45 Degree Roof Solar Panels

[Evin]

Hi,

I live in Auburn CA with a 12:12 pitch roof. My roof is about 12X47 feet and I'm looking for the best solution with this pitch roof. I have slight shade on one side during the morning and night. Many installers will not work on this roof. Is there a panel that works best with this pitch roof? I'm looking for around a 7kW system.

Thanks

[posplayr]

Hi,

I live in Auburn CA with a 12:12 pitch roof. My roof is about 12X47 feet and I'm looking for the best solution with this pitch roof. I have slight shade on one side during the morning and night. Many installers will not work on this roof. Is there a panel that works best with this pitch roof? I'm looking for around a 7kW system.

Thanks

To get a meaningful answer you will need to describe how the 12:12 pitch is oriented with respect to N-S. If the ridge is oriented N-S for example this could be a difficult install. Unless you split the panels E-W and had morning and afternoon arrays. It all kind of depends on how the roof is oriented which could lead to a variety of other factors to consider.

[Evin]

Doc1.jpgThe main side faces South.

[posplayr]

[ATTACH]7353[/ATTACH]The main side faces South.

To be clear, despite the orientation of the house in the picture, the roof is geographically aligned with N/S? In other words the main side as you call it faces direct south or just nominally?

In degrees how far off is it?

[Evin]

The main roof is 158 degrees SE.

[posplayr]

The main roof is 158 degrees SE.

So you understand the issue?

Go to this website to determine optimal angle for solar harvest.

http://www.solarpaneltilt.com/

Use one of these formulas to find the best angle from the horizontal at which the panel should be tilted:

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.

For your latitude: Coordinates: 38°53′55″N 121°4′28″W

I get Tilt angle = .76*39+3.1=32.7 degrees

Your roof is at 45 degrees and skewed from true south so your mounting plane for an array is out of plane with the roof 45-32.7=12.3 degrees in tilt and 180-158=22 degrees in azimuth. The 12.3 degrees in tilt would not be too hard to adjust for. Just mount the panel further from the lower part of the roof than the upper. The 22 degrees of Azimuth is another bigger challenge. If it was a west error instead of an east error you could probably just accept it as improved afternoon harvests. To adjust for this the panel now goes out of plane with the roof. The challenge is to find a set of mounting points, that can be built over that result in all mounting rails to be in plane but out of plane with the roof. That necessarily means all of the standoffs are a different length (assuming a regular spacing).

If you can lay the lower rail, on running east west but on a slant to the roof, then you just have to figure out the lengths of the opposite side (all the same) that give you the proper tilt angle referenced to N-S. The panels will be staggered, but as it is based on a small angle the spacing should not be too band. Not to try and make this less complicated than it is, obviously the installer would be trying to keep the mounting hardware to a minimum and so how many mounting points, how large the panel arrays are as well as the associated loading are all factors that come to play. There is another thread about "side tilt" and a picture of a staggered design similar to what your's would be.

I'm assuming the typical Solar Super Cuts installer is apparently not going to take the time to figure it all out. Being an engineer I would figure it out myself or get some software to do the design. I don't know what your skill set is whether you can balance all of the objectives and the constraints and come up with a workable plan that someone could bid to. I am pretty sure that the more you study it the more you will understand. There reaches a point however where a 3D Computer Aided Design (CAD) package is required.

[J.P.M.]

Hi,

I live in Auburn CA with a 12:12 pitch roof. My roof is about 12X47 feet and I'm looking for the best solution with this pitch roof. I have slight shade on one side during the morning and night. Many installers will not work on this roof. Is there a panel that works best with this pitch roof? I'm looking for around a 7kW system.

Thanks

Solar PV panels do not exhibit a strong efficiency differential as a function of tilt (elevation) angle. In other words, given the same P.O.A. (Plane Of Array) irradiance, say 800 W/m^2, if panel X has an operating efficiency of say, 18% under a certain set of conditions, it will have just about the same 18% efficiency at any tilt angle for the same conditions. That's not to say the P.O.A will be the same for different tilt angles if the panels are mounted non parallel to the roof using racking designed for that purpose.

[J.P.M.]

Evin: Adding to my first post: Avoid the shaded portion of the roof as much as possible. Panels can be mounted at just about any orientation, even vertical if/as necessary. Roofers are probably reluctant mostly because of the pitch. Mount the array parallel to the roof with about 6" clear between the panels and the roof. Use good racking and a good vendor. a 7 Kw system will take up about 450 - 500 ft.^2. You'll probably need 3 ft. clear at the top and on at least 1 side of the array. Might be kinda' tight.

[sensij]

With a 158 deg azimuth and a 45 deg tilt, PVWatts model for annual production (assuming premium panels, fixed roof mount, and 10% loss) is 1643 kWh / kW. The absolute best model output I could get by experimenting with azimuth and tilt was 1693 kWh / kW (at 33 deg tilt and 180 deg azimuth). A difference of 50 kWh / kW * ~7 kW array = 350 kWh difference total. 350 kWh @ $0.20 / kWh = $70 / annually.

So... you could just stick with flush mount, and try to stay out of the shadows... or you could pay someone to try to engineer a custom mounting solution, figure out how to install it on a 12:12 roof using semi-custom hardware, and probably be forced to put panels in your partially shaded section because of the additional space between rows that would be required by something other than a flush mount... to try to extract another ~$70 annually.

In other words... your roof orientation is just fine for panels as it is. The thought experiment about what it would take to get the absolute maximum amount of Plane of Array irradiance is interesting, but ultimately, one of those things where perfection is not worth the effort to achieve it. There is no particular panel that should perform better or worse at that pitch... most arrays with the same STC rating will perform about the same. Your time will be better spent finding an installer that is comfortable with your roof and meets your other vendor criteria than worrying much about the panels used or any of the rabbit holes that were mentioned in an earlier reply.

[posplayr]

With a 158 deg azimuth and a 45 deg tilt, PVWatts model for annual production (assuming premium panels, fixed roof mount, and 10% loss) is 1643 kWh / kW. The absolute best model output I could get by experimenting with azimuth and tilt was 1693 kWh / kW (at 33 deg tilt and 180 deg azimuth). A difference of 50 kWh / kW * ~7 kW array = 350 kWh difference total. 350 kWh @ $0.20 / kWh = $70 / annually.

So... you could just stick with flush mount, and try to stay out of the shadows... or you could pay someone to try to engineer a custom mounting solution, figure out how to install it on a 12:12 roof using semi-custom hardware, and probably be forced to put panels in your partially shaded section because of the additional space between rows that would be required by something other than a flush mount... to try to extract another ~$70 annually.

In other words... your roof orientation is just fine for panels as it is. The thought experiment about what it would take to get the absolute maximum amount of Plane of Array irradiance is interesting, but ultimately, one of those things where perfection is not worth the effort to achieve it. There is no particular panel that should perform better or worse at that pitch... most arrays with the same STC rating will perform about the same. Your time will be better spent finding an installer that is comfortable with your roof and meets your other vendor criteria than worrying much about the panels used or any of the rabbit holes that were mentioned in an earlier reply.

If you are going to convert that to a $ figure you might want to consider the TOU rate schedule which was the predominate reason for altering the array orientation .

What do you assume is making installers shy away from the roof?

[posplayr]

I guess sensij did not come back to answer the questions I posed.

I don't disagree with sensij's analysis using PVWatts other than what he did not do and his conclusion. My reference to "afternoon harvest", is a reference (perhaps cryptic??) of Time of Use rate considerations. I dont know what your are but I did look at SCE as I was a long term customer. SCE have approximately a 50% rate hike that starts at 2 pm and extends to 8 pm on most all weekdays. Something like $0.30 going to $0.46.

It is always important to start with your top line production in your highest yielding periods. It is not easy to make up for losses here and the aggregate totals will tend to follow. Since you are in California and your predominate solar production will be in the summer, Day Light Savings time will apply which will be used to determine the appropriate TOU rate.

A true south solar array will have a peak solar production at 11:00 AM(Daylight Savings Time or 12m STD time) . A solar panel at 158 degrees is 22 degrees east of south and will have a solar production peak at 24/360*22=1.47 hr. So in round terms a panel mounted right flat to your roof will peak at 9:30 am (in the morning) producing power that by straight retail Net Metering rates would be credited at $0.30. The rate change to $0.46 is 4 1/2 hours later. So even with a 9 hour solar day the vast majority of the solar production will be at the off peak $0.30 rate.

On the other hand if you had a panel at 202 deg which is now SW rather than SE the while solar production would be close to the same, the economic production of that panel would be far greater. The solar production peak is now 1:30 pm just a 1/2 hour before the rate change. It would not be surprising when you run the details that the pointing angle would extend even further West. Just Talking in round terms, if the angel was extended so that solar peak was at 2:00 pm when there is a rate shift, then 1/2 of your production will be at $0.30 (before 2:00 pm ) and the other 1/2 will be at $0.46 (after 2:00 pm) . That means that the weighted Value of the SW panel situated for the rate hike is (0.30+0.46)/2=.38 v.s an average production value of $0.3 for the SE panel. That means your simple panel will at 27% below the SW panel in topline production value. 127%=0.38/.30.

You will have a different rate schedule, or possibly choose from many but the but a flat panel mount on the roof at 158 deg is going to be limited as compared to what could be achieved with other fixed mount arrangements. Perhaps it is obvious, but this is what I suspect is the primary reason you can not get much interest from many installers. I'm all ears to other considerations.

[ButchDeal]

On the other hand if you had a panel at 202 deg which is now SW rather than SE

Great analysis. Just want to point out that 202deg would be SSW. SW would be 225deg.

and from the photo there seems to be western tree shadows preventing much peak time production as well.

[posplayr]

Great analysis. Just want to point out that 202deg would be SSW. SW would be 225deg.

and from the photo there seems to be western tree shadows preventing much peak time production as well.

It is a DIY forum, not a Phd defense; I try to keep it in small bites.

Yes the morning sun seems less blocked, and more trees to the west but are probably secondary factors.

For example:, in the SCE rates the $0.30 rate starts at 8 am from an overnight $0.11. That means the early morning rate transition is only 1.5 hours from the production peak at 158 deg. That is going degrade the $0.30 average production rate assumption for the 158 degree panel.

[foo1bar]

What do you assume is making installers shy away from the roof?

IMO that it's a 12:12 roof.

A 4:12 roof (~18 deg) you need to still be careful, but it's not a big deal to walk around.
A 12:12 roof on the other hand...

[ButchDeal]

It is a DIY forum, not a Phd defense; I try to keep it in small bites.

Yes the morning sun seems less blocked, and more trees to the west but are probably secondary factors.

For example:, in the SCE rates the $0.30 rate starts at 8 am from an overnight $0.11. That means the early morning rate transition is only 1.5 hours from the production peak at 158 deg. That is going degrade the $0.30 average production rate assumption for the 158 degree panel.

I only pointed out the SW / SSW as the OP originally claimed S when it was really SSE and as you showed that makes quite a difference in the production in a TOU situation.