Photovoltaic Module Outdoor Exposure Testing: Standards, Procedures, and Evaluation Criteria

Outdoor exposure testing is a critical reliability and safety assessment that subjects photovoltaic (PV) modules to real-world climatic conditions in order to evaluate their long-term durability and electrical performance. Governed primarily by IEC 61215-2 (MQT 13) and IEC 61730-2 (MST 61), this test verifies that modules can withstand natural sunlight, temperature fluctuations, humidity, and environmental stressors with minimal power degradation. This article provides a comprehensive technical overview of outdoor exposure testing, including applicable standards, sample requirements, test procedures, pass criteria, and practical applications for manufacturers and system developers.

Technical Overview

Photovoltaic module outdoor exposure testing is a natural weathering assessment in which complete PV modules are mounted outdoors and exposed to authentic environmental conditions until they accumulate a defined cumulative solar irradiation dose. Unlike accelerated laboratory aging tests such as damp heat or thermal cycling, outdoor exposure evaluates module behavior under the combined and synergistic effects of solar radiation, ambient temperature, humidity, rainfall, wind, and atmospheric pollution simultaneously, providing a realistic benchmark for field performance.

The principal standard governing this test is IEC 61215-2, which specifies the outdoor exposure test as MQT 13 within the design qualification and type approval sequence for terrestrial PV modules. The companion safety standard IEC 61730-2 defines the corresponding safety requirement as MST 61. In addition, ASTM E1596 describes test methods for solar radiation weathering of PV modules and materials, offering alternative procedures for extended-duration natural weathering studies. Under the IEC framework, modules must typically receive a total solar irradiation of 60 kWh/m², measured in the plane of the array, after which the modules undergo visual inspection, maximum power measurement, insulation testing, and wet leakage current testing to determine whether degradation remains within acceptable limits.

Outdoor exposure serves two complementary purposes: it validates that a module design can survive short-term natural environmental stress as part of certification, and it generates degradation data that help manufacturers, researchers, and independent laboratories understand aging mechanisms such as encapsulant discoloration, delamination, corrosion, and backsheet embrittlement under site-specific climatic profiles.

Testing Samples

The test is performed on complete, production-representative modules that include all components of the final design, such as glass or polymer front sheets, encapsulant, cells, backsheet or rear glass, junction box, cables, connectors, and frame. Typically, a minimum of two modules of the same design are subjected to exposure, and the samples must be electrically and optically complete, since partial or non-representative coupons cannot reproduce the integrated degradation behavior of a full module.

Crystalline Silicon Modules

Monocrystalline and multicrystalline silicon modules in glass-backsheet or glass-glass constructions constitute the majority of samples tested. These samples allow evaluation of common failure modes including potential-induced degradation (PID), solder joint fatigue, encapsulant yellowing, and backsheet cracking under natural UV and thermal stress.

Thin-Film Modules

Cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-Si) thin-film modules are also tested. Because several thin-film technologies exhibit metastable electrical behavior, thin-film samples generally require preconditioning and controlled stabilization procedures before and after exposure so that measured power changes reflect genuine environmental degradation rather than transient recovery effects.

Bifacial, Flexible, and Specialty Modules

Bifacial modules may be exposed with rear-side irradiance considerations and albedo conditions documented, while flexible modules, building-integrated PV (BIPV) laminates, and lightweight polymer-fronted modules are tested to assess UV stability of polymer front sheets, adhesion of encapsulants, and resistance of edge seals, which are often the dominant degradation pathways for these product categories.

Testing Items

Outdoor exposure testing involves both the exposure phase itself and a defined battery of pre- and post-exposure characterization tests. The core testing items include:

• Initial characterization: visual inspection, maximum power determination at standard test conditions (STC), and measurement of temperature coefficients as applicable
• Continuous or periodic monitoring of in-plane irradiation, ambient and module temperature, relative humidity, and rainfall during the exposure period
• Cumulative irradiation accumulation to the required dose, typically 60 kWh/m² per IEC 61215-2 MQT 13
• Post-exposure visual inspection to identify major visual defects such as bubbles, delamination, cracks, discoloration, or junction box damage
• Post-exposure maximum power measurement to quantify power degradation relative to initial values
• Insulation test (MQT 03) to verify dielectric integrity after weathering
• Wet leakage current test (MQT 10) to confirm safety under wet conditions
• Optional extended diagnostics, including electroluminescence (EL) imaging, infrared thermography, and UV fluorescence inspection, to characterize internal defects

Testing Methods

Site Selection and Mounting Conditions

Exposure is conducted at a documented outdoor site with unobstructed solar access and no significant shading or reflective interference. Modules are mounted on open racks in an attitude approximating the intended installation, commonly at a tilt angle close to the site latitude or as specified by the applicable standard. The mounting configuration must permit free air circulation behind the modules for glass-backsheet designs, or replicate the intended thermal boundary conditions for building-integrated products. The site climatic class, such as the IEC 61215-1 climate zone definitions, should be recorded because the severity of temperature and humidity stress varies significantly between desert, tropical, temperate, and alpine locations.

Exposure Procedure and Irradiation Measurement

Modules remain electrically open-circuited or connected according to the standard requirements during exposure and are left in place continuously through day and night cycles so that they experience natural diurnal temperature swings, dew formation, and precipitation. In-plane solar irradiation is measured using a calibrated pyranometer or reference cell conforming to ISO 9060, mounted coplanar with the modules, and the cumulative dose is integrated over the entire exposure period. Exposure continues until the modules have received a total solar irradiation of at least 60 kWh/m², which may be achieved in approximately one to two months at high-insolation sites but can take longer at temperate or cloudy locations. Environmental parameters, including module temperature and ambient conditions, should be logged to enable correlation of observed degradation with climatic stress levels.

Post-Exposure Evaluation and Pass Criteria

After the required dose is reached, the modules are carefully removed, cleaned if necessary, and allowed to stabilize before final characterization. The evaluation sequence and acceptance criteria per IEC 61215-2 are:

• Visual inspection: no major visual defects as defined in IEC 61215-1, chapter 8, must be present
• Maximum power: degradation relative to the initial measured value must not exceed 5 percent
• Insulation test: dielectric withstand requirements must be satisfied without breakdown or surface tracking
• Wet leakage current: measured leakage must remain within the limits specified for the module class and area

A module design passes the outdoor exposure test only when all applicable criteria are met. When the test is performed under IEC 61730-2 MST 61 for safety qualification, the emphasis is placed on verifying that no safety hazards, such as accessible live parts, compromised insulation, or structural failure, have developed as a result of natural weathering.

Applications

Outdoor exposure testing is applied across the entire PV value chain for multiple purposes:

• Certification and design qualification: fulfilling the MQT 13 requirement of IEC 61215-2 and the MST 61 requirement of IEC 61730-2 as part of type approval in accredited third-party laboratories
• New material validation: assessing novel encapsulants, backsheets, anti-reflective coatings, and cell interconnect technologies under authentic UV and thermal loads before mass production
• Comparative aging studies: benchmarking module designs or manufacturing batches side by side at one or multiple climatic sites to support product development decisions
• Bankability and due diligence: providing degradation evidence that investors, insurers, and engineering firms use when evaluating long-term energy yield and warranty risk
• Failure analysis research: investigating field-reported failure mechanisms such as backsheet chalking, hot spots, and contact corrosion by reproducing them under controlled natural conditions
• Quality control: verifying that production modules from ongoing manufacturing runs continue to match the weathering performance of the qualified design

FAQ

How long does the outdoor exposure test take? The duration depends entirely on site insolation. Because the pass condition is based on a cumulative dose of 60 kWh/m² rather than a fixed calendar period, the test may conclude in roughly four to eight weeks at high-irradiance desert sites but require several months in temperate or overcast climates.

Which standards apply to PV module outdoor exposure testing? The primary references are IEC 61215-2 (test MQT 13) for design qualification, IEC 61730-2 (test MST 61) for safety, and ASTM E1596 for solar radiation weathering of PV modules and materials. Regional and national standards families, including EN and GB standards, generally adopt or align with the corresponding IEC test procedures.

How does outdoor exposure differ from accelerated laboratory aging? Accelerated tests such as damp heat, thermal cycling, and UV preconditioning apply a single intensified stress factor at controlled levels, whereas outdoor exposure applies the full spectrum of natural stresses in combination and at realistic intensities. Outdoor testing therefore captures synergistic effects and validates that accelerated test results correlate with real-world behavior, but it requires longer wall-clock time and cannot decouple individual stress factors.

What happens if a module exceeds the 5 percent power degradation limit? Exceeding the limit constitutes a failure of the design qualification sequence. The manufacturer must investigate the root cause, such as inadequate encapsulant stabilization or moisture ingress paths, implement design or process corrections, and resubmit the module for testing to demonstrate compliance.

Can modules be tested at multiple climates? Yes, extended research programs frequently expose replicate modules at geographically diverse sites representing hot-dry, hot-humid, temperate, and cold climate zones. While IEC 61215-2 requires only a single adequate exposure, multi-site data significantly strengthen lifetime predictions and are increasingly requested in commercial due-diligence studies conducted by accredited third-party laboratories.