60psf snow load on panels?

machineman
machineman Solar Expert Posts: 129 ✭✭✭
in Solar Beginners Corner #1
I'm working out the moutning rack for Evergreen ES-A 200w panels (65" x 37.5"). They will be portait and side by side roof mounted using non-tilted unirac rail system. The question is, are the solar panels strong enough to only be supported by 2 rails if my area has a 60lb snow load rating? Also what is the optimum rail spacing, if using 2 rails do they go a quarter (1/4 of 65") panel distance from the edge, meaning rail at 16.25" from the top and the other rail 16.25" from the bottom. I'm using top mount clamps.

Thanks.

Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
    Re: 60psf snow load on panels?

    I have no answer to your real question,, but it begs one of my own.

    60 psf snow load is huge. I understand the need if you are not going to clear the snow over a significant time,, but I can't imagine if you are using a system, that you wouldn't sweep them after most snow falls.

    That said,, unless you live in the very high Cascades or Sierras where you get huge wet snows,, I can't imagine a case where you might get 60 psf in one event,,, though I suppose it could happen. I can't remember my tables by 60 psf is a lot of snow,,, even if it is saturated with rain.

    Tony
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #3
    Re: 60psf snow load on panels?

    http://www.evergreensolar.com/upload/010908%20WEB%20LITERATURE/Mounting%20Guides/MG-ES-A_US_010908_ETL_FINAL%5B1%5D.pdf

    I mount my panels using end mounting with rails parallel with the long length of the panel. Your suggesting offset mounting so you will only be supporting the panels a 4 contact points.

    The above document says you can do offset for a COMBINED load of wind/snow of 60lb/sf ... so I would say no, you can't do 60sf of just snow as you have to factor in high winds as well

    By the way, that's about 1 cubic foot of water that would be standing on the panel per square foot ...
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: 60psf snow load on panels?

    That pesky Florida snow,,, has such a high water content too!

    Tony
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,746 ✭✭✭✭✭✭
    #5
    Re: 60psf snow load on panels?

    The good news is that if the location of the panel allows somewhere for the snow to gravitate to, it would be extremely rare that the snow could build up and not be blown down if high winds were to be a factor. Snow does drift! Give it someplace to go!

    If the roof has places where the snow can be trapped or your mounting angle is not high enough you might consider a pole mount or tracked array. It seems to be a waste of time and dangerous to be clearing snow from roof panels.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #6
    Re: 60psf snow load on panels?
    icarus wrote: »
    That pesky Florida snow,,, has such a high water content too!

    Tony

    No, we have to design to 125mph wind loads ... But I lived half my life in MA/NH/VT so I'm pretty aware of the cold white stuff

    As for the water content, I'd say even in the biggest down pours, maybe a 1/4" of water is on the panels at anytime on my arrays
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  • machineman
    machineman Solar Expert Posts: 129 ✭✭✭
    #7
    Re: 60psf snow load on panels?

    My place is in the southern cascades but only at 3500ft elevation. My property line boerders the 40psf and 60psf boundary set by the building department. However it likely will never see >40Psf snow load. The most snow I've seen standing on the roof (4:12 pitch) at one time is 2ft. However in the winter I'm not there all the time so nobody to sweep it off. Oh and it can also get windy on occasion.

    Thats why I was consdiering 3 rails instead of 2. Maybe I should consider adding some more tilt other than the roof pitch alone. A 4:12 pitch is only 15deg and my optimum position would be more like 40deg.

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
    Re: 60psf snow load on panels?

    Remember if you add tilt,,and you want the panels to shed snow,,, there has to be some place for the snow to slide to. 40 degrees, while it will still allow snow to accumulate, will shed fairly quickly once the sun comes back and begins to heat the panels. Once you get a film of water under the snow,, they'll slide.

    Tony
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #9
    Re: 60psf snow load on panels?

    And if you tilt, make sure one panel doesn't dump the snow under the lower panel .. this will ice dam and destroy the lower panel
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: 60psf snow load on panels?

    Check your winter time panel angle:
    http://www.macslab.com/optsolar.html

    Up here where I am the snow doesn't stick because the panels are so near vertical (75 deg).
    And yes, be sure there's someplace for the snow to slide to.
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  • machineman
    machineman Solar Expert Posts: 129 ✭✭✭
    #11
    Re: 60psf snow load on panels?
    Solar Guppy wrote: »
    And if you tilt, make sure one panel doesn't dump the snow under the lower panel .. this will ice dam and destroy the lower panel

    I'm only doing 1 row of 4 panels. So the snow will just slide up against the snow already on the roof.

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #12
    Re: 60psf snow load on panels?

    Ice dams can be an issue with a single row ... if snow can get under the panels, the panels will heat the snow and turn to slush-ice ... repeat the cycle a few times and then panels get broken from the ice expanding from underneath

    If your having snow measured in the feet per week, that's a real concern
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  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    #13
    Re: 60psf snow load on panels?

    90 PSF ground snow load in my town , Gorham NH and the adjacent town Randolph NH. We are in Northern NH right next to Mt Washington.

    Like anything else snow load is not straight forward and structural engineers make their living off the details. Roughly, if the panels are set above the definition of a shallow roof pitch ( I dont have my guide at hand but it somewhere around a 3/12), then the load is applied to the horizontal projection of the roof (or panel) rather than the actual length. So take the cosine of the pitch and multiply by the panel length. Of course if the panel or roof is within a certain distance of a higher roof, than there is usually a 30% increase in the effective snow load due to posibility of snow building up on the lower roof from the upper roof.

    I have adjustible tilt on my panels and snow will still build up at a 60 degree angle (from horizontal), but usually not much more than 6" during a major storm. The biggest hassle is snowblowing in front of the panels so the bottom edge isnt covered with snow. The plus side is that on a cold sunny day, the snow acts to reflect the sun onto the panels and the output goes well over the panels rating.

    My smaller set of panels are mounted at 1/3 and 2/3 the length of the panel. My larger panels are mounted at 1/4 spacing.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #14
    Re: 60psf snow load on panels?

    peakbagger,
    i wouldn't doubt your weather conditions are worse than many of our canadian users here as i've heard stories of mt. washington. i would generally say that what you typically have sliding off of the pv is in addition to what has accumulated below the pv so if you have say a 6" snowfall you wil get 6" accumulated on the surface below the pv and 6" falling off of the pv making it an easy 12" of snow total. granted some will have melted on the pv, but that which is on the surface below the pv is subject to further accumulations and may not have melted from the time before. it's a guessing game, but the more allowance between the pv bottom and the surface below, the better your chance is of having the pv clear. all of that assumes a good angle for snow sliding, which is not a problem for many in the more northern latitudes.
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  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    #15
    Re: 60psf snow load on panels?

    The bottom edge of the panels at the winter angle is 4' above the ground. Snow can and does get deeper than 4', but it was a trade off on what I had around the house for steel, the amount of concrete I wanted to mix, plus some concern about the bending moment with the pivot point so high up given the wind issues that occur on occasion. I get some wind drifting that works to my advantage most of the time

    The picture shows the panel at spring/fall angle (approx 45 degrees).

    The panels are older Siemens M-75's I bought at a garage sale in VT. Price was right!. The "Pole" is actually two rectangular structural tubes. The rack is unistrut. The array is nicely balanced so that I can change the tilt angle quite easily.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #16
    Re: 60psf snow load on panels?

    most times you should be fine and if worse comes to worse there's a shovel or better yet a snowblower.:D
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