Snow Shedding and Photovoltaics

When Alex and I built our net-zero-energy house in Seattle, we didn't have to worry much about snow. Seattle gets just under 6 inches of snow per winter. The Methow Valley couldn't be more different. Unlike mild Seattle, it is a place of extremes: extremely hot in the winter, very cold in the winter, and lots of snow—56 inches average per year.

With that in mind, we've come to be very conscious about our roof pitches. When mounting solar panels, this is always a concern, since there is a latitude-specific angle that theoretically captures the most usable solar energy. Dave calculated that at our latitude, a fairly steep roof (35–40 degrees) is ideal.

Solar Production at Various Angles of Tilt

We had originally hoped to have what's called a clerestory roof, which is like a gable with the roof sections offset at the peak. One of the original designs we considered had this roofline, which had a relatively shallow slope on the southernmost section of the roof, which is where we would mount our solar panels. We realized that this would slightly reduce our annual solar production, but we were willing to live with that in exchange for the modern aesthetic of the clerestory roof.

 An option with a clerestory roof. Notice the shallow slope.

An option with a clerestory roof. Notice the shallow slope.

Our thinking changed, however, when we started considering shading from snow. As we dug a little deeper into the topic, we realized that at that shallow roof angle, snow would likely pile up on our panels and bring our production down to almost zero for large parts of the winter. And because this house would be a nightly rental, we couldn't count on anyone to try to clear snow from the panels during the winter.

After doing a little research, we found that others had studied the snow-shading problem and confirmed our worries. Renewable Energy World summarizes a 2012 study on snow losses:

Depending on tilt angle, wintertime energy losses of 40%-60% and annual energy losses from 12%-18% were noted in the first year of operation. . . . The lost energy due to snow buildup in the seven-month winter season ranged from as little as 25% for the 39° tilt to as much as 42% for the flat orientation. The seasonal results project to losses in annual output of 12%, 15%, and 18% for the 39°, 24°, and 0° tilts, respectively.
— Renewable Energy World

Winthrop-based photovoltaic vendor Ellen Lamimam further reinforced our concerns that a shallow roof angle could have a big negative effect on our annual production. We asked her what pitch we needed for snow to reliably shed from our panels, and she provided some very useful information specific to Winthrop.

A 6/12 roof is the minimum slope I recommend. At 27 degrees it is optimum for May, June, and July energy production, but not a good angle for winter energy production, and the snow can hang up longer especially if the top of the solar array is not quite close to the ridge line. A 9/12 or 10/12 roof would be better and is the slope most used for grid-tied only systems. A 12/12 roof is optimum for snow shedding and year-round energy production (meaning you get more production in the winter, but the total for the year will be a little less than for a 9/12 slope).
— Ellen Lamimam

To translate what she said into degrees, a 9/12 roof is about 34 degrees, and a 12/12 roof is 45 degrees.

With these slope considerations in mind, we estimated that the best angle for our south-facing roof would be about 35 or 40 degrees, which would give us good year-round production and shed snow. That took us back to the drawing board and led us to choose a more traditional gabled roof over the more modern clerestory.

 In this section drawing, the roof on the right is facing south. We are still exploring various roof profiles to shed snow, minimize building costs, and maximize solar production.

In this section drawing, the roof on the right is facing south. We are still exploring various roof profiles to shed snow, minimize building costs, and maximize solar production.

If snow still doesn't shed off of our roof, we may have to encourage our guests to try the old trick of bouncing a tennis ball off of our panels to start a mini-avalanche.