Many of our customers have cottages, some are used throughout the winter, some are left to their own devices. Both cases present a challenge when there is an off-grid electrical system: Panels get covered in snow, and therefore don’t contribute to charging the batteries. At the same time, the Achilles heel of lead-acid batteries is that they do not like sitting at partial charge for long periods of time. What can we do about this?
It is surprising how little snow cover it takes to bring solar panel output to a grinding halt. Just a dusting of snow and there’s zero output! One solution is to put solar panels up at a steep angle, making it more likely for snow to slide off, and we have been selling (and in the past also installing) FabRack racking units that allow for the tilt-angle to be changed to 60 degrees in winter. At that angle snow does indeed slide off pretty well. It is the times where there is just a bit of snowfall, enough to blanked the panels, but not enough to get to the weight needed to slide down, that manual clearing is still needed.
A solution I have been seeing used by some cottage owners is to put some (or all) panels up at a full 90-degree tilt-angle. Vertical in other words! That solution does indeed work very well: Snow is much less likely to settle on the panels to begin with. In case you wonder, even at this angle there are times when snow clings to the panels (and for some reason in particular to the bottom, which is an argument to mount panels in landscape rather than portrait, so they still produce 2/3 of what they would do when clean, whereas in portrait the output would be zero when the bottom is covered). It is rare though, and panels will be snow-free for most of the time through winter.
What comes to mind is how the tilt-angle affects energy yield. In particular at other times of the year. I thought it would be helpful to put some numbers to this. One thing to keep in mind is that the steeper the tilt-angle of the panels, the more it matters that they actually face true south. So, for south-facing panels with no other sources of shading, what is the effect of tilt-angle on energy yield throughout the year? Here are some numbers for our location (at 45 degrees latitude):
What does this show?
All the numbers are relative to each other; I simply declared the highest energy yield to “100%” and related all the other numbers to it. The table shows that 30-degree tilt-angle in July produces the most energy, so that is 100%. By the way, for us the best year-around tilt-angle for energy yield is right around 40 degrees. Of course, that is when the snow is kept off the panels in winter, and this is not the best angle for winter yield when off-grid use is most desperate.
What the table shows is that at 60 degrees the panels do very well in winter (if they are kept clean)! You might even be tempted to just leave them at that angle, in summer you still get 80% of the best yield, and usually summer has energy coming out your off-grid ears anyway.
So how does 90 degrees tilt-angle fare? Mid-winter energy yield is very good, right around the maximum that can be made. Snow won’t be a problem at that angle, so that is not a bad way to mount panels! Pretty decent early-spring (and fall) yield too, but summer is not so good, bringing in just about the same energy as in winter. It could be that’s enough for your needs, though usually things are pretty tight in winter and off-grid life gets better when more energy comes in from spring through fall.
One possibility could be to mount some panels vertically, with their own charge controller, to improve winter energy yield. In particular for a cottage that is not used in winter this can work to keep the batteries charged; even a single panel with a little charge controller would go a long way.
Hopefully this article helped put some numbers to the problem and provide food for thought. A very good source of solar yield information is the US government Web site PV-Watts. It uses actual weather data and works for any location on earth. It provides monthly insolation values based on location, panel azimuth, and tilt-angle. Those insolation numbers are representative of energy yield, making it easy to compare various scenarios.
If you have any questions, or need an off-grid system designed for your particular case, feel free to contact us!