Adjustable tilt roof mount spacing

Animag771 · February 2018

I'm planning out 3 rows of panels on my roof, adjusting twice a year. I've tried doing the trigonometry to figure everything out, but I'm getting confused. I'm not normally someone who likes to be spoon fed information, but I'm stumped here. All of these angles are throwing me off and I'm not sure if the angles are based on the sun's angle or the horizon for some of these calcs. To add to the confusion, all of the info I have found is for ground mounting and I'm mounting on a sloped roof.

I used this site for my math:
https://www.civicsolar.com/support/i...er-row-spacing

The panels are 65" in length, my coordinates are 33.16, -97.76 and my roof slope is 14 degrees.

So what I did was take my winter angle of 33.41 degrees on December 21 and subtracted my roof angle, which gave me my new angle of 19.41 degrees.

Using this information I got a height difference of 22" which gives me a module row spacing of about 64". Finally I account for the difference in azimuth angle of 43 degrees for a 9-3 window and I end up with a minimum module spacing of about 47".

I've done the math, but it doesn't seem right. With all of the confusion going on between my wife (the math teacher) and me I think we messed something up.

Will someone who actually knows what they are doing PLEASE take a look at this and check if it's correct? I think I've given enough information to plug the numbers in and figure out what the correct winter angle and minimum panel spacing should be.

softdown · February 2018

Given that engineers may call my 20 degree angle a 70 degree angle, confusion easily reigns.

Given that adjustable racking is more expensive and problematic while being inherently weaker, I opted for fixed racking in spite of being somewhat mechanical....I built my own 8000 ft shop for example.

You likely know this but no harm in re-iteration. 33 degrees yields the highest numbers in yearly output. Add 15 degrees for optimum winter output. Subtract 15 degrees for optimum summer output. 33 degrees doesn't seem to favor winter or summer output from where I am sitting.....and very roughly speaking. That could be either a hot or a cold location.

If you use a lot of A/C then favor summer time output. I never use A/C so I favored winter time output. Sitting at 37 degrees latitude, I employed 45 degree racking so I do better during the winter. Why? Because we get ~"twice as much" sun during the summer.

A roof slope of 14 degrees is on the mild side. Safe enough.....most of the time. Yet working on roofs reigns as a significantly dangerous activity. On a one story I would not be afraid of adjusting racks. Two stories and I'm out since I am 59 and would not want to fall two stories.

You may seem to be over thinking this from here. I may recommend skipping the adjustable racking and opting for more panels with the savings.

You may find that board members rarely recommend adjustable racking....in my experience.

Welcome to the board, this is a rare one where manners still reign supreme. (Unless you let the "waterfarmer" know of your politics).

Estragon · February 2018

AFAIK, the general rule of thumb for a south facing array is latitude plus 15° for fall/winter (33+15=48 in your case), and lat minus 15° (33-15=18) in spring/summer. These are relative to the horizon, so subtract roof angle. Row spacing would be based on winter angle.

Animag771 · February 2018

> @softdown said:
> Given that engineers may call my 20 degree angle a 70 degree angle, confusion easily reigns.
>
> Given that adjustable racking is more expensive and problematic while being inherently weaker, I opted for fixed racking in spite of being somewhat mechanical....I built my own 8000 ft shop for example.
>
> You likely know this but no harm in re-iteration. 33 degrees yields the highest numbers in yearly output. Add 15 degrees for optimum winter output. Subtract 15 degrees for optimum summer output. 33 degrees doesn't seem to favor winter or summer output from where I am sitting.....and very roughly speaking. That could be either a hot or a cold location.
>
> If you use a lot of A/C then favor summer time output. I never use A/C so I favored winter time output. Sitting at 37 degrees latitude, I employed 45 degree racking so I do better during the winter. Why? Because we get ~"twice as much" sun during the summer.
>
> A roof slope of 14 degrees is on the mild side. Safe enough.....most of the time. Yet working on roofs reigns as a significantly dangerous activity. On a one story I would not be afraid of adjusting racks. Two stories and I'm out since I am 59 and would not want to fall two stories.
>
> You may seem to be over thinking this from here. I may recommend skipping the adjustable racking and opting for more panels with the savings.
>
> You may find that board members rarely recommend adjustable racking....in my experience.
>
> Welcome to the board, this is a rare one where manners still reign supreme. (Unless you let the "waterfarmer" know of your politics).

Thanks for the response. I did try posting this on another forum and so far the common responses were along the lines of don't do it or just get more panels.

I'd really rather not use more panels though. I have the
kind of mindset where I would rather increase efficiency than increase the number of panels. Why use 20 when 15 will do. Plus, I already have my panels and I have Iron Ridge XR100 racking with S5! clips that I had planned to do a DIY tilt mount with, similar to the one Iron Ridge sells (just much cheaper). I was just dumb and excited to get my panels on and forgot take spacing into consideration.

I'm still building the house, but I'm living on the property (in a camper) while I finish it, so the idea was to get the panels up as soon as the roof was on (it is now). The sooner it's producing power, the sooner it pays for itself.

I appreciate the ease of using the rules of thumb for figuring out the angles, but I prefer something more accurate, even if it's more work. In my climate there are almost the same amount of heating degree days as there are cooling. So I'm not even sure if the rule of thumb works out. I may be over thinking things, but with the costs associated with solar panels, I'd rather overthink something than mess it up.

softdown · February 2018

The possibility exists that people neglect to account for unexpected snow, ice, and high winds when employing adjustable equipment. Your 3/12 roof can easily slide one off the roof if encountering invisible ice that may result from the dew that tends to develop almost every day...it seems. I'm also not sure that adjustable racking would have survived the wind speeds exceeding 100 that I received in 12/2016. The security camera mounting bolts literally snapped in half for example....on smallish security cameras.

I suppose all of us would still use adjustable racking if they already had it. Though I can't recall a board member who recommends it. Do your adjustments later in the day....after the patches of invisible ice have melted off.

softdown · February 2018

Bolts are usually the weak point. Grade 8 bolts are readily attainable. Then again, stainless steel may be the way to go. At any rate, fractured mounting would be disastrous.

I have seen a global race to the bottom in manufacturing quality as pricing and marketing have supplanted trustworthy manufacturing names.

Aguarancher · February 2018

Bottom line is that ANY shading on your panels will drastically reduce their output and should be avoided at any cost.

mike95490 · February 2018

Remember, to not set your winter angle for Dec 21, but something more like Jan 30, where the angle will be more effective for more months.

Same for June 20, get an average summer angle, not the peak of summer angle.

My pole mounts are tilted for winter harvest, since that's the weakest season, I need the most. Summer, I'm in float several hours, and the off angle does not hurt (and it's easier to wash dirt off)

SupraLance · February 2018

This is one of those math problems that can be greatly simplified by getting some graph paper and drawing it. Start with a sloped line representing your roof, then draw a quick representation of your racking and a panel at full winter tilt attached to the racking, accounting for the space the racking will hold the bottom corner off the roof. Then make several lines corresponding to sun angles at various times/days that just touch the top edge of your panels and continue past to the roof slope behind them. Since it is all drawn to scale, you can then measure your distance between panels along the slope of the roof (notice this is different than the horizontal distance, which is what the calculators give you and just confuses things). You really have to draw it to get this right, and this will also allow you to easily see which hours/days will be shaded with a simple line at sun angle to horizon, since it will be tough to avoid at least some early morning/late day shading unless your roof is big enough.

softdown · February 2018

This is a good source for more exact figures on likely output levels: http://pvwatts.nrel.gov

By the way, I believe there are seasonal variations in solar output as soon as on leaves the equator. I probably misread this statement: "In my climate there are almost the same amount of heating degree days as there are cooling."

Estragon · February 2018

As supralance notes, a scaled elevation would be a good idea, even if just to sanity check numbers.

Another tool I found useful was a simple wood angle marked with elevations, with a level attached to the horizontal bottom leg. From the array location, it showed where trees etc would present shade issues.

softdown · February 2018

Been reflecting on the fact that all of my memories of professional on grid solar installations are panels attached to the existing roof angle. With that angle usually being around 15-25%. Denver being at ~40 degrees latitude.

I also may wonder about the temperature a few inches behind a black, glass panel?

westbranch · February 2018

In my view, Mike hit the nail on the head, He , and I both have angled our panels for the period of lowest incoming PV production. In my case, Dec 1 .through March due to clouds and snow... Bright days realy put out the power due to reflection off the snow but still doesn't make up for all the dark days...
So figure out when you will be short on incoming power and choose your appropriate angle with NO shade

Estragon · February 2018

I also have most of my panels set for winter angle all year - have adjustable racks - but don't bother adjusting as spring/summer production is fine at winter angle. This is probably true for most off-grid, but obviously a grid tied system would want to maximize spring/summer as well.

Animag771 · February 2018

Wow thanks everyone for the responses.

Mike - I'm glad you mentioned that, that's exactly what I plan to do.

Supralance - I think I may try something similar to what you suggest, although not on graph paper. When I was drawing up my house, I used Google SketchUp and did everything to scale. There is an add-on that simulates the sun's path and allows me to input a date and coordinates. Using that, I should be able to draw up the solar panels on the roof and run the simulation and if there's any shading, I can adjust accordingly.

Soft down - I have used PVWatts before, but it's been a while. I'll have to play with it again. Also I think you did misread my earlier statement... What I mean is I do have seasonal fluctuations, but as far as heating and cooling I would have to use them both about the same amount to maintain a constant year round temp. (If that makes any sort of sense. My words aren't working today.)

Estragon - I'm on grid so I'm trying to maximize my test round production.

Estragon · February 2018

I wondered if you might be grid-tied. Don't have time to look it up now, but I recall reading a compelling argument for flatter (ie < lat - 15°) summer angle to maximize production. IIRC, the less optimal noon angle is more than made up for by longer early and late day production (absent shade issues). FWIW.

Animag771 · February 2018

I'm going to have to look that up now.

softdown · February 2018

mike95490 said:

Remember, to not set your winter angle for Dec 21, but something more like Jan 30, where the angle will be more effective for more months.
Same for June 20, get an average summer angle, not the peak of summer angle.

My pole mounts are tilted for winter harvest, since that's the weakest season, I need the most. Summer, I'm in float several hours, and the off angle does not hurt (and it's easier to wash dirt off)

Makes sense to me. Could that mean adding about 10 degrees for winter and subtracting about 10 degrees for summer? Or does that still leave us around the popular plus and minus 15 degrees that is so commonly touted? It is too early to figure it out myself.

Animag771 · February 2018

Well I haven't been able to find that article, but I guess it doesn't matter. I've done some more math (I hate trigonometry) and I've figured out that in order to tilt the panels it would take up the majority of my roof space and there would be no room left for additional panels due to the spacing. So I guess I'll have to mount them along the slope. I built my place with a single roof slope to the south and it's only 22x32, so for now I'm going with 3 rows of 5 panels in portrait. This will give me 15 panels now and if they aren't producing enough for my needs at least I'll have enough room left over to mount up to 10 more in the future. Also if I'm only slightly low on my production I think I could still find a way seasonally tilt the highest 5 panels to eek out just a bit more power.

Kind of sucks though... I'm big on trying to maximize the efficiency of what I've got, versus just getting more stuff. Heck, that's why I'm building my house as small as it is. I guess in this case though, the most efficient way to do it is by leaving enough room now to add more later.

mike95490 · February 2018

May be a case where the high watt / sq ft sunpower modules are needed, if you need every last watt.

Mount the panels with tall standoffs, lets a bit more air for cooling under them (and more wind uplift force on your roof)

You may also be able to reverse tilt a row on the backside of your roof, although that starts to get tricky with wind forces.

Dave Angelini · February 2018

As Mike said on the Sunpower. The other choice is Panasonic. LG has a high end module that is close to Sun Power and Panasonic.
I use Panasonic on my trackers for clients and get excellent pricing. made in New York.

The 330w Sunpower and the 330 Panasonic
are about 41" x 61". Both are backed with 25 year product warranty which is unheard of anywhere else.

Estragon · February 2018

The flatter-in-summer tilt I referred to above:
http://solarpaneltilt.com

softdown · February 2018

Estragon said:

The flatter-in-summer tilt I referred to above:
http://solarpaneltilt.com

That yields very different numbers than the common plus or minus 15 degrees. Especially during the summer....for us northerners anyway.

Since many of us struggle during the winter in spite of steep "winter oriented" panels, I may continue with my winter time bias. Though the figures certainly augment arguments for following shallow roof lines on a grid tie installation.

Animag771 · February 2018

I've read that article before (and again today), but I didn't see anywhere that it mentioned using a low angle for year round production. Maybe I missed it somehow.

Estragon · February 2018

In the section for adjusting 2x/yr, winter angle at 30° is shown as ~7°, vs 30-15=15° "rule of thumb". Summer at 45.5 is about the same.

softdown · February 2018

If your latitude is below 25°, use the latitude times 0.87.
If your latitude is between 25° and 50°, use the latitude, times 0.76, plus 3.1 degrees.
If your latitude is above 50°, see Other Situations below.

This table gives some examples for different latitudes. It also shows the average insolation on the panel over the year (in kWh/m2 per day), and the energy received compared to the best possible tracker.

Latitude	Full year angle	Avg. insolation on panel	% of optimum
0° (Quito)	0.0	6.5	72%
5° (Bogotá)	4.4	6.5	72%
10° (Caracas)	8.7	6.5	72%
15° (Dakar)	13.1	6.4	72%
20° (Mérida)	17.4	6.3	72%
25° (Key West, Taipei)	22.1	6.2	72%
30° (Houston, Cairo)	25.9	6.1	71%
35° (Albuquerque, Tokyo)	29.7	6.0	71%
40° (Denver, Madrid)	33.5	5.7	71%
45° (Minneapolis, Milano)	37.3	5.4	71%
50° (Winnipeg, Prague)	41.1	5.1	70%

softdown · February 2018

I had been taught that 37 degrees would yield best year round production at my 37 degree latitude. The above referenced data says that 31 degrees would be better. About a 16% difference.

Yet this may only be of interest to on the grid applications. Many of us northern off gridders do not yearn for more summer time yields. Winter being a different story.

SupraLance · February 2018

I'm also at 37 degree latitude and double-checked their suggestions against local PVwatts data. The tilt angles in that article do work better, including the fact that 57 degree tilt yields better winter production. But I think those who end up leaving their pv at winter tilt all year are better served by less tilt or they may find summer production at considerably less than winter. Fixed at 31 degrees yields the highest year-round total, but 37 (or even higher) is a better choice for off-gridders not wanting to give up so much winter yield even though they make back more in the summer than they lose in the winter.

Interestingly, according to PVwatts data, summer production is actually higher with a 10 degree tilt E/W split array than with a 10 degree tilt due South array...

westbranch · February 2018

was that:
Interestingly, according to PVwatts data, summer production is actually higher with a 10 degree tilt E/W split array than with a 10 degree tilt due South array...

E/W a Virtual array?

SupraLance · February 2018

Try it for your locale, but if I have pv watts calculate monthly production for, say, 1000w due south at 10 degree tilt; and then compare that to the sum of 500w due east at 10 degree tilt and 500w due west at 10 degree tilt; the production of the split array (split 500w east and 500w west) is greater than the production of 1000w due south for the months of May-June-July. It makes sense if you think about it, since the sun makes a much greater sweep across the sky in the summer and the split array starts producing earlier and produces later into the day with both panels seeing most of the midday sun. This does NOT hold true for winter production, when split arrays produce very poorly. And if you increase the slope beyond about 15% on the split array the due south array wins.

softdown · March 2018

Estragon said:

The flatter-in-summer tilt I referred to above:
http://solarpaneltilt.com

Anybody know what happened to solarpaneltilt.com? I keep getting this message:Connection refused: solarpaneltilt.com:80

Adjustable tilt roof mount spacing

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