Photovoltaic cells are rigorously tested for hail and impact resistance using standardized international protocols that simulate extreme weather conditions. The primary method involves firing ice spheres of specific diameters and velocities directly at the panels in a controlled laboratory setting. This testing is governed by standards like the International Electrotechnical Commission’s IEC 61215, which includes a dedicated test sequence (MST 16) for hail impact. The goal isn’t just to see if the glass cracks, but to ensure the module continues to generate power safely afterward. For a photovoltaic cell to pass, it must suffer no major visual defects—like broken glass that exposes the cells—and experience minimal power loss, typically less than 5% of its rated output.
The foundation of all reliable hail testing is the creation of a realistic hailstone. Labs don’t just use random ice cubes; they produce highly uniform ice spheres. The standard hailstone diameters defined in IEC 61215 are 25 millimeters and 50 millimeters. The ice spheres must have a density between 0.88 g/cm³ and 0.92 g/cm³, mimicking the hardness of natural hail. They are launched using an air cannon, a device that uses compressed air to propel the ice ball with precise and repeatable force. The velocity is critical. For the 25 mm hailstone, the impact speed is 23.0 meters per second (approximately 82.8 km/h or 51.4 mph). For the more severe 50 mm hailstone, the speed is increased to 30.0 m/s (108 km/h or 67.1 mph). These speeds are not arbitrary; they are calculated to represent the terminal velocity of hailstones falling from a thundercloud.
| Hailstone Diameter (mm) | Impact Velocity (m/s) | Impact Velocity (km/h) | Kinetic Energy (Joules) |
|---|---|---|---|
| 25 | 23.0 | 82.8 | ~ 2.1 J |
| 50 | 30.0 | 108.0 | ~ 17.7 J |
The test procedure itself is meticulous. The solar panel is mounted in a frame that mimics its typical installation angle, often set at the most vulnerable position, which is usually perpendicular to the path of the hailstone. Technicians then target specific, critical areas of the module. These are not random strikes. The standard requires impacts on or near the corners, the edges of the frame, and directly over the silicon solar cells themselves, especially areas where busbars (the metallic strips on the cells) are located. This is because these are points of high mechanical stress. A module must withstand multiple impacts—usually 11 strikes across its surface—in a single test sequence to pass.
But the test doesn’t end when the ice shatters. The real evaluation begins with a thorough post-impact inspection. This is a multi-step process:
Visual Inspection: Engineers first examine the module for any visible damage. This includes checking for cracks in the front glass, delamination (where the layers of the panel separate), and any damage to the backsheet or frame. The criteria are strict; hairline cracks over the cells might be acceptable if they don’t propagate, but any crack that exposes the internal components to the elements is an immediate failure.
Insulation Test: Safety is paramount. The module undergoes a high-voltage test, often called a dielectric withstand or hipot test. This involves applying a high voltage (e.g., 1000V plus twice the system voltage) between the module’s internal current-carrying parts and its frame. If there is any compromise in the electrical insulation caused by the impact, this test will detect it by showing a current leak, resulting in a failure.
Power Performance Verification: Finally, and most importantly, the module is retested for its electrical output. It is placed under a solar simulator, and its maximum power output (Pmax) is measured. According to IEC 61215, the power loss after the hail impact must not exceed 5% of the initial pre-test measurement. A module with shattered glass might still function, but if microcracks have damaged too many internal cell connections, its efficiency will plummet, causing it to fail the test.
The resilience of a panel to hail is not a matter of luck; it’s a direct result of its material composition and engineering. The first line of defense is the front glass. Most hail-resistant panels use tempered glass, which is significantly stronger than standard annealed glass. Tempered glass is heat-treated to create a surface under high compression, making it about 4 to 5 times stronger. The thickness of this glass is a major factor; residential panels typically use 3.2 mm thick tempered glass, while panels designed for more severe environments might use 4 mm glass. Furthermore, some manufacturers employ anti-reflective coatings that are also engineered to add a degree of surface hardness.
Beyond the glass, the way the panel is laminated is critical. The solar cells are encapsulated within a polymer, most commonly Ethylene-Vinyl Acetate (EVA). This layer acts as a shock absorber. When hail strikes, the EVA distributes the impact force across a wider area of the cell, preventing the concentrated stress from shattering the fragile silicon wafer. The quality of the lamination process—ensuring there are no air bubbles and that the bond between the glass, EVA, and cells is perfect—is essential for impact performance. The aluminum frame also plays a role by providing rigid support and preventing flexing that could exacerbate damage.
While the IEC 61215 standard is the international benchmark, some manufacturers and regions subject their panels to even more strenuous testing. For instance, the UL 61703 standard used in North America involves similar hail impact tests but may include additional environmental stress sequences. Some panel datasheets now boast resistance to hailstones larger than 50 mm, such as 55 mm or even 60 mm, indicating they have been tested beyond the minimum requirements. These “extended hail” tests often involve higher velocities or larger ice balls to demonstrate superior durability, a key selling point in hail-prone areas like the American Great Plains or parts of Europe.
Independent testing laboratories like UL Solutions, TÜV Rheinland, and Intertek are the neutral parties that verify a manufacturer’s claims. They purchase panels from the open market or receive samples directly from manufacturers for “type approval” testing. When a panel model passes, it receives a certification mark. This third-party validation is crucial for installers and consumers, providing confidence that the product has been tested to a rigorous, unbiased standard. The entire process, from creating the perfect ice ball to the final power measurement, is about de-risking a significant investment and ensuring that solar power plants can operate reliably for decades, even when faced with nature’s fury.