Are solar panels hail resistant?

In short: every module certified to IEC 61215 has survived 25 mm ice balls fired at 23 m/s, which is about 2 joules a hit. A 45 mm hailstone carries ten times that, so the certificate covers ordinary hail, not the storms that break roofs. Of the modules in our catalogue that state their glass, 47 % have a single front sheet of 3.2 mm and 45 % two sheets of 2.0 mm, and in a hail region I'd buy on a tested hail class rather than on the glass.

Hail is the one weather risk a buyer can't size away. Wind and snow get a load rating on the datasheet, and you can pick a module rated for your roof. Hail gets one line, "IEC 61215", which reads like an answer and is really a floor. Here is what that floor is, how far real hail sits above it, and what a module that is meant to take more has to show.

What the hail test fires

The hail test in IEC 61215 is simple to describe: freeze a ball of clear ice in a mould, put it in a launcher and shoot it at the glass. The standard's table pairs each diameter with a mass and a test velocity. The velocity stands for a hailstone of that size falling at its terminal speed, so the test stands for a stone hitting a module square on, with no wind.

The first three columns are the standard's own; I worked out the last one. The table is the one IEC 61215 printed in 2005, which I read in its Indian adoption, IS 14286. The current edition runs the test as MQT 17 in IEC 61215-2, and IEC TS 63397 describes it as "spherical ice balls of a minimum diameter of 25 mm, or optionally larger". Kiwa PVEL's hail sequence fires its 35, 45 and 55 mm balls at the same speeds today.

25 mm is the standard size. Larger balls exist in the table "for special environments", and a manufacturer can choose to pass one, but nothing obliges it to. In the 2005 edition the ball hits eleven places: a corner of the glass, an edge, the edges of cells near an electrical joint, the narrowest gaps between cells, the points where the module is clamped, points far from all of those, and one point the lab expects to be weak. Errors of up to 10 mm from the aimed spot are allowed.

To pass under the current edition, as IEC TS 63397 quotes it, the module needs no open circuit during the test, no major visual defect afterwards, a wet leakage current within limits and its power within the standard's loss limit. The 2005 edition put the hail test's power limit at 5 %. Cracked cells that don't cost that much power yet are not a fail.

Our catalogue holds which standards a datasheet names, and 72 % of the modules whose datasheet lists any certification name IEC 61215. The catalogue doesn't record which ball size a module passed, so it has no figure for that. If a datasheet doesn't name a size, assume 25 mm.

Bigger stones hit far harder

Hailstone energy grows much faster than its diameter. A ball twice as wide holds eight times the ice, and it also falls faster, so a 45 mm stone, under twice the diameter of the test ball, carries over ten times its energy. Move the diameter and watch the bar.

The jump is the whole point of this article. A module that passes at 25 mm has shown it survives about 2 J in eleven places. That says little about 20 J, and a storm that drops golf-ball hail drops it on every module of the array, not on eleven chosen spots.

Isolated hits are not the full story either way. Sandia National Laboratories fired standard ice balls at modules and, presenting the work in a webinar for DuraMAT, a US Department of Energy research consortium, reported that "most modules will survive 55 mm standardized ice impacts at normal incidence over homogeneous glass area". The failures that keep a module's qualified size down came from particular places: the glass edge, where cracks start, and the area over the junction box. Sandia also found that a cell which already had a crack saw that crack grow with every later hit, even hits away from it. Kiwa PVEL, which tests modules for buyers, writes the same from the field: modules without broken glass may have extreme cell cracking.

Glass in the catalogue

Our catalogue holds the front glass thickness for 12,780 of 14,005 modules. Two thicknesses stand for the two builds:

The rest are other thicknesses. A front sheet as thin as 2.0 mm comes with glass behind the cells too, since the thickness a datasheet states is one sheet's. By that reading, 50 % of modules stating a thickness are glass-glass. That is 6,395 modules against 6,332 with a single sheet of 3.0 mm or more. A bifacial module needs a clear back, which a second sheet of glass gives it, and the catalogue has 7,743 bifacial modules against 6,262 monofacial ones.

So does thicker glass help? Not in any way a datasheet shows. A single 3.2 mm sheet is thicker than either sheet of a 2+2 module, but the glass-glass module has two, with the cells between them. The standard doesn't rank one build over the other. Both pass the same 25 mm test to be sold. Sandia's tests point the same way: cells rarely cracked without the glass breaking too, in either build, and the quality of the cells and the lamination looked more important than the packaging.

Two figures the datasheets do state can be held group against group:

The catalogue-wide median front load is 5,400 Pa. If the two groups show the same front load, the thicker single sheet buys no higher load rating on paper. If one group has a longer product warranty, that is a term the manufacturer chose, and it is unlikely to matter for hail: a product warranty covers how the module was made, not what hits it. The warranty article explains what counts as a defect and what does not.

There is one difference the catalogue can't see. Kiwa PVEL fires smaller ice balls (35, 40 and 45 mm) at modules whose glass isn't fully tempered, and larger ones (45, 50 and 55 mm) at fully tempered glass. Its example of the first kind is a 2.0 mm glass-glass module; its examples of the second are 3.2 mm glass with a backsheet and 2.5 mm glass-glass. Our catalogue doesn't record which kind a module has, and a datasheet that says only "glass" leaves it open, so ask.

Hail classes beyond IEC

Two documents go where IEC 61215 stops.

The Swiss hail register. Switzerland's cantonal building insurers, through their association VKF, run a register of building products tested against hail, roof tiles, blinds and facade plaster as well as solar modules. A product gets a hail resistance class, HW 1 to HW 5, for the largest stone it took without losing its function, which for a module means without broken glass:

The register works from energy, with its own ice density and drag, so its masses and speeds differ a little from the IEC table. HW 3 already sits above the IEC test: 3.5 J or more against 2 J.

The register's rule for modules is the closest thing to an official answer to the glass question. A module with one sheet of fully tempered glass at least 3 mm thick, or a framed glass-glass module with two fully tempered sheets of at least 2 mm each, that has passed IEC 61215, gets HW 3 on request without a further test. Both builds in the catalogue table above meet that thickness, but the rule asks for fully tempered glass, and Kiwa PVEL's example of glass that isn't is the 2.0 mm glass-glass module. A 2+2 module qualifies only if its maker can show both sheets are fully tempered and the module is framed. Everything else has to be tested, and HW 4 and HW 5 always need a test, whatever the glass. An entry lasts five years, and a thinner glass means a new assessment.

IEC TS 63397. Published in 2022, this technical specification is for modules going where hail is a known risk. It changes the order. The module spends 200 hours in damp heat, takes the hail, then goes through cyclic mechanical load, thermal cycling and humidity freeze, and its power loss is compared with modules that went through the same sequence without the hail. Electroluminescence images go in the report too. That is the test for the hidden damage Sandia describes: cracks that open later, under wind, heat and frost. The specification is not part of IEC 61215, and the IEC doesn't plan to add it there, since not every module goes where hail is heavy.

What to ask for in a hail region

If your area gets large hail, look past the glass to the test behind it.

1. Ask which ball size the module passed. A test report or certificate names it. 35 mm or 45 mm on the IEC table is a real step above the floor, 25 mm is the floor.
2. Look for a tested class. In Switzerland that's HW 4 or HW 5 in the hail register, the classes no module gets without a hail test. Elsewhere, a report to IEC TS 63397 or a manufacturer's own larger-ball test is the nearest equivalent.
3. Ask what glass it is. Thickness, and whether it is fully tempered or heat-strengthened. Our catalogue holds the first for most modules and not the second.
4. Check your insurance first. Ask what it covers and whether it asks for a hail class. Hail is weather, so the product warranty is unlikely to pay for it; the product and performance warranty article explains what that warranty does cover.
5. Have it checked after a big storm. A cracked cell under unbroken glass shows only on an electroluminescence image or, later, as lost power. The datasheet guide shows where the certification and glass lines sit on the sheet.

Outside hail regions, the 25 mm floor is enough, and neither glass build is a reason to pay more. Compare two candidate modules on the rest of the sheet in the panel comparison calculator.

What this can't tell you

The catalogue records the glass thickness and load ratings a datasheet prints, not how a module behaves when hit. It holds no hail test results and no ball size. The energies above come from the standards' tables and assume a round stone hitting square on. Real hail is lumpy, can have sharp points, falls at an angle with the wind, and varies in density far more than lab ice, so the table is a scale for comparing tests, not a forecast of damage.