Solar Panel Hail Performance: What Class IV Modules Actually Survive in Colorado

The solar industry uses one number to talk about hail performance — IEC 61215 certification, the international standard every residential and commercial module is tested against. The number it gives you is 25 millimeters, which is just under one inch, hitting the panel at terminal velocity. Every photovoltaic module sold in the United States passes that test before it gets to market.

The Front Range threw 2.5-inch hail across the south metro in May 2017, 2.75-inch hail across Colorado Springs in 2018, and 2-inch-plus events somewhere in the state in five of the last seven years. The standard solar test does not cover Colorado weather. This article is about what does — what Class IV modules actually do differently, what your insurance carrier thinks of all this, and what we install on a hail-zone home.

What IEC 61215 Actually Tests

IEC 61215 is the omnibus reliability standard for crystalline silicon photovoltaic modules. The hail portion is one of about a dozen sub-tests covering thermal cycling, humidity, mechanical load, UV stability, and so on. The hail sub-test specifies:

• An ice ball with a 25 mm diameter (approximately the size of a US quarter).
• Mass of approximately 7.5 grams.
• Velocity of 23 meters per second, which is the calculated terminal velocity of that size hailstone.
• Eleven impact points distributed across the module face — each corner, each edge midpoint, and the center.
• Pass criterion: no visible damage and no power degradation greater than 5 percent under a follow-up flash test.

That is a real test. It rules out modules that would fail in a moderate hailstorm. But the energy content of a hailstone scales with the cube of its diameter. A 2-inch hailstone is not twice as bad as a 1-inch hailstone — it is roughly eight times as much kinetic energy at impact. The standard test does not predict 2-inch performance.

Class IV and the FM 4476 Standard

The construction industry uses a different scale for shingle hail rating — UL 2218 Class 1 through 4. Class 4 is the highest, tested with 2-inch steel balls dropped from 20 feet (a stand-in for hail). When solar manufacturers started building modules for hail-zone markets, several brands adopted FM 4476, which is the equivalent factory-mutual hail standard for solar.

FM 4476 grades modules at 1-inch (Class 3) or 2-inch (Class 4) ice impact. A few manufacturers — REC Alpha Pure-R, SolarEdge Energy Bank panels paired with optimizer/inverter selection, Q CELLS Q.PEAK DUO ML-G11+, Tesla — publish FM 4476 Class 4 ratings on specific modules. The premium versus a standard 25mm-rated panel is typically 8 to 18 percent depending on the module and the season.

What changes structurally between a Class 3 and a Class 4 module is usually some combination of:

• Thicker tempered front glass — typically 3.2 mm versus 3.0 mm, sometimes with chemical strengthening.
• A redesigned cell layout that reduces the area of any single cell exposed to a single impact.
• Reinforced backsheet or full glass-glass construction.
• Frame and mounting redesign for higher mechanical-load tolerance.

Tesla Solar Roof — which is a roof tile rather than a module on rails — is tested under UL 2218 Class 3 (the original spec) and also under UL 2218 Class 4 with a separate impact test program. Tesla publishes the Class 4 result publicly, and we have seen real-world Tesla Solar Roof performance through the May 2024 Front Range storms.

Real-World Front Range Storm Survival

The May 2024 storms across Aurora, Centennial, and southeast Denver Metro produced 1.75 to 2.25-inch hail in a swath that hit several solar arrays we had installed. We walked roofs in the affected ZIP codes the week after. Here is what we observed across about 40 systems:

• Standard 25mm-rated modules on hit homes had visible cell-fracture patterns — the small dark lines under the glass that indicate microcracks. About one-third had visibly cracked front glass. Power output on the cracked modules was down 8 to 15 percent on the next sunny day.
• Class IV / FM 4476 modules on the same streets had no visible cracking and full power output. We pulled IV curves on a sample as a sanity check — within manufacturer tolerance.
• Tesla Solar Roof tiles in the same swath had zero replaced tiles in our portfolio after that storm. The energy storage tiles (visible glass) and the look-alike non-energy roof tiles came through equivalently.

One nuance: visible damage and microcracks are not the same thing. Microcracks reduce output gradually and can grow under thermal cycling. A panel that looks fine after a hailstorm might be down 10 percent in three years. The only reliable test is an electroluminescence (EL) image, which is not something a homeowner can do at home. After a major hail event in a known storm cell, a professional EL scan of every panel is the only way to know what you actually have.

Insurance Underwriting and Coverage Gaps

Most homeowner policies in Colorado cover the solar array as a fixture under the dwelling structure. That generally means it gets repaired or replaced under a hail claim along with the roof. The key word is "generally." Several carriers have introduced cosmetic-damage exclusions and named-peril restrictions specific to solar, and the carrier mix in Colorado has shifted in the last 18 months.

Practical implications for a homeowner adding solar in 2026:

• Ask your carrier in writing whether your solar array is covered as part of the dwelling or requires a separate endorsement. Do not assume.
• Verify coverage extends to lost production during repair — many policies cap loss-of-use at the structure itself and do not cover net metering shortfall.
• Ask whether the carrier requires Class IV / FM 4476 modules to write or renew. A few carriers in hail-prone Colorado now do.

For deeper claims mechanics see understanding your roof insurance claim. For the broader hail-rating economics conversation, see Class 4 shingle insurance discounts.

What We Specify on a Hail-Zone Home

For a residential install in a Colorado ZIP code with documented 2-inch-plus hail history within the last decade, we default to FM 4476 Class IV modules and we walk the homeowner through the cost difference openly. The pure economics — premium price minus expected insurance reduction minus avoided future replacement — works out roughly even or slightly positive on Front Range homes. The peace-of-mind value is the additional return.

Specific products we currently install on hail-priority projects:

• Q CELLS Q.PEAK DUO ML-G11+ for traditional racked installs — strong hail rating, good aesthetics, US warehouse stock.
• REC Alpha Pure-R when the homeowner prioritizes efficiency per square foot.
• Tesla Solar Roof when the homeowner is also re-roofing and wants a single integrated system rather than a roof plus a rack-mounted array. Our Tesla Solar Roof page has more on this.
• Solar shingles in tile-pattern (CertainTeed Solstice, GAF Energy Timberline Solar) as a middle path — better hail performance than rack-mount panels, lower cost than Tesla Solar Roof, no module penetrations.

For homeowners considering solar at the same time as a re-roof, the cleanest install is when the roof and the solar are one project rather than two — flashing, penetrations, warranty, and rack alignment are all easier when nothing is retrofit. Our solar overview covers the integrated approach.

Bottom Line

The standard 25 mm IEC test is a reasonable baseline. It is not enough for Colorado. If you live in a Front Range or Eastern Plains ZIP code that has seen 2-inch-plus hail in the last decade — and that is most of the I-25 corridor and the south metro — Class IV modules or Tesla Solar Roof are the right specifications, and your insurance carrier may already require it for new policies.

If you are debating an array spec or trying to figure out whether your existing 2018-era system survived the last storm intact, we are happy to walk the roof and run an electroluminescence scan. There is no cost for the inspection itself.