Warranted by one, controlled by many: the reality of PV module quality

As the PV industry expands, the pressure to lower prices increases, potentially leading to threats to module quality. Factory audits, incoming inspections, certificates, and finished-product testing by third parties help to ensure quality, but they only go so far. The reliability of a module depends on a complex upstream supply chain that module manufacturers cannot fully control through documentation and small incoming quality testing sample sets. Responsibility for long-term module performance extends beyond the module factory gates. Purchasers must evaluate whether the module manufacturer has sufficient visibility, contractual leverage, and technical controls over the materials incorporated into the module.

IQC is essential, but it is not sufficient

Incoming quality control, or IQC, is an important part of a PV module manufacturer’s material-quality system. It can verify supplier documentation, identify obvious deviations, detect certain shipping or storage problems, and provide an early warning before material enters production. However, IQC does not give a manufacturer complete control over incoming material quality. It reduces risk, but it cannot eliminate risks.

A typical incoming inspection program may include a review of the certificate of analysis and manufacturing records, together with IQC checks. However, a certificate of analysis or conformity provides limited assurance unless supported by contractual specifications, traceability, and defined supplier liability, particularly when failures emerge after field deployment.

Statistical significance of the glass IQC test

Consider a shipment of 50,000 glass pieces. Assuming each module requires two pieces of glass, the shipment can supply approximately 25,000 modules, equivalent to 14.5 MW at 580 Wp per module. Under the specified ISO 2859-1 Special Inspection Level S-1 and AQL 1.5 plan, eight specimens are inspected, with Ac = 0 and Re = 1. The lot is accepted only if all eight specimens conform and rejected if one nonconforming specimen is found. However, eight specimens represent just 0.016% of the 50,000-piece shipment.

Assuming nonconforming glass pieces are randomly and uniformly distributed, the probability of finding at least one is:

𝑃(detection)=1−(1−𝑝)8 

where 𝑝 is the nonconforming fraction. If 1.5% of the shipment is nonconforming, this corresponds to 750 glass pieces, potentially affecting up to 750 modules depending on how the pieces are used. Despite this exposure, the eight-sample plan has only an 11.4% probability of detecting the issue and an 88.6% probability of missing it and accepting the lot. Approximately 31.2% of the shipment would need to be nonconforming before eight random samples provide a 95% chance of detection. Therefore, AQL 1.5 is a sampling-plan index, not a guarantee that 1.5% nonconformity will be detected.

Put simply: Testing eight pieces can catch obvious or widespread problems, but a passing result provides limited assurance about the remaining 49,992 pieces. As such, typical sampling-based testing serves as an effective screening tool rather than a comprehensive guarantee of overall product quality.

What happens when incoming material fails IQC?

A technically sound IQC program should:

• Quarantine failed material
• Initiate a nonconformance report
• Conduct confirmatory testing
• Inform the supplier
• Identify root cause
• Determine whether the lot should be returned, reworked, downgraded, or released under a documented deviation

In practice, the decision is rarely made in a vacuum. PV factories often operate against production reservations, shipping dates, delivery milestones, and contractual liquidated damages. If a material problem is identified shortly before scheduled production, rejecting the shipment may stop the line and jeopardize the module-delivery schedule.

Resolving a supplier claim can require repeat testing, exchange of samples, review of test methods, a supplier investigation, and negotiation over replacement material. If alternative approved material is unavailable, the manufacturer may face a difficult choice between production delay and conditional use of the questioned lot. This commercial pressure does not remove the manufacturer’s responsibility, but it can weaken the practical effectiveness of IQC unless the supply agreement contains clear test methods, response times, replacement obligations, and consequences for nonconforming material.

Limited visibility into upstream manufacturing capability

Module manufacturers are regularly asked to open their factories to customers and third-party quality organizations. Auditors may inspect equipment, process controls, calibration systems, storage conditions, traceability records, incoming inspection, production monitoring, and pre-shipment inspection. Production-monitoring programs can also verify whether the materials used for a particular order conform to the contracted bill of materials (BOM). Such oversight is now an established risk-control measure in large PV procurements.

Equivalent transparency may be more difficult to obtain from upstream material suppliers. Encapsulant formulations, additive packages, glass processing, backsheet constructions, and process settings can be treated as proprietary information. The module manufacturer may receive a technical data sheet, certificate of analysis, and approved product designation without receiving the supplier’s complete formulation or detailed manufacturing instructions. Even where a supplier audit is permitted, access to formulation records, sub-suppliers, and process-change history may be restricted.

Supply-chain concentration adds another dimension. The International Energy Agency Photovoltaic Power Systems Programme reported that China held dominant shares of PV manufacturing across wafers, cells, and modules in 2023 and also dominated important materials such as glass and encapsulants. Policy restrictions, trade measures, and rapid sourcing changes can further complicate supplier qualification and material continuity. At the same time, the recent market has also experienced significant overcapacity and intense price competition, so bargaining power is not uniform across every material, supplier, or time period. The more defensible conclusion is that access and leverage depend on the criticality of the material, availability of approved alternatives, supplier concentration, contractual terms, and the manufacturer’s purchasing scale.

The lack of full upstream transparency can leave the module manufacturer responsible for long-term warranty performance without having complete visibility into every variable that affects that performance. This is especially significant because reliability is determined not only by individual materials but also by their interaction as a BOM. The encapsulant must be compatible with the cell architecture, glass, backsheet, metallization, flux residues, and lamination profile. A material that performs acceptably in one construction may not provide the same result in another.

Responsibility must be shared, even when accountability is not

These constraints do not rule the module manufacturer out of responsibility. The manufacturer selects the suppliers, approves the BOM, defines incoming controls, operates the production process, releases finished modules, and issues the warranty. The module manufacturer remains the primary accountable party from the purchaser’s perspective.

These constraints do, however, show that the manufacturer’s control is significant but incomplete. Module quality is produced by a chain of organizations: raw-material suppliers, component suppliers, module manufacturers, testing laboratories, auditors, logistics providers, installers, and purchasers. The manufacturer is the central integrator of this chain, but it cannot reliably control upstream quality through certificates and a small number of incoming samples alone.

Recommendation: procurement must look beyond module price

Due diligence should extend upstream to the suppliers of critical raw materials. Module manufacturers should implement the following risk-based controls:

• Supplier qualification: Evaluate suppliers according to the criticality of the material, the supplier’s quality-management system, manufacturing capabilities, process controls, and historical performance.
• Formulation and change control: Establish agreements requiring suppliers to disclose and obtain approval for changes to formulations, raw-material sources, manufacturing processes, production locations, or key sub-suppliers.
• Batch-level traceability: Maintain traceability from each incoming material batch to the specific modules or production lots in which it was used.
• Risk-based incoming testing: Develop testing plans based on the material’s criticality, physical form, production history, supplier performance, and potential failure modes.
• Periodic supplier audits: Audit critical suppliers at intervals determined by material risk, supplier performance, and any history of quality issues or unexplained process changes.
• Retain samples: Preserve representative samples from critical incoming batches under defined storage conditions and for an appropriate retention period. These samples provide a reference for future testing and can help determine whether a field or production failure originated in the supplied material, changed during storage, or arose during module manufacturing.
• Contractual investigation rights: Include provisions giving the module manufacturer appropriate access to supplier records, relevant production and quality-control data, retained samples, and manufacturing facilities when investigating suspected material-related failures. The agreements should also define expectations for root-cause analysis, corrective action, and cooperation with independent testing when necessary.

Sampling plans should be designed around the physical form and production history of the material, not simply around the shipment’s total capacity.

Module purchasers must also move beyond a procurement process dominated by price, datasheet power, and warranty length. A low module price can be attractive at financial close, but it does not reveal whether the BOM has been independently tested, whether raw-material suppliers are controlled, whether substitutions are permitted, or whether a warranty provider will remain capable of honoring claims decades later. Independent reliability testing, factory audits, production monitoring, BOM verification, pre-shipment inspection, and batch testing should be treated as complementary controls rather than optional extras.

Ultimately, PV module quality should not be viewed as the responsibility of only one company. The module manufacturer must remain accountable for the product it sells, but purchasers must evaluate whether that module manufacturer has sufficient visibility, contractual leverage, and technical controls over the materials incorporated into the module. A rigorous module procurement process should therefore assess not only who makes the module, but also who makes its critical materials, how those materials are controlled, and what evidence demonstrates that the complete BOM can survive its intended operating life.