Snow on Solar Panels

Snow on solar panels is an energy loss and a structural load, and the two are not solved by the same advice. NREL’s snow model, documented for use in the System Advisor Model, treats sliding as the dominant way snow leaves a tilted module. Melting and wind removal are not in the model, aside from a noted exception for flat fixed-tilt. New snowfall is assumed to cover the array. That is a model of coverage, not an instruction to climb a roof with a broom.

Applied across weather files, the same work found fixed-tilt snow losses from about 0 to 16 percent of annual energy in the continental United States, for the tilt-equals-latitude and 20-degree cases that were mapped. Some Alaska sites in that study came out as high as about 40 percent. A national range that wide is the result. A single “snow cuts solar by 10 percent” is not.

How snow on solar panels removes itself in the model

Sliding depends on tilt. A steeper module sheds earlier in the model than a low-tilt module. A flat roof array is the case the model has to treat differently, because there is no slope to slide on. This is one reason a low-tilt commercial roof in a snow climate is not the same plant as a latitude-tilt ground mount a few miles away. The mount choice is discussed in roof-mounted versus ground-mounted solar. The snow model does not make the roof safer to walk on. It makes the annual energy number more honest if someone includes it.

The model is not a melting calculation. If your modules clear because the dark surface heats and the snow lets go at the interface, that mechanism may be real on the roof and still absent from the model. Do not “correct” a proposal by deleting the snow loss because you once saw a panel clear at noon. Do not add a second loss on top without knowing whether the model already counted coverage. Ask which weather file and which tilt were used. If snow was omitted, the continental range above is the scale of what was omitted, not a number to paste in blindly. Alaska is not Ohio. A 20-degree roof is not a 40-degree ground row.

Bifacial modules see a second effect. Snow on the ground can raise the reflectivity the rear side uses, while snow on the front blocks the front. NREL’s bifacial field material describes spectral differences between snow and grass. Those are not the annual energy losses from the snow model, and they should not be added to the 0 to 16 percent band. Rear-side design is in bifacial solar panels.

What snow on solar panels does to the structure

Coverage loss ends when the snow slides or is removed. Load does not care about the kilowatt-hours. The module, the clamps, and the roof or the piers have to be rated for the snow the site actually gets. A datasheet mechanical load is the number. A procurement spec that says framing should resist snow, wind, and hail, as one Xcel-related technical specification does, is a buying sentence. It is not a hailstone diameter and it is not your local ground-snow load. The engineer of record uses the local load. This page will not invent one.

Sliding snow leaves the module and lands somewhere. On a roof that somewhere may be a walkway, a gutter, or a lower roof. Pacific Northwest National Laboratory’s carport and roof notes in the federal installation guidance call out gutters, snow, and icicles as design items, including a Y-shaped layout where snow has to be kept off a path. A ground mount dumps snow into the aisle you meant to walk. Leave the aisle. Do not treat the modeled slide as permission to stand under the drip line.

Access in snow is worse than access in July. PNNL ties maintenance cost to how hard the array is to reach. A roof that needs a lift on a dry day needs a different plan when the surface is ice. The electrical equipment does not become safe because production is down. Modules that are partly clear are still live. The shutdown order remains the AC disconnect, then the DC disconnect, by someone trained. Brushing snow off a live, icy module with a metal tool is how a maintenance idea becomes an incident. This article does not provide a clearing procedure.

Snow on solar panels versus a fault

A monitoring drop in a snow week is first snow. A drop that remains after the modules are clear, in sun, outside the ordinary weather band PNNL describes of about plus or minus 10 percent around average insolation, is when you look for a real fault. Inverters, fuses, and shade do not take the winter off. Shade from a tree is still shade when the snow is gone. That diagnosis is in how shade affects PV output. A performance warranty does not pay for snow cover. It pays, if it pays, when a qualifying test shows the module itself below the contract floor. File the weather with the production record so the two are not confused. Warranty reading is in what a solar panel warranty covers.

If you are estimating a year, include a snow loss that matches the tilt and the climate file, and say so. If you are standing on the site in January, do not clear modules unless the people, the method, and the warranty allow it. Production lost to a safe delay is smaller than the cost of a fall or a cracked laminate. The model’s job is the annual number. Your job in a storm is to leave the array alone.

Frequently asked questions

Should snow on solar panels always be removed?

No. The NREL model expects tilted modules to shed by sliding, and it does not instruct anyone to climb. Removal is a safety and warranty decision. A low-tilt array sheds less. That is a reason to account for the energy, not a reason to send an untrained person onto ice.

How much energy does snow on solar panels cost?

In the national fixed-tilt study, about 0 to 16 percent of annual energy across the continental United States for the tilts that were mapped, and as high as about 40 percent at some Alaska sites in that same study. Your file can sit anywhere in a range that wide. Do not use 10 percent as a default.

Does snow on the ground help bifacial solar panels?

It can raise rear-side reflectivity while snow on the module blocks the front. Those are different surfaces. Do not add a spectral snow-versus-grass note to the annual snow-loss percentage. Model the front coverage and the rear albedo separately.

Sources and references

Sliding as the modeled removal, the flat-tilt exception, full cover on new snowfall, and the 0 to 16 percent continental range with Alaska sites as high as about 40 percent: NREL, snow-coverage energy loss in SAM. Spectral snow and grass notes, which are not those energy losses: NREL bifacial field slides. Insolation band and access: Pacific Northwest National Laboratory. Gutters, icicles, and snow paths at installation: US Department of Energy, FEMP installation guidance. Framing expected to resist snow, wind, and hail in one buyer’s specification, without a hail diameter: PSCo all-source technical specifications for solar.