Roof mount at 45-deg or pole mount at better 60-deg

Grinnin · October 2014

I joined this forum because I'm finally building the "real" house on this property.

I'm thinking of mounting 4kW of PV on the 45-deg roof and just accepting the winter loss of power. PVWatts calculates the hit as 6% in the worst months compared to a 60-deg pole mount. Adding an additional 10% inefficiency for snow increases the hit to 15 to 16% in the worst months. PVWattsIt also adds that 10% loss in July and August so I'm not sure about the weather info it uses.

While a pole mount would be more efficient through the dark months, there's not a very good place to put that much PV. I am not interested in mounting panels at 60-deg on the 45-deg roof because of the snow trap they would become.

I won't be able to easily clean snow off the panels on the roof. With a pole mount it might be possible. The panels I have now are mounted at 60-degrees and an easy reach for cleaning. They WILL shed snow pretty quickly if left alone. The roof-mounted panels will be warmer to shed snow quicker, but not as steep so may shed slower. This will be my first south-facing roof, so I don't really know how fast it will shed snow.

The cost of PV has dropped so much that I'm inclined to just fill the roof. That'd be 4kW to 5kW compared to the 408W I've used for the past several years. I may not need it all immediately, but I'm sure I can grow into it.

I have used a generator for about 8 hours total in 13 years and I have had good battery life with the system. I am not interested in the added hastle of a generator for the new system either.

I COULD find a place to put a smaller second array on a pole mount within reach for cleaning. I'd then choose a charge controller that can handle 2 separate arrays.

This post actually has no questions but I would be interested in your experience with

Snow on 45-deg panels vs 60-deg.
Snow on roof-mounted panels vs ventilated.
Designed-in inefficiencies.
Split arrays.

Thanks.

mtdoc · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

Based on my experience with an array at 45 deg on a shed roof and a pole mounted array whose tilt I vary from 30 to 80 degrees based on the season, I would say you won't see a lot of difference between snow sticking at 45 versus 60 degrees - BUT it really depends on the type of snow you get. Here in the PNW - most of the time the snow is fairy heavy with a high moisture content and more prone to stick. With lighter, dryer snow there would be more difference between 45 and 60. My recollection from my years living in northern Vermont is that snow in your neck of the woods is similar to that here most of the time (unlike the Rockies where the snow is mostly dry and light).

Comparing my 2 arrays when the tilt is the same - I haven't noticed much difference based on roof mounted versus better ventilated pole mounted.

Cariboocoot · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

Panels mounted on a roof do not have to be at the same angle as the roof, nor do they have to be fixed at one angle.

Grinnin · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

mtdoc wrote: »

Here in the PNW - most of the time the snow is fairy heavy with a high moisture content and more prone to stick.

I agree that snow here also has high moisture content. It does vary, but rarely is it a light, dry snow.

Thanks for your comments on 45 vs 60 and roof vs pole.

vtmaps · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

My experience is with pole mount. I have a DPW top of pole mount. The most it tilts is 65° (25° from vertical). I keep it 65° during snow season and I frequently have to scrape off snow or ice.

If I had it to do over, I would have purchased a mount that can go all the way to vertical. The reflection of the sun (off the snow on the ground) makes up for the less-than-optimal angle to the sun.

--vtMaps

jtdiesel65 · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

In VT and I have two pole mount which I adjust ~15 to ~65 degrees. Any reason not to be concerned about summer at a fixed 45 or 60 degrees?

One reason not to do roof mount is heat. Performance goes down when temp goes up.

My experience with snow on panels @ 65 degrees is if it's light, dry and cold out, it usually slides off by 9-10AM and so it's often not worth brushing them off.

If it's a bit warmer but still cold and the snow melts on the main part of the panel, the melt will run down the panel and then refreeze on the edge which has a shadowing effect. Which is why folks mount them in landscape. At 45 degrees, I could see this becoming more of an issue. If it's warmer yet, it won't refreeze and just melt off.

Sometimes, the snow will only partially melt off and big chunks of snow stay on the panels. It's not warm enough to melt enough for the snow to slide off. You really need to brush them at this point, because if you don't those big chunks will refreeze onto the panel and it's harder to remove at that point.

Freezing rain followed by snow usually results in a thick ice crust on the panel topped with some hardish snow. I brush the snow off and wait for the sun to melt off the ice.

I don't scrape my panels. IMO, it's not worth potential damage.

Wind is probably just as much of a concern as anything. If you're roof mount, you need to bolt into the rafters or you may kiss your roof goodbye. Pole mount needs to be sturdy and you may want to be able to go completely horizontal.

I have two arrays and use two midnite charge controllers in "follow me" mode with no issues.

Of course, instead of pole you could make a frame to mount them on the ground.

zoneblue · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

We are in a similar situation. Our temp accomodation had no suitable angled roof, so we went with a wooden ground mount rack, fixed at latitude. When we build the house, the question is the same as yours, we have an ideal oppurtunity to design the array into the the roof. However here latitude is 39 so thats easier to build. If it wasnt for the 6 inch clearance it would be a no brainer. Aesthetically im just no sure its right.

BTW the summer derating is based on high AOI (angle of incidence), and high temp. Even though the Air mass ( suns path through the atmosphere) is less in summer the angle the sun hits the panel is more oblique then during teh equinoxes. High temps also lower output, by lowering the Vmp.

Obviously snow is a factor for you northern folk. Other than that, studying your load pattern v insolation pattern is the usual thing to do. If your load pattern is flat year round, latitiude is best. If you have heavy summer loads (air con) flattening the array some can help with that, because your tilting it toward a strong source. However generally steepening the array to try to improve winter performance is generally futile per se, because with high winter airmass, its a 50% of 50% of 50% kind of thing. BUT, thats where the vertical/snow thing does help, because you are catching more light.

Grinnin · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

Thanks, all, for your replies.

Three of you have mentioned rack mounts that tilt to vertical. It would work here for dumping snow, but I've cleared trees only a short distance to the south so the shadow in winter comes pretty close to the buildings. The point is that there's nearly no reflected light available here and it'll probably stay that way. A neighbor with a similar-enough yard has 2 adjustable pole mounts and he no longer bothers to adjust.

Loss of efficiency in summer is not an issue. Even the few panels I have now provide lots of juice in summer. PVWatts showed the loss for roof-mount vs. pole mount, and it's perfectly acceptable in the summer. No AC needed this close to the Atlantic in Maine. Those trees help too.

Unlike jtdiesel, though, snow doesn't slide off by 9-10. Between our wet snow and the low sun angle, it often takes half a day. With an ice storm it can take days. I have used only a "T" broom on a long handle. While my panels are now mounted within reach, they were mounted high at the back of the ridge of a building for about 4 years. They were out of range of routine cleaning. That was a problem a few times those first few years in certain weather conditions.

Again, THANKS for your replies.

gww1 · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

I went with more of winter facing non=adjustable ground mount. The panels are at 45 degree and 55 would probly be more winter but I did face them a little westward off of south. Pv watts didn't show much differance but in real life my panels produced better daily in winter then the long days in summer. My over all summer average was a bit higher due to not as many cloudy days.

I do lose a little due to winter shading at about three oclock but hit most of the best of the day. Ten to twenty more feet out in the feild and I wouldn't even have that, I just guessed wrong.

You made the point that the neibor no longer moves his. I would say that that is more mormal then not.

gww

peakbagger · October 2014

Re: Roof mount at 45-deg or pole mount at better 60-deg

I have two systems that can be adjusted for angle and one roof mount at a 4.5/12 pitch. Even at 60 degrees tilt on the adjustable systems, if the snow conditions are right, it can take a day to clear off. With the roof mount, it could be several days. I would consider a landscape mount and microinverters as frequently the top of the array clears long before the base does.

If you can just clear a small portion of a snowed in panel it will clear more rapidly. My roof panels are on a second story roof but I can reach the bottom with roof rake. If I can clear the bottom one foot and the space in front of the array , they melt out quicker.

My two tilting arrays consist of a traditional pole mount and a wall mount that works like an awing. The wall mount melts far quicker than the pole mount as heat reflected off the house heats up the back of the panel quicker. I think wall mounts tend to be ignored but if they fit your house design, they are a nice option.

Roof mount at 45-deg or pole mount at better 60-deg

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