Why Can a Solar Module Lose Imp While Isc Remains Stable

A solar module can lose Imp even when Isc barely moves. In practice, this usually means the problem is affecting the part of the I-V curve where the module actually produces power, rather than the short-circuit point. Higher series resistance, weak electrical connections, cell mismatch, electrically significant cracks, and some shading patterns can all cause this kind of result.

Take a simple case. A module changes from 14.0 A Isc and 13.3 A Imp to 13.9 A Isc and 12.2 A Imp. Isc is down only about 0.7%, but Imp is down about 8.3%. If Vmp also drops from 42.0 V to 40.5 V, output falls from 558.6 W to 494.1 W, a loss of about 11.5%.

Those numbers are useful only if the test conditions are comparable. A change in sunlight or module temperature can move the readings even when nothing is wrong with the module. IEC 60904-1 defines the measurement of photovoltaic current-voltage characteristics for this reason.^[1]

Confirm Isc Before Diagnosing a Fault

If a technician records 13.9 A today and 14.0 A yesterday, that alone is not enough to say Isc is stable.

Suppose yesterday's test was done at 1,000 W/m² and today's test at 900 W/m². The readings are not directly comparable. Module current changes with irradiance, and temperature, sun angle, spectrum and sensor accuracy can also shift the result.

IEC 60891 provides procedures for translating measured I-V curves between different irradiance and temperature conditions and includes methods related to series-resistance determination.^[2]

In real fault finding, treat Isc as “stable” only when:

• irradiance is comparable or correctly normalized;
• module temperature is known;
• the same module or same model is being compared;
• test connections are reliable;
• measurement uncertainty is smaller than the suspected change.

There is no universal rule saying that a 1%, 2% or 3% Isc difference is always acceptable.

Use Isc, Imp, Voc, Vmp and FF Together

One current reading cannot tell you where the loss is coming from. Isc, Imp, Voc, Vmp and fill factor each describe a different part of module behavior.

Maximum power is:

Pmax = Imp × Vmp

Fill factor is:

FF = Pmax ÷ (Voc × Isc)

A widely used PV electrical model includes light-generated current, a diode, series resistance and shunt resistance. Each part affects the I-V curve in a different way.^[3]

NREL long-term performance analysis has also shown that Isc, Imp, Voc, Vmp and Pmax do not necessarily degrade at the same rate.^[4]

Series Resistance: 0.03 Ω Can Matter at 13 A

When Isc changes very little but Imp, Vmp and fill factor get worse, higher series resistance is one of the first things worth checking.

The extra resistance may come from several places:

• cell fingers;
• busbars or collection wires;
• cell interconnections;
• soldered or bonded joints;
• ribbons;
• junction-box contacts;
• connectors and cables.

The basic electrical relationship is straightforward:

Voltage loss = Current × Resistance

Heat produced in the resistance is:

Heat = Current² × Resistance

At 13 A, the effect of a small added resistance looks like this:

These figures describe the loss in the added resistance itself. They do not mean final module Pmax must fall by exactly 1.69 W, 5.07 W or 8.45 W, because the operating point also moves as resistance changes.

For high-current modules, a resistance change that looks small on paper can therefore matter in the field. Tongwei discusses the same diagnostic issue in its article on rising solar-module series resistance, where repeated Rs growth is separated from a single abnormal reading.

Stable Isc with Falling FF Points Away from Simple Light Loss

A useful way to spot this type of problem is to compare two modules that have the same Isc and Voc but different maximum-power performance.

Power has fallen by about 68.5 W, or 12.4%, even though Isc and Voc are unchanged.

That is not the pattern you would normally expect from a simple uniform reduction in sunlight. The loss is happening in the working part of the I-V curve.

A lower FF still does not identify one exact fault. Series resistance, shunt leakage, cell mismatch, cracks and weak electrical connections can all reduce it.

Cracks: Electrical Isolation Matters More Than Crack Size

For a customer or field technician, the visible size of a crack is not the main question. What matters is whether the crack has cut electrical paths.

A crack may:

• break metal fingers;
• increase local resistance;
• disconnect part of a cell;
• damage contact with an interconnection;
• create mismatch with other cells.

A long crack can have little immediate electrical effect if enough conductive paths remain. A smaller crack may be more serious if it disconnects an active part of the cell.

NREL field testing has shown that cracked modules can develop additional power loss during outdoor aging and that EL images can reveal increasing electrical isolation of cracked cell fragments.^[5]

Cell design also changes the outcome. Tongwei's TWMNH-66HD, for example, uses 0BB flexible interconnection and multiple collection paths. A crack in that structure does not necessarily cause the same loss as a similar-looking crack in an older few-busbar cell.

For that reason, power loss should not be estimated from crack length or visible cracked area alone.

One Weak Cell Can Shift the Module Operating Point

Cells connected in series have to carry the same current at a given operating point.

Imagine most cells can work efficiently around 13 A, while one damaged cell becomes strongly limited around 12 A. The healthy cells cannot continue at 13 A on their own while the weak cell stays at 12 A.

The whole connected group has to settle at a common current. That can pull module Imp down even though most of the cells are still healthy.

Mismatch can come from:

• cracks;
• local shade;
• uneven cell aging;
• damaged contacts;
• temperature differences;
• manufacturing variation.

Weak Joints, Ribbons and Fingers Raise Resistance

Soldered or bonded joints: repeated heating and cooling can weaken a contact. Some damaged joints only become noticeably worse after the module heats up.

Ribbons and interconnections: corrosion or partial cracking can reduce the conducting area even while the path remains connected.

Cell fingers: when collection lines break, current has to travel farther before it reaches an intact conductor.

IEA PVPS identifies cracked cells, disconnected interconnect ribbons, corrosion, connection problems and bypass-diode faults among established PV module failure modes.^[6]

Temperature can help narrow this down. If the same module performs normally when cool but shows lower Imp or FF after heating, contacts, cracks and interconnections deserve a closer look before the change is written off as normal temperature loss.

Partial Shade: Look for Steps, Not Just Lower Current

If the entire module receives 20% less irradiance, both Isc and Imp normally fall substantially. Local shade can look very different because only a few cells in one series-connected section may be affected.

Typical causes include:

• bird droppings;
• leaves;
• cables;
• roof edges;
• poles;
• vegetation;
• uneven dirt.

Sandia PV performance material shows that partial shading can create different submodule operating regions and activate bypass diodes.^[7]

On the I-V curve, the practical signs are:

• a step in the curve;
• more than one knee;
• a large Vmp change that appears only under certain shading conditions.

Partial shade does not always leave Isc unchanged. The result depends on which cells are covered, how much of each cell is shaded, the substring layout and bypass-diode state.

Bypass Faults: Watch for Missing Voltage

Normal bypass-diode operation under shade and a permanently failed diode should not be treated as the same fault.

Normal bypass operation: when one substring becomes strongly limited, the diode can let current bypass that section while the module is under load. The trade-off is voltage.

For a simplified three-substring example:

• normal Vmp = about 42 V;
• each equal substring contributes about 14 V;
• one substring is bypassed;
• remaining operating voltage may be around 28 V.

At a simplified current of 13 A:

42 V × 13 A = 546 W

Compared with:

28 V × 13 A = 364 W

This does not mean every three-substring module loses exactly one-third of its power. Real substring voltages, current and MPP all move during bypass operation.

Shorted bypass diode: if the diode itself fails short circuit, the associated substring can stay bypassed even when there is no shade. Voc and Vmp may then lose a clear section of voltage.

Open bypass diode: the module may look normal under uniform sunlight, but when the protected cells become severely shaded, the intended bypass path is no longer available.

A diode that temporarily conducts under load should not be expected to cause the same Voc loss as a permanently shorted diode. Voc is measured with essentially no output current.

IEA PVPS field assessments identify bypass-diode failures among faults capable of causing major module power loss.^[8]

A 30°C Temperature Rise Can Look Like Power Degradation

Hot modules normally lose voltage and power even when there is no defect. For crystalline-silicon modules, higher cell temperature usually reduces Voc, Vmp and Pmax, while Isc changes much less.

The right way to quantify that effect is to use the temperature coefficient for the actual product.

Assume a 550 W module has a Pmax temperature coefficient of -0.30%/°C.

If cell temperature rises from 25°C to 55°C:

55°C - 25°C = 30°C

Simple estimated power change:

30 × 0.30% = 9%

Estimated power at the higher temperature, assuming the same irradiance:

550 W × 0.91 = 500.5 W

The difference is about 49.5 W. In this example, that loss comes from temperature, not module degradation.

The -0.30%/°C value is only an example. Use the correct coefficient from the relevant module datasheet. Current Tongwei module specifications are available through its module download center.

800 W/m² Can Cut a 14 A Isc to About 11.2 A

Irradiance has a direct and visible effect on current.

If a module produces about 14.0 A Isc at 1,000 W/m², a simple first estimate at 800 W/m² is:

14.0 A × 800 ÷ 1,000 = 11.2 A

A field reading near 11.2 A under 800 W/m² does not mean the module has lost 20% of its current capability.

The calculation is a quick check, not a full correction method. Temperature, diode behavior, shunt effects and other factors also change as irradiance changes.

If Isc stays close to its corrected expected value while Imp falls much more, uniform irradiance loss becomes a weaker explanation.

Rule Out Test Error Before Replacing the Module

Before a module is removed, returned or put into a warranty process, check whether the test setup itself could be creating the apparent loss.

Bifacial modules need extra care because useful light reaches both sides. IEC TS 60904-1-2 gives additional requirements for measuring bifacial I-V characteristics.^[9]

This matters when comparing bifacial products such as Tongwei's TWMNH-66QD. The product has bifacial power generation, so equal front irradiance does not guarantee equal total irradiance if the rear-side conditions are different.

Read the I-V Pattern

The Imp/Isc ratio can make a change easier to compare between tests.

Example:

13.2 ÷ 13.9 = 0.950

After degradation:

11.9 ÷ 13.8 = 0.862

Relative change in the ratio:

(0.950 - 0.862) ÷ 0.950 = about 9.3%

There is no universal Imp/Isc pass/fail value. It is most useful when the same module is compared over time or when identical modules are tested under comparable conditions.

Use Thermal Imaging and EL to Confirm the Fault

Once the electrical data points to a real module problem, thermal imaging and EL can help locate it.

With thermal imaging, check for:

• one cell hotter than neighboring cells;
• a hot interconnection;
• a hot connector;
• abnormal junction-box heating;
• bypass-diode heating.

IEC TS 62446-3 defines requirements for outdoor infrared inspection of operating PV modules and plants, including environmental conditions, inspection procedures and interpretation.^[10]

A fixed rule such as “10°C hotter always means failure” should not be used. Wind, irradiance, electrical load, reflections and camera angle all affect the image.

EL imaging is better suited to cracks, broken fingers and electrically disconnected cell areas.

A module can therefore pass visual inspection but fail an I-V curve test. The glass and surface may look fine even when electrical paths inside the laminate are damaged.

Tongwei's PV testing center includes module reliability testing and failure-analysis capabilities, allowing visible defects and electrical faults to be checked separately.

Do Not Diagnose PID from Low Imp Alone

Low Imp with stable Isc is not enough to classify a module as PID-affected.

PID becomes more credible when the low power also lines up with:

• module position in a high-voltage string;
• system polarity;
• changes in Voc or FF;
• matching EL patterns;
• temperature and humidity conditions that support PID stress.

NREL describes several PID mechanisms and shows that system voltage, humidity, temperature and module materials can all affect the result.^[11]

Compare Aging with the Module Baseline

A slow change over years is different from a sudden drop after a specific event.

NREL notes that PV modules can gradually lose output as they spend years exposed to heat, moisture, UV radiation and mechanical stress.^[12]

Take a closer look when:

• Imp changes suddenly after a storm;
• FF drops after a connector or junction box overheats;
• performance becomes worse only when the module is hot;
• one module separates clearly from identical modules nearby;
• the I-V curve develops a new step or rounded knee.

For warranty decisions, use the exact applicable module warranty and product documents instead of assuming one degradation percentage applies to every product.

Inverter Current Cannot Identify One Bad Module

A string-current value on the inverter does not tell you the Imp of each individual module.

Modules connected in series carry the same string current at a given operating point. If one module becomes weak, it can move the operating point of the whole string.

Low string current may come from:

• one weak module;
• several mismatched modules;
• partial shading;
• damaged wiring;
• connector resistance;
• fuse problems;
• MPPT behavior.

To prove that one specific module has lost Imp, module-level I-V data or another module-level test is usually needed.

Test in This Order

1. Record irradiance and module temperature. Repeat the measurement if conditions are changing quickly.
2. Check local shade and contamination. Include cables, bird droppings, leaves and uneven dirt.
3. Check accessible connectors and test leads. Rule out external resistance.
4. Run one complete I-V sweep. Record Isc, Imp, Voc, Vmp, Pmax and FF.
5. Check the curve shape. Separate rounded knees, steps, tilted high-current regions and missing voltage sections.
6. Use thermal imaging when resistance or abnormal heating is suspected.
7. Use EL when cracks, broken fingers or disconnected cell areas are suspected.
8. Check bypass circuitry when a large voltage section is missing.
9. Check the string, wiring and MPPT if the individual module tests normally.

PV strings can remain at dangerous DC voltage whenever modules are illuminated. IEC 62446-1 covers inspection and commissioning tests used to verify grid-connected PV systems.^[13]

Do not short a PV string with improvised wiring, disconnect DC connectors while they are carrying load current, or use meters and leads below the required DC rating. String-level electrical work should be performed by properly qualified personnel.

Worked Example: 0.6% Isc Loss but 13.6% Pmax Loss

Two identical 550 W modules are measured under comparable irradiance and temperature.

Isc and Voc have changed by only about 0.6%, while Imp is down 9.5% and Pmax is down 13.6%. Once irradiance and temperature have been checked, a large uniform loss of sunlight is not the best match.

A permanently shorted full substring is also less likely because Voc has barely moved.

The stronger suspects are:

• higher series resistance;
• weak electrical interconnections;
• cell mismatch;
• electrically significant cracks;
• damaged current-collection paths.

If the I-V knee is smooth but more rounded than the healthy reference, check resistance and connections first. If the curve has a step, check local shading, mismatch and bypass behavior. Thermal imaging can help locate hot resistance points, while EL can show electrically isolated cracked areas.

Finally

A small change in Isc can sit beside a much larger loss in usable power. In the 550 W example, Isc falls only 0.6%, while Imp falls 9.5%, Vmp falls 4.6%, FF drops from 79.5% to 69.5%, and Pmax falls 13.6%. Once irradiance and temperature have been ruled out, the curve shape becomes the fastest way to narrow the fault. A rounded knee points more toward resistance or connection damage.

Steps point toward shade, mismatch or bypass operation. A large missing voltage section points more toward a shorted bypass path or lost substring. An Isc reading on its own is not enough to decide whether a module should be repaired, replaced or sent for further testing.