Acceptance testing of photovoltaic modules: why mobileLAB protects your investment
The moment the modules arrive on site is critical: even if the manufacturer declares compliance with standards, the reality of the supply may hide risks that become costly over time. For this reason, acceptance testing of photovoltaic modules with a mobile laboratory (IV + EL) have become an essential tool for EPCs, investors, and property owners who want certainty before installation. With our mobileLAB, the only one in Italy, MRP brings a TUV Rheinland-calibrated and certified, class A+A+A+ LED solar simulator to the construction site, providing traceable, repeatable data that can be used for any contractual disputes.
What is acceptance testing of photovoltaic module?
When we talk about acceptance testing of photovoltaic modules, we are not referring to a superficial check: we mean complete IV curves (Pmp, Isc, Voc, FF) performed with metrological traceability and electroluminescence (EL) analysis acquired according to declared parameters. These tests allow us to compare the material received with the contractual specifications and reveal defects that might not emerge until months or years after installation.
Curve IV: protecting expected yield
IV curves provide the actual electrical behavior of the module. We often find supplies where the measured power is consistently at the lower end of the tolerance range or, in the worst cases, below the negative tolerance specified by the manufacturer. Having independent IV measurements and a statement of uncertainty allows us to determine whether a batch is acceptable or needs to be rejected. Without this data, it is very difficult to obtain replacements or refunds, which has a direct impact on the performance of the plant and bankability considerations.
It is important to keep in mind that the yield of a photovoltaic plant throughout its useful life is largely based on the actual initial power of the installed modules. Simulation tools, such as PVSyst, are estimating production based on PAN files, often provided by the manufacturers themselves, which take the nominal power rating as a reference. But if, in reality, the modules already start at -3% compared to the declared value, that gap translates into a loss of production that continues throughout the life of the plant. Added to this is the risk that the actual annual degradation is worse than declared—as we have found in several cases with recently installed modules—further amplifying the overall economic damage.
A case from the field: underperforming modules
Quite often, we are coming across modules with actual power ratings lower than the nominal value. In this example of large-format high-efficiency modules with a power rating of 715 Wp, all of our measurements taken on modules taken directly from the pallets on site were lower than the nominal power rating. Of these, 1 in 5 modules provided a Pmax lower than the nominal value by more than 3%, despite the “positive tolerance” of -0/+5 Wp indicated in the manufacturer’s datasheet. As many as 5 out of 95 modules provided power lower than 691 Wp, i.e., approximately 25 W below the minimum expected by the datasheet.
Electroluminescence: not all images are the same
EL is a powerful tool for detecting microcracks, contact defects, and interconnection problems, for example; however, the quality of the image acquisition is crucial, during in the acceptance testing of photovoltaic modules. We have encountered cases where the manufacturer has provided images that have been “patched-up” through automatic post-processing that hide defects, or images taken with test currents that are too low and with deliberately reduced contrast: conditions that turn real problems invisible.
EL images are like a fingerprint: an end-of-line EL inspection must provide the same photograph that is observed in the field, and this becomes a guarantee for correct analysis by the manufacturer.
In this first example, we received a database of EL images from the manufacturer that had been acquired at such low currents that they did not reveal the contact defects in the cells: the module had passed the end-of-line inspection, even though according to the acceptance criteria it should have been classified differently.
Photovoltaic module acceptance testing: how EL manipulations appear and why they are dangerous
In another recent, even more serious case, we received EL images that apparently showed perfectly healthy modules; by comparing those images with acquisitions made with our mobileLAB, we discovered that the manufacturer had applied filters to all the images that erased evidence of microcracks and other defects at source. These practices are not only unfair to the customer: they compromise the expected performance and increase the risk of litigation.
The example shows one of these modules where it is clear that the manufacturer has automatically applied a “mask” to all EL images from its end-of-line (pre-shipment) inspection system. By doing so, it has hidden the real problems with the modules (in this case, contact problems) even from its own automatic evaluation system for final ELs.
How we set up acceptance testing of photovoltaic modules with mobileLAB
To be effective, tests must follow a clear procedure agreed upon in the specifications. MRP adopts a four-step approach:
- definition of the sampling plan and AQL criteria per batch;
- IV measurement with traced equipment and declared uncertainty, compared with the datasheet and, if available, with a golden sample;
- EL acquisition according to documented parameters (injection current, exposure, SNR[1]), without the use of post-processing that modifies the content;
- Complete report and data archive (IV curves, raw EL images, serial number traceability), ready for use in any disputes or warranty claims.
This strategy produces a defensible report, which reduces ambiguity in negotiations with the supplier and supports the mitigation of technical and economic risk.
The signal-to-noise ratio (SNR) is an index that measures the quality of the image in electroluminescence (EL). A high SNR means that the useful signal (the light emitted by the module) is much stronger than the electronic and background noise, making any defects (cracking, PID, inactive cells, etc.) clearly visible. If, on the other hand, the SNR is low (for example, because the module has been polarized with too low a current), the image appears “noisy” or dark, and even serious defects may remain hidden.
Direct advantages for EPCs, investors, and lenders
Performing acceptance tests means significantly reducing exposure to risk: it allows to reject non-compliant pallets, obtain replacements before installation, and decrease the likelihood of production losses that impact the project’s cash flow. In addition, standardized reports and EL images increase the evidentiary value in the event of contractual disputes, improving the bankability of the operation in the eyes of insurers and lenders.
What to include in the Specifications to be protected
To avoid ambiguity, we recommend including clear contractual clauses: the key phrase to use is “photovoltaic module acceptance testing with mobileLAB (IV + EL).” Some key items to include are, for example, minimum EL parameters, uncertainty statements for IV measurements, the sampling plan per batch, and traceable reports.
Would you like a model specification ready to be included in your offers or purchase contract? MRP is your partner in protecting your investment, both during the contract phase and during acceptance testing of PV modules, thanks to IV/EL criteria, a sampling plan, and a report template that can be defended before insurers and lenders.
Article by Giovanni Guiotto and Mark Rossetto – MRP Srl
September 2025
