Photovoltaic (PV) module hail testing, defined primarily by IEC 61215-2 (Test MQT 10) and ASTM E1038, verifies that solar modules can withstand ice ball impacts without glass breakage, cell cracking, or electrical failure. This technical guide explains the test samples, testing items, equipment, procedures, and acceptance criteria commonly applied by accredited third-party laboratories. Understanding hail impact testing helps manufacturers, project developers, insurers, and lenders confirm the mechanical and electrical reliability of PV modules deployed in hail-prone regions.
Technical Overview
Hail impact testing is a mandatory mechanical robustness test within the IEC 61215 series, the internationally recognized design qualification and safety approval standard for terrestrial photovoltaic modules. During the test, ice balls of specified diameters are propelled at the module surface at calibrated velocities, simulating the kinetic energy delivered by natural hailstones. The test exposes potential weaknesses such as front glass fracture, hidden cell micro-cracks, interconnect damage, backsheet perforation, and insulation degradation.
The principal test method is MQT 10 of IEC 61215-2, which replaced and harmonized earlier national procedures. In North America, UL 61730 references the same hail impact approach as part of module safety evaluation, while ASTM E1038 provides an alternative standardized method for determining hail resistance and associated power loss. China has adopted the IEC methodology in the GB/T 61215 national standard series. Newer editions of IEC 61215-2 introduce hail severity classes, allowing larger ice balls (up to 75 mm) to be specified for sites with known severe hail exposure.
The default qualification condition uses a 25 mm ice ball with a mass of approximately 7.5 g launched at 23.0 m/s, delivering roughly 2 J of kinetic energy to each impact point. At the upper end of the severity scale, a 75 mm ice ball of about 203 g traveling near 40 m/s delivers over 160 J, which is sufficient to challenge even heavy-duty glass-glass constructions used in high-risk climates.
Testing Samples
Samples should be selected from regular production or from a pilot line fully representative of production materials and processes, as required by IEC 61215-1. Modules are typically tested in the as-manufactured state with their intended frame, glass, encapsulant, cells, backsheet, and junction box.
Crystalline Silicon Modules
Monocrystalline and multicrystalline modules, including PERC, TOPCon, and heterojunction designs, represent the majority of hail test submissions. These modules usually employ 3.2 mm tempered or heat-strengthened front glass, and their brittle silicon cells are highly sensitive to impact-induced micro-cracking, making them the primary target of the test.
Thin-Film Modules
Cadmium telluride (CdTe), CIGS, and amorphous silicon modules are tested in both superstrate and substrate configurations. Because their glass or flexible substrates differ structurally from conventional crystalline designs, impact response and failure modes are evaluated against the same MQT 10 procedure with geometry-adjusted impact point placement.
Bifacial, Glass-Glass, and Special Constructions
Glass-glass bifacial modules, building-integrated PV (BIPV) laminates, lightweight modules with polymer front sheets, and flexible modules can also be assessed. For bifacial products, project specifications or extended test programs may additionally require rear-side impact evaluation, since the rear glass or backsheet may also be exposed to hail in certain mounting configurations.
Testing Items
A complete hail test program evaluates both the immediate mechanical response and the residual electrical performance of the module:
- Ice ball impact resistance at all defined impact locations, using the specified diameter, mass, and velocity
- Visual inspection for major defects such as glass breakage, cracks through cells visible from the front, bent or broken external surfaces, and junction box damage
- Maximum power (Pmax) measurement before and after impact to quantify power degradation
- Wet leakage current (wet insulation resistance) testing to confirm electrical insulation integrity after impact
- Electroluminescence (EL) imaging, widely used as a diagnostic supplement to reveal hidden cell cracks not visible to the naked eye
- Optional insulation resistance and dielectric withstand checks where required by the applicable safety standard
Testing Methods
Equipment and Ice Ball Preparation
The test apparatus consists of a compressed-air launcher (ice ball cannon) with a barrel matched to the ice ball diameter, a rigid module support fixture, and calibrated velocity measurement sensors such as light gates positioned close to the impact plane. Ice balls are molded from distilled or deionized water, must be free of cracks and trapped air bubbles, and are conditioned in a freezer until they reach the required temperature of -4 °C ± 2 °C. Ice ball diameter, mass, temperature, and launch velocity must all fall within the tolerances tabulated in IEC 61215-2, and temperature verification is performed with a thermocouple embedded in a representative ice ball.
Test Procedure
The module is securely mounted at ambient laboratory temperature, and the launcher axis is aligned perpendicular to the module surface. The general sequence is as follows:
- Perform initial measurements: visual inspection, maximum power determination, and baseline EL imaging where required by the test plan
- Select the ice ball diameter corresponding to the agreed severity class (25 mm by default, or up to 75 mm for severe hail requirements)
- Impact the module at the fixed pattern of impact points defined in IEC 61215-2, which covers corners, cell edges and corners near the frame, mid-points of long and short edges, and the module center
- Verify and record the velocity of each launched ice ball with the calibrated measurement system
- Inspect the module visually immediately after each impact for glass damage or surface penetration
- After all impacts, repeat visual inspection, maximum power measurement, and the wet leakage current test
Acceptance Criteria
The module passes the hail test when all of the following conditions are satisfied: no evidence of major visual defects as defined in IEC 61215-1; the wet leakage current requirement is met after impact; and, where power is measured, the post-test maximum power remains at or above the level permitted by the standard, with no more than 5 percent degradation typical for qualification purposes. Hidden cracking detected by EL imaging, while not always a formal pass-fail criterion in the base standard, is frequently evaluated in purchaser and bankability specifications because micro-cracks can propagate under subsequent thermal and mechanical cycling.
Applications
Hail testing results are used across the entire PV value chain:
- Design qualification and safety certification of new module types prior to market introduction
- Project bankability reviews, in which lenders and technical advisors verify hail resilience for plants in hail-prone climates
- Insurance underwriting and loss-prevention assessments for utility-scale and commercial rooftop installations
- Validation of hail stow strategies for single-axis trackers, which angle modules to reduce direct impact exposure
- Product development and benchmarking of glass thickness, encapsulant choice, and frame design against severe hail classes
- Factory production monitoring and periodic re-qualification after design, material, or process changes
- Procurement specifications, where buyers define ice ball size classes exceeding the 25 mm baseline for high-risk sites
Because hail losses have become one of the leading causes of insured damage in large PV plants, an increasing number of developers in regions such as North America, Australia, and parts of Europe now specify 35 mm or 45 mm severity levels, supported by extended testing performed by accredited third-party laboratories.
FAQ
Which standard governs PV module hail testing?
The core method is Test MQT 10 of IEC 61215-2, which is referenced by UL 61730 for the North American safety framework and adopted by China in the GB/T 61215 series. ASTM E1038 provides an alternative hailstone impact method commonly cited in research and procurement documents.
What ice ball size and speed are used by default?
The standard qualification condition is a 25 mm ice ball of about 7.5 g launched at 23.0 m/s. Higher severity classes use diameters up to 75 mm at correspondingly higher velocities specified in the IEC 61215-2 table.
Why is the ice ball held at -4 °C?
The -4 °C ± 2 °C condition approximates the temperature and hardness of natural hailstones at the point of ground impact, ensuring repeatable and realistic impact energy across laboratories.
Does passing the hail test guarantee immunity to hail damage?
No. The test demonstrates resistance only to the specific impact energy and ice ball size evaluated. Severe supercell hailstorms can exceed standard test severities, which is why enhanced classes, stow strategies, and extended stress programs are specified for high-risk sites.
How is hidden cell cracking detected after the test?
Electroluminescence imaging is the industry-standard diagnostic. It reveals micro-cracks and fractured cell regions that are invisible under visual inspection but can cause progressive power loss under field operation.