The temperature sensor must also be thermally mounted in a suitable way on the module being tested.
Measuring ambient air temperature does not automatically replace measurement of the module temperature.
Measure a Module or the Complete String?
Both measurements have different advantages.
String measurement
A complete string I-V curve is well suited for:
- rapid comparison of several strings,
- commissioning,
- regular periodic testing,
- initial localization of power deviations.
If only one abnormal I-V curve is found among several identical strings, troubleshooting can then be focused specifically on that string.
Module measurement
Measurement of individual modules then allows much more precise localization.
It is useful if:
- a particular string is abnormal,
- thermography indicates a suspicious module area,
- bypass diodes or cell groups are to be checked,
- a module is to be evaluated after damage or replacement.
An efficient test procedure therefore often consists of:
compare strings → identify abnormal string → selectively test modules
.
Practical Example: One String Delivers Significantly Less Power
A PV system has eight identical strings.
All strings contain:
20 identical modules
with the same orientation.
The inverter indicates that String 6 consistently generates less power.
Step 1: Visual inspection
No obviously missing or covered modules are found.
Step 2: Compare Voc
The open-circuit voltage of String 6 is only slightly below that of the neighboring strings.
No clear fault can yet be determined from this.
Step 3: Measure I-V curves
All strings are measured under irradiance conditions that are as comparable as possible.
The I-V curve of String 6 shows a clear step in the middle voltage range.
The other strings, in contrast, show smooth I-V curves.
Step 4: Check for shading and soiling
A closer inspection reveals a locally heavily soiled section of one module.
After cleaning, the I-V curve is measured again.
Step 5: Repeat the measurement
The first step in the curve has become smaller but is still present.
This makes it clear that:
soiling was part of the cause
but:
not the only cause
.
Step 6: Investigate individual modules
The I-V curves or supplementary tests are narrowed down to individual modules.
One module continues to show a characteristic deviation in one of its substrings.
Step 7: Thermography and electrical testing
The additional inspection reveals an abnormal cell group or junction box.
The affected module is then investigated further or replaced in accordance with the manufacturer’s instructions.
The example demonstrates the advantage of I-V curve measurement:
It does not necessarily tell you immediately which component is defective, but it clearly shows that the electrical behavior of the string does not match the expected condition and in which region of the I-V curve the deviation occurs.
Combining I-V Curve Measurement with Thermography
The I-V curve shows electrical behavior.
A thermal imaging camera, on the other hand, shows local thermal abnormalities.
The two methods therefore complement each other very well.
I-V curve measurement can show, for example:
- mismatch,
- reduced power,
- steps caused by active bypass paths,
- unusual resistance effects.
Thermography can then provide indications of:
- hotspots,
- abnormal cell groups,
- heated connectors,
- abnormal junction boxes,
- local electrical losses.
An abnormal I-V curve combined with a thermal anomaly on the same module provides considerably more diagnostic information than either measurement method on its own.
Measuring PV Strings Safely
PV generators produce DC voltage whenever they are exposed to irradiance, even when the inverter is switched off.
In large installations, string voltages can reach:
1,000 V DC
or:
1,500 V DC
.
I-V curve measurement may therefore only be carried out by appropriately qualified personnel using a measuring instrument suitable for the voltage, current and installation category involved.
The following must be observed, among other things:
- manufacturer instructions for the PV measuring instrument,
- permissible DC voltage and permissible DC current,
- properly rated test leads and connectors,
- specified isolation or disconnection procedures,
- protection against unintentional reconnection,
- risk of DC arcing.
PV connectors must not simply be disconnected under load unless this is explicitly intended and approved for the respective installation.
The connection sequence specified in the instructions for the I-V curve tester must be followed.
Systematic Test Procedure
- Check the circuit diagram, string configuration and module data.
- Ensure that the I-V curve tester is suitable for the maximum voltage and maximum current of the string.
- Disconnect the PV string from the inverter or operating system according to the specified procedure.
- Enter the correct module type and number of modules in the measuring instrument.
- Position the irradiance sensor in the module plane.
- Attach the temperature sensor correctly to the module.
- Document current shading and visible soiling.
- Record the I-V curve.
- Check Voc, Isc, Vmpp, Impp, Pmax and, where applicable, the fill factor.
- Compare the measured I-V curve with the expected curve or the curve corrected to reference conditions.
- Look for steps, notches and changes in slope.
- Compare the result with identical neighboring strings.
- If abnormalities are found, first rule out shading and soiling.
- Then narrow the suspicious string down to individual modules.
- If necessary, supplement the investigation with thermography, visual inspection and further electrical tests.
- Save measurement conditions and I-V curves for future periodic testing.
Common Errors in I-V Curve Measurement
- Measuring only Voc: Faults in the MPP region remain hidden.
- Measuring only Isc: Changes in the I-V curve shape are not detected.
- Not measuring irradiance: Low current is incorrectly interpreted as a module fault.
- Using only ambient temperature: The actual module temperature may be significantly higher.
- Selecting the wrong module type: The expected I-V curve does not match the generator being tested.
- Entering the wrong number of modules: Expected voltage and power are calculated incorrectly.
- Incorrectly aligning the irradiance sensor: The measured irradiance does not correspond to the module plane.
- Measuring during rapidly changing cloud cover: Irradiance and the electrical I-V curve may correspond to different conditions.
- Interpreting every step as a defective bypass diode: Shading, soiling or cell damage can also activate a bypass path.
- Overlooking shading during measurement: Even small shadows from railings, antennas or the technician can influence the result.
- Looking only at peak power: The shape of the I-V curve contains additional diagnostic information.
- Directly comparing strings under different conditions: Different irradiance and temperatures distort the comparison.
- Disconnecting PV connectors under load: This can create a dangerous DC arc.
Suitable PV I-V Curve Testers
For professional I-V curve measurement on modern PV modules and strings, the I-V600 is one suitable option.
The instrument provides, among other things:
- I-V curve measurements on PV modules and strings,
- measurements up to 1,500 V DC and 40 A DC,
- determination of open-circuit voltage VOC and short-circuit current ISC,
- determination of characteristic power values,
- measurements on monofacial and bifacial PV modules,
- evaluation using an extensive PV module database,
- temperature and irradiance measurement with the SOLAR-03 data logger,
- graphical display of the I-V curve directly on the instrument.
I-V curve measurement is carried out according to the procedures provided for evaluating and correcting PV I-V curves in the application.
For installations with lower maximum voltages, the I-V400w is also available.
The instrument enables I-V curve measurements on individual modules and strings up to 1,000 V DC and displays, among other things:
- current power,
- peak power,
- short-circuit current,
- open-circuit voltage,
- graphical I-V curve.
Further instruments for commissioning, troubleshooting and testing photovoltaic installations can be found under Photovoltaic Instruments / PV Measuring Instruments at ICS Schneider.
Further electrical test and measuring instruments can be found under Electrical Test and Measuring Instruments.
Conclusion
An I-V curve measurement provides significantly more information about the actual condition of a PV module or string than measuring only the open-circuit voltage or short-circuit current.
Shading, uneven soiling and mismatch can create characteristic steps or notches in the I-V curve when bypass diodes become active.
Increased resistance, reduced current generation and other module deviations can also appear in the shape of the curve.
However, the I-V curve must not be considered in isolation.
Irradiance and module temperature also have a significant effect on current, voltage and power. These values must therefore be recorded as simultaneously as possible and taken into account when comparing measurements with manufacturer data.
Comparing several identical strings under conditions that are as similar as possible is particularly informative. An abnormal string can then be narrowed down step by step to individual modules.
Thermography, visual inspection and further electrical tests complement I-V curve measurement and help determine the specific cause of an identified deviation.
The decisive question is therefore not simply: “Does the string reach its open-circuit voltage?”, but rather: “Does its complete current-voltage behavior under the actually measured irradiance and temperature conditions correspond to the expected behavior?”
FAQ: PV I-V Curve Measurement
What is an I-V curve for a solar module?
The I-V curve shows the relationship between current and voltage of a PV module or string over the complete operating range from short-circuit current to open-circuit voltage. It can be used to determine, among other things, Isc, Voc and the Maximum Power Point.
Why is open-circuit voltage alone not sufficient for testing a PV module?
Open-circuit voltage describes only one operating point without load current. Faults such as increased series resistance, mismatch or activated bypass paths can have a much greater effect under load or within the I-V curve.
What does shading look like in an I-V curve?
Partial shading can cause steps, notches or significant changes in slope. These are caused by current mismatch between cell groups receiving different irradiance levels and, where applicable, activation of bypass diodes.
Does a step in the I-V curve mean that a bypass diode is defective?
No. A step initially indicates mismatch and possible bypass activation. The cause may be shading, local soiling, cell damage or a bypass diode fault. Further testing is required for a definitive diagnosis.
Why must irradiance be measured during I-V curve testing?
The current of a PV module depends strongly on solar irradiance. Without knowing the actual irradiance, low current cannot reliably be distinguished from the normal response to reduced sunlight.
Why is module temperature important?
Module voltage is temperature-dependent. As cell temperature increases, the open-circuit and MPP voltages of typical crystalline silicon modules decrease in particular. A hot module can therefore operate well below its STC voltage even when functioning correctly.
What does STC mean for photovoltaic modules?
STC stands for Standard Test Conditions. These include, among other things, irradiance of 1,000 W/m² and a cell temperature of 25 °C. Manufacturer rated values are specified under defined reference conditions.
What is the MPP?
The Maximum Power Point is the operating point at which the product of voltage and current, and therefore the electrical output power, is at its maximum. It is described by VMPP and IMPP.
What does mismatch mean in a PV string?
Mismatch means that modules or cell groups have different electrical characteristics or operating conditions. In a series connection, a module with lower current capability can affect the output of the entire string.
Can soiling produce an I-V curve similar to shading?
Yes. Uniform soiling often reduces mainly the current. Local soiling can affect individual cell groups to different degrees and thereby create steps or notches similar to partial shading.
How can increased series resistance be detected?
Increased series resistance can distort or round the I-V curve, particularly in the MPP region and towards the open-circuit voltage. Usable power decreases while the open-circuit voltage may still appear relatively normal.
Should a complete string or each individual module be measured?
For rapid system testing, comparing complete strings is generally useful first. If an abnormal string is identified, troubleshooting can then be narrowed down to individual modules.
Can I-V curve measurement clearly identify a module defect?
It can reveal electrical deviations very effectively, but not every I-V curve shape automatically identifies a specific defective component. Visual inspection, thermography, bypass diode testing or further electrical measurements may be required.