Why Does a Solar Module Show Stable Power but Rising Series Resistance

Rising series resistance (Rs) can show up before a clear power drop. On a 450 W module, a 2 W loss is only 0.44%, so the module may still look normal on a power reading even though Vmp, fill factor, or the I-V curve has started to change. The Rs increase is worth taking seriously when it appears again under comparable temperature and irradiance and is backed up by other electrical or physical signs.

What Power Should You Compare?

For an Rs check, the most useful number is module-level Pmax from comparable I-V tests. Inverter output and monthly energy production are much broader figures. They mix the behavior of many modules with weather, MPPT control, clipping, soiling, and system losses.

IEC 60904-1 defines procedures for measuring photovoltaic I-V characteristics, while IEC 60891 covers correction of measured curves for differences in temperature and irradiance.^[1][2]

Clipping is a good example of why inverter power can mislead. Once the inverter reaches its AC limit, DC-side power can move without changing the AC plateau. Tongwei covers this situation in how inverter clipping can hide module faults.

How Small Can the Power Loss Be?

Start with the basic relationship:

Pmax = Vmp × Imp

A module operating at:

40.0 V × 10.0 A = 400 W

may later operate at:

39.9 V × 10.0 A = 399 W

That is only a 0.25% loss. In field work, a change this small can easily fall within normal measurement variation unless the test setup is tightly controlled.

For a 450 W module, the scale looks like this:

So a real 1–3 W loss can already be present while the module still appears to have “stable power.”

Which Values Should Move With Rs?

Rs is much easier to judge when the surrounding electrical data are read at the same time. Vmp, Imp, Voc, Isc, fill factor, and Pmax each tell a different part of the story.

In practice, a rise in Rs is more convincing when Vmp and FF are moving down at the same time while Voc and Isc remain relatively steady.

What Does a High-Rs I-V Curve Look Like?

The full I-V curve usually gives a clearer answer than Pmax alone. With a mainly resistance-related problem, the change is often easiest to see around the knee and on the high-voltage side of the curve.

• Isc stays close to its earlier value.
• Voc stays close to its earlier value.
• Vmp falls.
• Fill factor falls.
• The knee becomes less square.
• Pmax eventually falls.

Tongwei's example of a module that passes visual inspection but fails an I-V curve test shows why curve shape can reveal hidden electrical problems that a visual check cannot find.

How Much Can Fill Factor Change?

Fill factor is calculated as:

FF = Pmax ÷ (Voc × Isc)

Voc and Isc stay the same in this example, but FF drops from 0.777 to 0.766. That fits a higher-resistance pattern, although FF by itself cannot confirm the cause. Low shunt resistance, mismatch, shading, cell damage, temperature, and measurement errors can also pull FF down.

Can Two Modules Have the Same Pmax but Different Electrical Behavior?

These two examples produce almost the same Pmax, but they reach that power at different Vmp and Imp values. A buyer, operator, or test engineer who looks only at Pmax would miss that difference.

How Much Can Temperature Hide?

Take an illustrative 450 W crystalline-silicon module with a Pmax temperature coefficient of -0.30%/°C. This is an example value, not a universal value for every module.

At 45°C, the simple estimate is 27 W lower than at 25°C. That difference is far larger than an early 1–3 W loss linked to rising resistance. Comparing raw power from two tests at different temperatures can therefore lead to the wrong conclusion.

IEC 60891 provides the reference framework for translating I-V measurements taken at different temperatures and irradiance levels.^[2]

How Much Can Irradiance Hide?

The same problem appears with sunlight. The example below uses a 450 W module and keeps temperature and other conditions fixed.

Real PV output is not perfectly linear with irradiance, but the size of the effect is clear. A moderate change in sunlight can move raw output by tens of watts, while an early Rs-related loss may still be only a few watts.

IEC 61829 covers field I-V measurements, weather measurements, and translation of array results to selected reference conditions.^[4]

How Do You Know the Rs Increase Is Real?

Rs is often fitted from an I-V curve together with other model parameters rather than measured as one isolated resistor.^[3]

Before calling the change degradation, verify that:

• the same I-V tracer was used;
• temperature measurement was consistent;
• irradiance was stable or correctly translated;
• software and fitting settings did not change;
• test contacts were clean and stable;
• the same voltage-sensing arrangement was used.

Tongwei's article on module power drift during flash testing covers several test-related causes that can move measured output without permanent module degradation.

What Does a Control Module Tell You?

If these modules are identical and were measured together, Module A is the outlier. That makes a module-specific problem more plausible.

Now compare that with a day when every module shifts by about the same amount:

In that case, the test system deserves attention before four modules are blamed at once.

Tongwei's guidance on maintaining reference solar modules covers the importance of stable reference devices, records, contacts, and calibration.

Where Does Extra Resistance Usually Come From?

NREL-supported field research linked rising module series resistance with solder-bond failure and current crowding in the modules studied.^[5]

Research on field-aged silicon cells has also shown that deterioration of metal/silicon current paths can contribute to higher series resistance.^[6]

IEA PVPS treats corrosion, interconnection failure, cell cracking, and junction-box faults as separate failure modes because their electrical signatures are not identical.^[7]

Connector faults should also be checked before the laminate is blamed. IEC 62852 defines requirements and tests for PV DC connectors.^[8] Tongwei's discussion of PV connector aging and cracking covers heat, moisture, mechanical stress, and contact damage in outdoor service.

How Much Heat Can a Small Contact Resistance Create?

For a known local contact:

P = I²R

A local resistance of 0.05 Ω may look small on paper, but at 13 A it can produce about 8.5 W of heat at one contact.

The module's complete fitted Rs should not be used as the resistance of one connector. Module Rs is spread across the measured electrical path; a local heating calculation needs a local resistance value.

What Can Be Mistaken for High Rs?

A high-Rs diagnosis is stronger when Isc and Voc remain relatively stable while Vmp, FF, and the high-voltage part of the curve get worse.

How Should You Read Percentage Change?

Case A has the bigger percentage, but Case B has nine times the absolute increase. Percentage alone is not enough. It has to be read alongside absolute ΔRs, repeatability, and the effect on the module's electrical performance.

What Does a Real Long-Term Trend Look Like?

Illustrative example only; it is not a normal degradation rate or specification for a particular module.

Across the four tests, Rs rises from 0.30 to 0.47 Ω, Vmp falls from 41.2 to 39.8 V, FF falls from 0.790 to 0.760, and Pmax falls from 412.0 to 396.4 W. That combination is much more useful than any one number on its own.

When Should You Use Thermal or EL Imaging?

Infrared imaging makes the most sense when the extra resistance may be concentrated in a connector, junction box, interconnection, or another small area.

IEC TS 62446-3 defines requirements for outdoor thermographic inspection of operating PV modules and plants.^[9]

The NREL-supported solder-bond study used DLIT together with I-V measurements and modeling to locate current crowding caused by failed solder connections.^[5]

When Is the Problem Serious Enough to Act?

PV modules can remain electrically live in daylight. IEC 61730-1 addresses module construction requirements intended to reduce electric-shock, fire, and mechanical risks.^[10]

Can Rising Rs Predict Remaining Life?

No. A 5% Rs increase in one year does not mean another 5% will appear every following year.

• Solder degradation may stay slow and later accelerate.
• A connector fault may worsen quickly.
• Corrosion depends on moisture, temperature, material, and location.
• Mechanical damage can cause a sudden change instead of a gradual trend.

DOE reliability work treats PV service-life prediction as a failure-mechanism problem, not as a simple straight-line extension of one electrical parameter.^[11]

Can the Module Still Pass Its Power Warranty?

In this example, the module is still 13 W above the hypothetical warranty threshold even though Rs has increased. Actual warranty decisions depend on the manufacturer's written terms and test requirements.

What Data Should You Keep?

• module serial number;
• test date;
• I-V curve;
• corrected Pmax;
• Rs;
• Voc and Isc;
• Vmp and Imp;
• fill factor;
• module temperature;
• irradiance;
• instrument and software used;
• thermal or EL images when available.

For commercial plants, comparing many identical modules makes Rs trending much more useful. Tongwei's overview of solar module reliability also separates power degradation from physical module failure instead of treating them as one metric.

Finally

A single Rs reading is not enough to condemn a module. On a 450 W module, a 2 W loss is only 0.44%, so early electrical deterioration can exist while Pmax still looks stable.

The stronger case is a repeatable rise in Rs together with lower Vmp or FF, a matching I-V curve change, and physical evidence such as a hot connector, junction-box fault, or abnormal EL region. If several identical modules show the same jump on the same day, check the test setup first. If one module is the clear outlier and the same pattern keeps returning, investigate the module and its connections.