Hail impact testing is a critical mechanical evaluation used to verify that photovoltaic modules, solar collectors, roofing systems, and other exposed products can withstand real-world hailstone strikes. This article provides a detailed analysis of the major hail testing standards, including IEC 61215, EN 61215, GB/T 61215, UL 2218, ASTM E822, ISO 9806, and JIS C 8990, covering test principles, sample requirements, procedures, and acceptance criteria. Read on to understand how accredited third-party laboratories apply these specifications to support product certification, insurance assessment, and market access.
Technical Overview
Hail testing is a destructive mechanical evaluation that simulates the impact of natural hailstones on products exposed to outdoor environments. During a hailstorm, hailstones with diameters ranging from a few millimeters to more than 75 mm can strike surfaces at velocities approaching their terminal velocity, delivering kinetic energy that may fracture glass, crack solar cells, puncture roofing membranes, or degrade structural integrity. The purpose of hail testing is to reproduce these loads in a controlled, repeatable laboratory setting so that design weaknesses can be identified before products reach the market.
The fundamental principle of hail testing is kinetic energy equivalence. A projectile — typically a manufactured ice ball, or in some roofing standards a hardened steel ball — is accelerated to a defined velocity and directed perpendicular to the specimen surface. By controlling projectile diameter, mass, and velocity, the test delivers a reproducible impact energy representative of a natural hailstone of the same size. Most international standards derive their velocity tables from the approximate terminal velocities of real hailstones in free fall.
Principal Standards Framework
Hail resistance requirements are specified in several international, regional, and national standards, each tailored to a product family:
- IEC 61215-2 (MQT 17) — Terrestrial photovoltaic (PV) module design qualification and type approval; defines the ice ball hail impact test for crystalline and thin-film modules, including the most widely used 25 mm / 23.0 m/s severity.
- EN 61215-2 — The European normative adoption of IEC 61215-2, applying identical hail impact requirements within the European market.
- GB/T 61215.2 — The Chinese national standard adopting IEC 61215-2, with the same MQT 17 hail impact procedure; the legacy standard GB/T 9535 also specified a 25 mm / 23 m/s hail test.
- UL 2218 — Standard for Impact Resistance of Roof Covering Materials; classifies asphalt shingles, tiles, membranes, and other roof coverings into Class 1 through Class 4 using a steel ball drop method.
- ASTM E822 — Standard practice for determining the resistance of solar collector covers to impact by hail using steel balls.
- ISO 9806 — Solar energy, solar thermal collectors, test methods; includes an external impact resistance test applicable to glazed and unglazed collectors.
- JIS C 8990 — The Japanese industrial standard for crystalline silicon terrestrial PV module design qualification, aligned with the IEC 61215 framework including hail impact.
Across all of these standards, the common technical elements are projectile preparation and conditioning, velocity calibration, defined impact locations, and post-test evaluation against visual, electrical, or functional acceptance criteria.
Testing Samples
Accredited third-party laboratories perform hail testing on a wide range of products intended for outdoor service. The specimen type determines which standard applies and how results are judged.
Photovoltaic Modules
Crystalline silicon and thin-film PV modules are the most common hail test specimens, evaluated under IEC 61215-2, EN 61215-2, GB/T 61215, or JIS C 8990. One module is typically subjected to the full MQT 17 procedure, with impacts distributed across the glass surface, cell corners, module edges, and other structurally vulnerable points.
Solar Thermal Collectors
Glazed flat-plate collectors, evacuated tube collectors, and unglazed polymer collectors are tested under ISO 9806 or ASTM E822. The evaluation focuses on cover plate integrity and whether impact damage leads to fluid loss, moisture ingress, or thermal performance degradation.
Roofing and Cladding Materials
Asphalt shingles, concrete and clay tiles, metal roofing, polymer membranes, and synthetic coverings are classified under UL 2218. Multiple specimens of each product configuration, including seams, joints, and edge terminations, are impacted to establish a Class 1–4 hail resistance rating.
Glazing and Transparent Components
Architectural glass, polycarbonate and acrylic glazing, skylights, greenhouse panels, and cover materials for outdoor equipment may be subjected to ice ball impact protocols adapted from the PV and solar thermal standards to verify resistance to cracking and penetration.
Automotive and Aerospace Components
Exterior body panels, lighting covers, radar and sensor housings, painted composite surfaces, and aircraft structures are evaluated against vehicle or airframe manufacturer engineering specifications that frequently reference ice ball or equivalent impact energies drawn from the same test physics.
Testing Items
A complete hail test program comprises several distinct measurement and evaluation items:
- Projectile verification — Confirmation of ice ball diameter, mass, and freedom from cracks or entrapped air; ice balls are molded from distilled or deionized water and conditioned in a freezer at approximately −10 °C ± 5 °C (or colder, where the applicable standard requires) before launch.
- Velocity calibration — Measurement of projectile speed at the specimen plane using calibrated optical light gates or ballistic sensors; launch pressure is adjusted until the standard velocity tolerance is achieved.
- Impact energy confirmation — Calculation of delivered kinetic energy (E = ½mv²) for each projectile size, verified against the standard's energy table.
- Impact orientation and location control — Impact applied perpendicular to the specimen surface at the defined strike points; IEC 61215-2 specifies 11 impact locations, including module corners, cell corners, cell edges, and points near the frame.
- Post-test visual inspection — Examination for major visual defects such as broken front glass, visible cracked cells, delamination, bubbles or blisters beyond allowable dimensions, backsheet tears, and junction box damage.
- Electrical performance measurement — Where the test sequence requires, determination of maximum power at Standard Test Conditions (STC), with degradation compared against the limit defined by the standard (typically not more than 5 % relative to initial measurements).
- Safety verification — Insulation resistance, dielectric withstand, or wet leakage current testing after the hail sequence to confirm that impact damage has not created an electrical hazard.
Testing Methods
The test method depends on the applicable standard, but all methods share a controlled projectile delivery system, environmental conditioning, and defined pass/fail evaluation.
Ice Ball Launch Method (IEC 61215-2 / EN 61215 / GB/T 61215 / JIS C 8990)
The specimen is stabilized at ambient laboratory temperature, typically 23 °C ± 5 °C. Ice balls are launched from a compressed-air cannon aimed perpendicular to the module surface. Before formal testing, the system is calibrated by firing at a target and measuring velocity, adjusting air pressure until the required speed is reached within tolerance. The standard severity table from IEC 61215-2 (MQT 17) is as follows:
- 12.5 mm diameter — mass 0.94 g — velocity 16.0 m/s — energy approximately 0.12 J
- 15 mm diameter — mass 1.63 g — velocity 17.8 m/s — energy approximately 0.26 J
- 20 mm diameter — mass 3.89 g — velocity 21.9 m/s — energy approximately 0.93 J
- 25 mm diameter — mass 7.53 g — velocity 23.0 m/s — energy approximately 1.99 J (the default qualification severity)
- 35 mm diameter — mass 20.7 g — velocity 27.2 m/s — energy approximately 7.66 J
- 45 mm diameter — mass 43.9 g — velocity 30.7 m/s — energy approximately 20.7 J
Each of the 11 specified locations receives one impact. After testing, the module undergoes visual inspection under the MQT 01 criteria, and, where required by the qualification sequence, maximum power measurement and electrical safety checks. Failure is declared if major visual defects are present, power degradation exceeds the standard limit, or any safety test fails.
For installations in regions with severe hail exposure, enhanced hail testing is frequently specified using the larger projectile sizes (35 mm, 45 mm, and in some market-specific programs up to 55–75 mm) to demonstrate higher impact resistance for insurers, project developers, and certification bodies.
Steel Ball Drop Method (UL 2218)
For roof covering materials, UL 2218 uses hardened steel balls dropped from a fixed height of 3.66 m (12 ft) onto the most vulnerable areas of the specimen, including intersections, seams, edges, and fastener locations. The four classification levels are:
- Class 1 — 31.75 mm (1.25 in) steel ball
- Class 2 — 38.1 mm (1.5 in) steel ball
- Class 3 — 44.45 mm (1.75 in) steel ball
- Class 4 — 50.8 mm (2.0 in) steel ball, representing the highest hail impact resistance rating
A classification is achieved when the covering exhibits no cracking, rupture, fracture, or opening through the exposed surface that would allow water penetration. Class 4 products are frequently recognized by insurers for reduced hail damage premiums.
Solar Collector Impact Methods (ISO 9806 / ASTM E822)
ISO 9806 specifies an external impact resistance procedure for solar thermal collectors using ice ball projectiles at diameters and velocities consistent with the PV hail methodology, or an agreed steel ball alternative. ASTM E822 covers steel ball impact on collector covers, defining apparatus, projectile sizing, and strike locations. Acceptance is based on the absence of cover breakage, loss of weather sealing, or measurable deterioration of collector performance.
Quality Control During Testing
- Ice balls must be rejected if cracked, misshapen, or containing visible bubbles, as these defects alter the effective impact energy.
- Velocity measurement devices must be calibrated traceably, and projectile speed must be verified immediately before or during the test series.
- Impact angle must remain perpendicular to the surface to ensure the specified energy is delivered to the strike point.
- Specimens must be securely mounted in a rigid fixture that reproduces the intended installation condition without absorbing impact energy.
Applications
Hail test results are used throughout industry for certification, engineering, and commercial decision-making:
- PV market access and certification — Successful completion of IEC 61215-2 / EN 61215 / GB/T 61215 / JIS C 8990 qualification, including the hail test, is a prerequisite for type approval and participation in international certification schemes for terrestrial PV modules.
- Utility-scale and commercial solar projects — Developers and lenders often require evidence of enhanced hail resistance for plants located in high-risk hail regions, beyond the baseline 25 mm severity.
- Insurance underwriting — UL 2218 Class 3 and Class 4 roofing classifications are widely referenced by insurers to quantify hail risk for residential and commercial buildings.
- Product research and development — Manufacturers use hail testing to compare glass thicknesses, heat-strengthened versus tempered options, encapsulant materials, frame designs, and protective coating systems.
- Failure analysis — After field hail events, laboratory reproduction of the impact allows engineers to distinguish hail damage from installation defects, thermal stress, or transport damage.
- Automotive and aerospace validation — Exterior components and composite structures are screened against impact energies derived from hail testing physics to confirm durability in service.
FAQ
What is the standard hail test for PV modules?
The default qualification severity under IEC 61215-2 (MQT 17), and its EN, GB/T, and JIS adoptions, is a 25 mm ice ball with a mass of 7.53 g launched at 23.0 m/s, delivering approximately 2 J of kinetic energy at each of 11 defined impact locations on the module.
Why do some standards use ice balls and others steel balls?
Ice balls best reproduce the actual mechanical response of a hailstone, including its fragmentation behavior during impact, and are therefore used for PV and solar thermal products. Steel balls provide a highly reproducible and conservative surrogate projectile, which is why UL 2218 and ASTM E822 use them for roofing and collector cover evaluation.
What constitutes a failure in the IEC 61215 hail test?
A module fails if post-test inspection reveals major visual defects such as broken glass, visibly cracked cells, or significant delamination, or if required electrical measurements show excessive power degradation or failed insulation, dielectric, or wet leakage safety tests.
Can a module be certified for larger hailstones than 25 mm?
Yes. The IEC 61215-2 severity table extends to 35 mm, 45 mm, and larger projectiles, and accredited third-party laboratories routinely perform enhanced hail testing at these severities for projects in severe hail regions or to satisfy insurer requirements.
How many samples are required for hail testing?
For PV module qualification, typically one module is allocated to the hail test within the overall IEC 61215 test sequence. For UL 2218 roofing classification, multiple specimens of each product configuration are tested so that the classification applies to the full installed system, including seams and joints.
Is hail testing mandatory for PV modules?
The hail test is a mandatory component of the IEC 61215 design qualification and type approval sequence, which is the primary basis for PV module market access in most regions. Roofing hail classifications under UL 2218 are typically voluntary but commercially essential in hail-prone markets.
How do the results support insurance and warranty decisions?
Documented hail resistance — whether a Class 4 UL 2218 rating for roofing or an enhanced 45 mm IEC hail qualification for PV modules — provides objective evidence of impact durability that insurers use to price risk and that manufacturers use to define hail-related warranty coverage.