Snow on solar panels: Six design considerations


editors-blog-entry3One of the things about going solar is that you learn as you go along. We’re definitely learning a lot about solar panels and snow this winter, our first with our 5.59 kW home solar system, which was installed in June 2010.

There are a number of things about snow on solar panels I would have like to have known before we went solar.

If you’re about to go solar somewhere in northern climate like ours – we live in Aurora, Colorado, here are a few things you might want to consider before, and when, you go solar:

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  • The pitch of your roof. The roof that houses our solar panels faces directly south – which is very good for production. However, it’s got a relatively shallow pitch – 19 degrees. This means snow doesn’t slide off our panels. It sits it until it melts. Our experience this winter has shown us that this melting process can take from three to four days, even in extremely sunny (but also cold) conditions. Our solar company, REC Solar, tells us that the general rule of thumb in terms of roof pitch and snow sliding easily and relatively quickly off your solar system is you need a pitch of 27 degrees or steeper to see snow slide off pretty quickly in sunny conditions. If your roof pitch is shallower than 27 degrees and you get moderate to large amounts of snow where you live, you might want to consider putting your system on tilt bars to increase their pitch (this will also increase the system’s overall production).

  • System design and access to your panels. Our system consists of two 13-panel strings, one on our lower garage roof, one on our upper house roof (see pictures). The lower 13 panel string blocks roof access to the upper string, making it impossible to sweep snow off our upper string, even with a 23-foot Mr. Longarm equipped with a rubber squeegee. It’s possible we could have designed our system so that it would have given us access to our upper, 13-panel string. This would have allowed us to more easily clear this string of snow in the winter, and made it easier to clean in the non-winter months. As you design your system, you may want to consider how your system design affects access to your panels so that you can free them of snow in the winter and clean them in the summer.

  • The system inverter. We have a central inverter (an SMA Sunny Boy 6000) and two strings of 13-panels, one on our upper roof, another on our garage roof (which is attached to our house). One of the advantages of a central inverter system is that it’s generally cheaper to install and build. One of the disadvantages is that with a “stringed”, central-inverter system, you’re locked into a lowest common denominator electricity production situation. This means the lowest producing panel on a string will force all of the rest of the string to produce at its low level. Snow on even a single panel in a string can essentially reduce your string’s production to zero. This means that sweeping your system free of snow won’t always reap immediate results. More on this below.

  • snow-upper-stringIndividual panel production. When we worked with REC Solar to design our system, I fully understood that a string-based, central-inverter based system means that a low-producing panel on a string will bring that whole string down to its level. What I didn’t quite get – but now understand thanks to our experience with snow and snow sweeping – is what happens in terms of an individual solar panel. At the module/panel level, anything that runs horizontally and completely across a vertically set panel will essentially eliminate that entire panel’s production. This means that even a sliver of snow, which might cover only 1 percent of that panel but which cuts all the way across that panel, will essentially shut that panel down! So, basically, a tiny bit of snow that cuts horizontally across a single panel on each of our 13-panel strings will shut down the entire system!

  • Micro-inverters. Micro-inverters are one solution to snow-slogged production you might want to consider if you live in a northern climate and you have a shallow-roof pitch such as ours (again, 27 degrees or higher and the snow will slide off quickly in sunny conditions). Micro-inverters regulate production on a panel-by-panel basis. This means that if one panel in a set of 13 is producing at a lower level because it’s partially covered with snow but the other 12 are free of snow, you will get the full production from each of these 12 panels rather than lose the production of all 13 because one of them is snow-covered. If you decided to sweep snow off your system, micro-inverters clearly reward you much more than a central inverter, stringed system. However, it’s my understanding — and this might be incorrect — that a horizontal line of snow, even if it covers 1 percent of a single panel’s surface area, will still essentially shut down that panel on a micro-inverter based system. Of course, with micro-inverters that panel with the sliver of snow won’t shut down the panel next to it, if that other panel is free of snow.

  • Tigo Energy module maximizers. REC Solar tells me that Tigo’s module maximizers, which seek to mimic micro-inverters on a central inverter based system, could help reduce, or even eliminate, the problem of a small amount of snow cutting across a single panel on a solar string and shutting down production of the entire string when all of the rest of the panels in the string are free of snow.

Are micro-inverters worth it?
A micro-inverter based system and a Tigo outfitted system cost more than a traditional, central inverter, string-based system. Can they make up for the extra cost by producing more electricity on an annual basis? Both Enphase, which is the major producer of micro-inverters, and Tigo Energy, claim their systems will outperform a central-inverter based system by 10, 20 percent or more.

micro-inverterIt’s not clear to me if these estimates take snow cover into account, or if Enphase and/or Tigo have closely examined the issue of snow. That’s something I’ll look at in a future entry. I’ll also shelve a cost-benefit analysis for a future entry.

From a pure production standpoint, it’s becoming readily apparent that a micro-inverter system or perhaps a Tigo outfitted system are definitely worth it in a northern climate with substantial snowfall — if you have a shallow roof pitch and you are into sweeping snow off some, or all, of your system.

That’s because even a relatively small amount of snow sitting the “wrong” way on a single panel on each of our two 13-panel strings brings our entire 5.59 kW system to a grinding halt.

To a certain extent, this makes my sweeping-the-snow-away effort questionable – at least if a quick ramp up to full production following a sweep is the goal, which it is.

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Snow frustrations
Of course, across several sunny but cold days a sweeping effort still pays off. That’s because even if it takes an entire day for the thin slices of snow to melt off a string, as it did this week for us, and that first day sees zero production – which it did, the string ramps up the next day, which, again it did (please see the table below). That same string would have produced absolutely nothing for 1 ½ days this time around if I had not swept it free of snow on the morning of the first sunny day after the snowfall (see table below for more on this).

A micro-inverter system or a Tigo outfitted system would also help increase production even if you don’t sweep snow off. Take our upper, 13-panel string as an example. I cannot reach it, or sweep it. Three completely sunny but also cold days after a six-inch snowfall, our upper string is still partially covered by snow, with the upper panels of the string melting more quickly than the lower panels. In fact, the upper panels on that string were essentially completely free of snow 2 ½ days after the original six-inch snowfall, but the entire string was still not producing anything at all because the lower panels were snow covered, albeit partially.

Sweeping the snow off solar – An ongoing tally of kWh gained & lost

Snowfall date(s)

Snowfall Amount

Date and time of snow sweeping (lower of 2 13-panel strings of 5.59 kW system only)

Estimated kWh gained from sweeping (gain may have been across multiple dates)

Dec. 30-31, 2010

2 ½ inches

Jan. 1, 2011
12:30 p.m.

7.3 kWh

Jan. 9-10, 2011

6 inches

Jan. 10, 2011, 8:30 a.m.

33 kWh

Jan 19, 2011

1 inch

Jan. 20, 8:15 a.m.

6 kWh

Jan. 31, 2011

2 inches

Feb. 1, 9 a.m. (roof too icy; failed to sweep much snow off)

0 kWh (failed snow clearing attempt due to icy roof conditions)

Feb. 5-6, 2011

5 inches

Feb. 6, 9 a.m. (from ladder with 23′ Mr. Longarm)

13 kWh

Feb. 7-8, 2011

6 inches

Feb. 9, 8 a.m. (from ladder with 23′ Mr. Longarm)

29 kWh

(12 kWh of additional production missed on 1st day due to slivers of snow left on on few panels — minuses of central inverter system)

Running total kWh saved as a result of sweeping snow off 13-panel garage roof string

88 kWh

Running total kWh lost as a result of not sweeping snow off 13-panel upper roof string

88 kWh

Running total of additional kWh lost as a result of limitations of central inverter system

20 kWh

Running total of estimated kWh that could have been produced with immediate, 100-percent post-storm clearing of snow from entire 5.59 kW system

196 kWh

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