Solar panels and snow: what happens in winter

Snow is the first worry most Canadians raise about solar. It is a fair question. A panel buried under snow makes almost nothing. But snow covers panels for fewer days than people expect, and it falls in the months when panels produce the least anyway. This guide explains what snow, ice, hail and wind actually do to a rooftop system, and what you should and shouldn't do about them.

How snow behaves on panels

Solar panels are smooth tempered glass, usually mounted at the same angle as your roof. That combination helps snow leave on its own.

• Sliding. On a typical pitched roof, snow often slides off within a day or two of a storm, especially once the sun comes out.
• Melting from the bottom. Panels are dark. Even a thin layer of snow lets some light through, which warms the glass and loosens the snow from underneath.
• Light snow blows off. Dry, powdery snow often clears with the wind.

Flat roofs and low-pitch arrays hold snow longer. If your panels will sit at a shallow angle, ask your installer to factor snow cover into the production estimate.

Snow on the ground can also help. When panels are clear, sunlight reflecting off a white yard adds a little extra light.

Should you clear snow off your panels

In almost every case, no. Leave it.

• Safety. Climbing on a snowy or icy roof is one of the most dangerous things a homeowner can do. The output you would gain on a short December day is worth a few dollars at most.
• Damage. Shovels, scrapers and ice picks can scratch or crack the glass and damage the frame. Hard ice is especially risky to chip at.
• Warranty. Damage caused by tools is usually not covered by the panel manufacturer's warranty. Check your warranty terms before touching the array.

If you really want to clear a ground-mounted array or panels you can reach safely from the ground, use only a soft foam roof rake made for the purpose and never try to remove ice. For roof arrays, call your installer if you are worried.

Ice dams and sliding snow

Snow that slides off panels can land in a big pile. Plan for it.

• Check where snow will fall. It should not dump onto a front step, walkway, driveway, gas meter or vent.
• Snow guards (small brackets or bars below the array) can hold snow back or break it up so it falls gradually. They are worth asking about above entrances.
• Panels don't usually cause ice dams. Ice dams form when heat escaping from the house melts snow that refreezes at the eaves. If your roof already gets ice dams, fix the attic insulation and air sealing first. Our guide on replacing your roof before solar covers roof condition.

Hail ratings

Panels are tested for hail before they can be sold. The international test, IEC 61215, fires 25 mm (about 1 inch) ice balls at 23 metres per second at 11 points on the panel, and the panel must pass without significant damage or power loss. In Canada, panels also need a safety certification, such as CSA C22.2 No. 61730.

Some manufacturers test with larger ice balls. If you live in a hail-prone area, such as parts of Alberta, look for panels tested beyond the minimum and ask the installer about it. Severe hail can still break panels, which is one reason to tell your insurer about your system.

Snow and wind loads

Heavy wet snow and strong winds put real force on panels and racking.

• Panel datasheets list a tested front load (snow pushing down) and rear load (wind lifting up), measured in pascals. Many common panels list front loads around 5,400 Pa and rear loads around 2,400 Pa, but values vary, so check the datasheet.
• The racking and roof attachments are designed for your local snow and wind loads under the building code. Your installer or an engineer should confirm the roof structure can carry the extra weight. This is part of the permit process.

What winter losses really mean

Here is where many people overestimate the problem. In our Toronto sample system (7.3 kW), December, January and February together produce about 600 kWh, roughly 8% of the year's output. Even if snow wiped out half of that, you would lose about 4% of annual production.

Research from the US National Renewable Energy Laboratory found that annual snow losses for fixed-tilt systems ranged from about 0% to 16% across the continental United States, with the highest losses in snowy states such as Minnesota and Michigan and steeper panels shedding snow faster. Your number depends on your local climate and roof angle.

With net metering, short winter losses matter even less. Summer surplus builds up as credits, and in Ontario those credits carry forward for up to 12 months. That bank covers part of your winter bill whether or not the panels were snowy that week. If your system takes Ontario's HRSP rebate instead and can't join net metering, winter output matters a little more, because there are no banked credits to draw on. See net metering vs the rebate.

Practical winter checklist

• Expect very low or zero output for a few days after a storm. That is normal.
• Don't go on the roof. Don't use tools on the glass.
• Watch where sliding snow lands and ask about snow guards if it is a hazard.
• Judge your system on a full year of output, not on a snowy week. Our guide on solar in Canadian winters explains why.

Next step

See how much your roof would produce month by month, winter included: check your roof.

Common questions

Do solar panels work when covered in snow?

Panels fully covered in snow produce little or nothing. Snow usually slides or melts off within a few days on a pitched roof, and winter is a small share of annual output anyway.

Should I clear snow off my solar panels?

Generally no. Climbing on a snowy roof is dangerous, tools can damage the glass, and tool damage is usually not covered by the warranty. Let the snow clear on its own.

Can hail break solar panels?

Panels must pass a standard hail test with 25 mm ice balls fired at 23 metres per second. Larger hail can still break them, so tell your insurer about your system.

How much output do I lose to snow in a Canadian winter?

It depends on your climate and roof angle. In our Toronto sample, December to February is about 8% of annual output, so even heavy snow cover costs only a few percent of the year.

Sources

• NREL: Integration, Validation, and Application of a PV Snow Coverage Model in SAM (2017)
• Sinovoltaics: IEC 61215 MQT 17 hail test explained
• Sinovoltaics: Mechanical load test for PV modules
• Standards Council of Canada: CSA C22.2 No. 61730-2:19, PV module safety qualification