Photovoltaic Module Hail Impact Testing: Standards, Methodology, and Acceptance Criteria

Photovoltaic (PV) module hail impact testing evaluates the ability of solar panels to withstand hailstone strikes without glass breakage, cell cracking, or dangerous electrical degradation. The test is defined primarily in IEC 61215-2 (Test MQT 10), with related requirements in IEC 61730-2, ASTM E1038, UL 61730, and the corresponding GB/T standards. This article explains the test principle, sample requirements, test items, procedures, acceptance criteria, and typical applications for accredited third-party hail resistance testing of PV modules.

Technical Overview

Hail impact testing is a mechanical robustness test in which ice balls of defined diameter are propelled at a PV module surface at a calibrated velocity, simulating the kinetic energy delivered by natural hailstones. The purpose is to verify that the front cover, encapsulation, solar cells, and electrical insulation survive hail impact without major visual defects, unacceptable power loss, or safety hazards such as loss of insulation integrity.

The internationally recognized method is specified in IEC 61215-2 "Terrestrial photovoltaic (PV) modules – Design qualification and type approval – Part 2: Test procedures", where hail impact is designated MQT 10. The safety standard IEC 61730-2 also addresses impact resistance within its module safety verification program and cross-references the hail impact method. Additional recognized methods include ASTM E1038 (hail resistance by propelled ice balls), UL 61730 (which superseded UL 1703 in many markets), and the Chinese national adoption GB/T 61215.2.

In the standard configuration, a 25 mm diameter ice ball with a mass of approximately 7.5 g is launched at 23.0 m/s, striking the module normal to its surface at a prescribed set of 11 impact points. For sites with more severe hail exposure, larger ice balls may be specified with correspondingly higher impact velocities; for example, IEC 61215-2 tabulates a 35 mm ball at approximately 27 m/s and a 45 mm ball at approximately 31 m/s, with sizes extending up to 75 mm for extreme environments.

Testing Samples

Crystalline Silicon Glass–Backsheet Modules

Standard framed or frameless modules with tempered or heat-strengthened front glass and a polymeric backsheet represent the majority of samples submitted for hail testing. Samples should be production-representative, incorporating the final glass type and thickness, encapsulant, cell technology, and frame configuration intended for market delivery.

Dual-Glass and Bifacial Modules

Glass–glass constructions and bifacial modules are tested using the same impact protocol, typically applied to the front (sky-facing) surface. Because rear-side glass may also be exposed on tracking systems or vertically mounted installations, additional rear-side impact programs can be agreed between manufacturer and purchaser.

Thin-Film and Special Application Products

Since the 2016 edition, the IEC 61215 series covers all terrestrial module technologies, including thin-film products. Building-integrated PV (BIPV), flexible laminates, lightweight modules, and vehicle-integrated solar products may also be assessed, with test plans adapted to their mounting and exposure conditions.

Sample Quantity and Condition

Testing is generally performed on complete, unused modules that have not undergone prior stress conditioning beyond the initial baseline measurements. Depending on the certification sequence, one or more modules are required, and baseline electrical, visual, and imaging records must be taken before impact so that post-test comparisons are valid.

Testing Items

A complete hail resistance evaluation combines the impact exposure itself with a defined set of before-and-after measurements:

• Initial visual inspection for pre-existing defects such as scratches, delamination, or glass edge damage.
• Baseline maximum power (Pmax) and current–voltage characteristic measurement at standard test conditions (STC).
• Optional electroluminescence (EL) imaging, per IEC TS 60904-13, to document the absence of pre-existing cell cracks.
• Hail impact exposure at the specified ice ball diameter, velocity, and impact point map.
• Post-test visual inspection to identify glass breakage, visible cell cracks, or other major visual defects.
• Post-test maximum power determination to quantify power degradation, which must not exceed 5% relative to the baseline value.
• Insulation resistance and wet leakage current testing to confirm that electrical safety is preserved after mechanical impact.
• Optional post-test EL imaging to reveal micro-cracks that are invisible to the naked eye but capable of progressive power loss in the field.

Testing Methods

Ice Ball Preparation and Conditioning

Ice balls are molded from purified or deionized water in precision molds corresponding to the required diameters. Each ball is inspected to confirm it is free of trapped air bubbles and cracks, and its diameter and mass are verified against the tolerances of the applicable standard. Balls are conditioned and stored in a freezer at approximately −10 °C until shortly before launch, and each ball is used only once to guarantee authentic ice material properties, since substituting steel or other projectiles does not reproduce the fracture behavior of real hail.

Launch Equipment and Velocity Control

A pneumatic hail cannon (compressed-air launcher) accelerates the ice ball along a calibrated barrel toward the clamped module. Optical velocity sensors located immediately in front of the sample measure the speed of each shot, and the launching pressure is adjusted until the required velocity is achieved within the specified tolerance. The shot must strike the module perpendicular to its surface, and the trajectory is verified so the impact point lands within the allowed radius of each target location.

Impact Procedure

The module is secured in a rigid mounting fixture in its normal orientation. A total of 11 impact points are struck, one impact per point, selected to cover structurally and electrically critical regions: the center of the module, each corner region, mid-span and edge locations, areas adjacent to the junction box, and locations near electrical terminations and mounting points. All impacts are completed in a controlled ambient environment so that the ice balls do not melt or deform prior to striking the sample.

Post-Test Evaluation

After the final impact, the module is allowed to stabilize and then undergoes the agreed post-test sequence: visual inspection, maximum power measurement, and insulation and wet leakage testing, supplemented by EL imaging where specified. The module passes if it exhibits no major visual defects, power degradation does not exceed 5%, and insulation integrity is maintained. Any glass penetration, exposed live parts, or systematic cell cracking constitutes a failure requiring design revision and retest.

Applications

Hail impact testing is applied across the entire PV product lifecycle:

• Design qualification and type certification: manufacturers obtain IEC 61215 and IEC 61730 certification through accredited third-party laboratories as the basis for market access in most regions.
• Site-specific engineering: project developers and lenders specify enlarged ice ball diameters (35 mm, 45 mm, or larger) for plants located in hail-prone climates, ensuring modules match the local hail threat.
• Insurance and risk management: verified hail resistance data supports underwriting decisions and loss-prevention programs for utility-scale and commercial installations.
• Research and development: engineering teams use hail testing to validate glass thickness, heat-treatment quality, encapsulant selection, frame design, and dual-glass constructions before product release.
• Change management: design or material changes to certified products trigger retesting to confirm continued compliance.
• Forensic and field-failure analysis: after real hailstorm events, comparative laboratory testing helps distinguish hail damage from manufacturing defects or other mechanical stresses in warranty disputes.

FAQ

Which standard governs PV module hail impact testing?

The primary method is MQT 10 of IEC 61215-2:2016, which is mirrored in regional adoptions such as EN IEC 61215-2 and GB/T 61215.2. Safety verification under IEC 61730-2 also includes impact assessment, and ASTM E1038 and UL 61730 provide recognized alternative procedures for specific markets.

What are the default test conditions?

The standard condition is a 25 mm ice ball launched at 23.0 m/s, striking 11 defined points on the module, one strike per point. Larger balls with higher velocities, up to 75 mm diameter, can be specified for harsher hail climates.

What are the pass criteria?

The module must show no major visual defects, must not lose more than 5% of its initial maximum power, and must pass insulation and wet leakage current tests after the impact sequence.

Does passing the standard test guarantee survival of any hailstorm?

No. The 25 mm condition represents a baseline design qualification level. Natural hailstorms can produce larger stones with higher impact energy, which is why enhanced hail programs with 35 mm, 45 mm, or larger ice balls are recommended for high-risk regions.

Why are real ice balls used instead of steel balls?

Real ice reproduces the density, hardness, and fracture behavior of natural hailstones. Steel or other rigid projectiles transfer energy differently and can produce non-representative damage, so standards mandate genuine ice projectiles conditioned and dimensionally verified before launch.

Can cracked cells still pass the hail test?

Cracks that cause power degradation above 5% or constitute a major visual defect result in failure. EL imaging is frequently added to the test plan because it detects micro-cracks that are not visible externally yet may degrade long-term field performance.