What snow and wind load rating does a solar panel need?

In short: the two pascal figures on a datasheet are test loads, what the module survived in the lab, and the load you may put on it is the test load divided by 1.5. The median module in our catalogue states 5,400 Pa on the front and 2,400 Pa on the rear. Whether that's enough depends on your roof's snow and wind, which you can turn into pascals yourself, and on the clamps: one manufacturer's manual rates the same module at less than half its front figure when it's clamped on the short sides.

A module on a roof carries two kinds of push. Snow presses on the glass, and wind does too when it blows at the roof, but wind passing over a roof also sucks the module upwards, away from the rails. Datasheets answer both with one number each, in pascals: newtons per square metre of glass. This article explains where those numbers come from, how to turn the snow and wind at your house into the same unit, and why the clamps matter as much as the module.

Two numbers on every datasheet

Look for a line called mechanical load, static load or maximum load. It usually prints two figures, a positive one for the front and a negative one for the rear, like +5400 Pa / -2400 Pa. The mechanical load glossary entry has the vocabulary; the datasheet guide shows where the line sits on the sheet.

The front figure is the one people call the snow load. It's a pressure on the glass, pushing the module towards the roof. The rear figure is the wind load in the sense that matters most: suction that tries to lift the module off its rails. Wind can push on the front too, but snow usually decides that side.

A pascal is small. A kilopascal (1,000 Pa) is about the weight of 100 kg spread over a square metre, so a module rated 5,400 Pa on the front survived the equivalent of roughly 550 kg on each square metre of glass in the lab. That last part, in the lab, is the one to hold on to.

Test load is not design load

The figure on the datasheet is normally the load the module was tested at, and the test has a safety factor built into it. LONGi's installation manual says it in one line: the load it describes "is for the test load", and "on the basis of IEC 61215-2021 installation requirements, when computing the corresponding maximum design load, a safety factor of 1.5 need to be considered". In its own shorthand: test load = design load × 1.5.

The test itself is the static mechanical load test of IEC 61215, MQT 16 in part 2 of the 2021 edition. The module is mounted the way its manual says and loaded on the front and on the rear; the pass criteria are the standard's, not the manufacturer's.

So the number to compare with your roof is the test load divided by 1.5:

A few datasheets print the design load instead, or both. If the sheet says "design load" next to the figure, don't divide it again. If it doesn't say, the manual's mechanical section will.

What the catalogue states

12,969 of the 14,005 modules in our catalogue state a front figure, and 12,678 a rear one. Read the percentile columns next to the median: where they match it, most modules print the same number and the rating hardly separates one from another.

11,098 modules state exactly 5,400 Pa on the front, and 10,163 exactly 2,400 Pa on the rear. Those are test levels manufacturers pick, not measurements of a breaking point: a module that passes at 5,400 Pa is not shown to fail at 5,500. So between two modules printing the same pair, the datasheet tells you nothing about which is stronger.

Compare the rear's 90th percentile with its median. If it sits higher, at least one module in ten is tested harder on the back than the common level. A higher rear rating is worth having on a windy site, and the worked example below shows why the rear can be the tighter side.

766 modules state a lower figure for the front than for the rear. That can be real, but I'd read the original sheet before trusting it: the two columns are easy to swap.

From the snow on the ground to pascals on the module

Snow loads on roofs in Europe are worked out with EN 1991-1-3, part 1-3 of Eurocode 1. It starts from the characteristic snow load on the ground, sk, in kN/m², which each country publishes in its national annex, usually as a map by zone and altitude. Denmark's annex, for example, sets one value for the whole country, sk = 1.0 kN/m². Your building authority or your installer can tell you yours.

The ground value then becomes a roof value: s = μ1 × Ce × Ct × sk. Ce (exposure) and Ct (thermal) are 1 on most houses; the JRC's worked example of the second-generation code uses exactly that, with μ1 = 0.8 on a flat roof. μ1, the shape coefficient, carries the roof's pitch, because snow slides off steep roofs:

• up to 30°: μ1 = 0.8
• from 30° to 60°: μ1 = 0.8 × (60 - pitch) / 30, falling in a straight line
• 60° and steeper: μ1 = 0

1 kN/m² is 1,000 Pa, so the roof value is already in the datasheet's unit. One more step: building codes put their own safety factor on the load as well, and the figure below uses 1.5, the usual factor on snow and wind in the Eurocodes. The comparison is then the factored load against the module's design load.

A worked example, with example numbers rather than any real site's:

• ground snow load sk = 1.5 kN/m², roof pitch 30°
• snow on the roof: 0.8 × 1.5 = 1.2 kN/m², or 1,200 Pa
• with the load factor: 1,200 × 1.5 = 1,800 Pa
• module tested at 5,400 Pa: design load 5,400 / 1.5 = 3,600 Pa, twice what the roof asks

The rear works the same way with the wind uplift your mounting supplier's calculation gives for the module's place on the roof. Say 1,000 Pa: × 1.5 is 1,500 Pa, against 2,400 / 1.5 = 1,600 Pa for a module tested at 2,400 Pa. That passes by less than a tenth. In this example the rear, not the snow, is the tighter side.

The figure below runs the same sums. The module starts at the catalogue's median test loads; the snow, the pitch and the wind are example inputs, so put in your own.

The sums leave out what the code adds for real roofs: drifts against a parapet or a dormer, snow piling where a steep roof meets a flat one, and stronger suction at the roof's edges and corners. National annexes can add their own cases on top: Denmark's has a drifted one for exposed pitched roofs, with no snow on the windward side and up to 1.2 times the ground snow load on the leeward side, half as much again as the 0.8 above. They also ignore that snow on a pitched roof presses partly along the module rather than straight into it, which makes the simple version err on the safe side. Treat it as a first check, not as the structural calculation.

Where the clamps go changes the rating

The datasheet figure belongs to one way of mounting the module, and the manual says which. LONGi's V19 manual prints a table for each module and each mounting method. For the LR5-54HPH, a 54-cell module:

The same module, clamped at the corners of its short sides, is rated less than half on the front. Run the worked example again with those figures: 2,400 / 1.5 = 1,600 Pa against 1,800 Pa of snow, and 1,800 / 1.5 = 1,200 Pa against 1,500 Pa of uplift. It fails on both faces, on a roof the same module carries easily when clamped on its long sides. The button for short-side clamps in the figure above does exactly that.

Larger modules do worse in the same manual. The 66-cell LR5-66HTH gets +1800 / -1400 Pa with rails across the short frame, and for the 72-cell LR5-72HTHF and LR7-72HTHF both short-side methods are marked "Not Allow". A longer module has more glass between supports on the short sides, so the same pressure bends it further. The module size article covers how much larger they've become.

The manual also sets the clamp itself: LONGi recommends clamps at least 50 mm long, requires them to overlap the frame by 10 to 12 mm, and says they must never touch the glass. And for snowy places it adds one line that has nothing to do with pascals: "adjust the height of the mounting system so that the lower edge of the module is not covered by snow". The manual doesn't give the reason; the plain reading is that snow should be able to slide off the bottom row rather than build up against it.

How the figures were read

Every catalogue figure here is what a manufacturer printed on a datasheet, read as printed, in pascals. Values below 1,000 or above 20,000 Pa are left out as extraction slips. The catalogue doesn't record whether a sheet calls its figure a test load or a design load, so a few modules in the table may carry a design load where most carry a test load. Counts are modules, not datasheets: one sheet usually covers a ladder of power ratings, and each rating is its own row. The full rules are under the article.

What to check when you buy

1. Find your ground snow load sk in your country's national annex, or ask the installer. Multiply by 0.8 for a roof up to 30°, less for a steeper one, and by 1.5 for the code's safety factor.
2. Ask the mounting supplier for the wind uplift at the modules' positions, edges and corners included, and multiply that by 1.5 too.
3. Divide the module's front and rear test loads by 1.5 and check both are above those two numbers. Check the rear even if snow is your worry.
4. Open the installation manual, not just the datasheet, and find the rating for the mounting your installer plans. If the rails run across the short side or the clamps sit at the corners, use that row.
5. Between two modules that print the same pair of figures, the load rating doesn't pick a winner. If one prints a higher rear figure and your site is windy, that one is worth a closer look in the panel comparison calculator.

Your installer or a structural engineer signs off the real calculation, with the roof structure under the modules as well, which this article doesn't touch. What you can do before that is make sure the module and the mounting you're quoted are rated for the numbers they'll use.