45 Degree Roof Solar Panels

[posplayr]

The model shows a 7 kW array with a 248 deg az with 28 deg tilt could generate $2956 of credit annually under TOU-D-A, or ~10535 kWh under a straight tiered plan.

The model shows a 7 kW array with a 158 deg az with 45 deg tilt could generate $2951 of credit annually under TOU-D-A, or ~11500 kWh under a straight tiered plan.

Any suggestion that the OP install panels on a face other than the SSE face originally suggested is misguided, at best.

Linked here is a spreadsheet showing the TOU analysis of the SW array. Perhaps some careful study will help explain the difference between these results and the incomplete, inaccurate analysis that suggested a different conclusion. Based on the little I can get out of what was posted, a fundamental misunderstanding of how daylight savings time works is responsible for at least part of the error, but with a basic mistake like that, really, anything is possible.

I'm sure if the OP comes back he will be appreciative to Sensij for doing this analysis. He has used his detailed TOU-D-A model to confirmed that the OP has a viable "nearly" equivalent option in that he can obtain virtually the same annual credit using his Northwest roof plane with probably minimal angling utilizing an effectively flat mounting arrangement. I'm not sure if he was aware or if he had ever contemplated it? Of course it doesn't offer the same space of the SE roof, but then again the OP is having a hard time finding anyone to bid that SE roof. Which brings us back to the "near equivalence" caveat mentioned above. How much is it going to cost to mount panels on a roof when you can't get a bid to mount panels on that roof?

We have heard that the 45 degree slope is probably the driver reducing any interest of OP's local installers. We also know that in large part due, to the 45 degree slope of the SE roof it is going to be more costly if not prohibitively do do any type of angling. In fact in a straight head to head comparison the mounting cost on the SW roof will be cheaper than the SE roof making the SW roof preferential purely looking a cost.

Any suggestion that the OP install panels on a face other than the SSE face originally suggested is misguided, at best.

Until the OP has a competitive bid in hand comparing the two, I would think you are in serious danger of prejudging the situation. While you see no issue with the roof, the local installer who have been offered a chance to bid have thought otherwise. We dont know why, but we do know there are other options.

I know that if I was in the OP's shoes, I would want to understand the cost drivers of a bid are and what issues (if any) were driving it. The OP apparently has been pursuing exactly the path Sensji is suggesting and that is what lead him here. Whether "No Bids" or "High Bids" he is looking for answers and I would suspect options.


Not to leave any stone unturned, let's get back to my prior analysis. The notion that TOU-D-A is a driver to annual credits is certainly valid. To use Sensji's numbers the SE panels at 45 degree flat mount developed (according to his model) $2951 annually while an optimized panel yielded would yield $3206 annually. While some might scoff, but when considering fixed solar installs even 8.6% gain is significant. Sure you can always throw more panels at the problems and solve orientation and mounting problems, but that assumes that you can find anybody to mount them (and at what cost). The issue I brought up was that the SE roof pointed east, and so was at a disadvantage for "afternoon harvest", the point being that other roofs would be better oriented and not suffer the same 8.6% loss. Is 8.6% significant, well yes. Is it prohibitive? No not so long as you know that you will under perform. The OP might just have his heart set on that SE roof just so he doesn't even have to see the panels from the front and he doesn't really care how many there are nor how much they cost.

Is the 8.6% penalty enough to get a NO bid from your favorite installer? It doesn't seem like it unless the installer figures the OP will ask for some angling in the already difficult 45 degrees slope which would get back to a chicken and the egg questions is the the angle or the azimuth or is it some of both. Maybe the OP can do an exit interview and take a poll with all of his NO BID installers.

In summary, the OP is looking for answers, doing the same thing is not working. He probably knows more now about his roof than he did before and he has another viable probably cheaper option (i.e. the SW roof). It was a good exercise for me as well. The decomposition of the PV watts data confirmed my suspicions, and now with a full model that Senseji has we have a full roll up to confirm the annualized results as well.

[J.P.M.]

The model shows a 7 kW array with a 248 deg az with 28 deg tilt could generate $2956 of credit annually under TOU-D-A, or ~10535 kWh under a straight tiered plan.

The model shows a 7 kW array with a 158 deg az with 45 deg tilt could generate $2951 of credit annually under TOU-D-A, or ~11500 kWh under a straight tiered plan.

Any suggestion that the OP install panels on a face other than the SSE face originally suggested is misguided, at best.

Linked here is a spreadsheet showing the TOU analysis of the SW array. Perhaps some careful study will help explain the difference between these results and the incomplete, inaccurate analysis that suggested a different conclusion. Based on the little I can get out of what was posted, a fundamental misunderstanding of how daylight savings time works is responsible for at least part of the error, but with a basic mistake like that, really, anything is possible.

FWIW, seems to make sense to me. I've found T.O.U time/rate structures to be pretty straight forward with a little effort.

[sensij]

The model shows a 7 kW array with a 248 deg az with 28 deg tilt could generate $2956 of credit annually under TOU-D-A, or ~10535 kWh under a straight tiered plan.

The model shows a 7 kW array with a 158 deg az with 45 deg tilt could generate $2951 of credit annually under TOU-D-A, or ~11500 kWh under a straight tiered plan.

Any suggestion that the OP install panels on a face other than the SSE face originally suggested is misguided, at best.

Linked here is a spreadsheet showing the TOU analysis of the SW array. Perhaps some careful study will help explain the difference between these results and the incomplete, inaccurate analysis that suggested a different conclusion. Based on the little I can get out of what was posted, a fundamental misunderstanding of how daylight savings time works is responsible for at least part of the error, but with a basic mistake like that, really, anything is possible.

[posplayr]

I used PVwatts and did a quick run without optimization to compare a SE array 7K watt @152 Az and 43.7 Tilt to a SW array @225 and the same tilt 43.7.

I downloaded the full years worth of hourly data into two separate spreadsheets so I could clip out only the Day Time savings data. Then I sorted by hour and plotted the data for the SE and SW arrays. I applied the primary rate schedule under TOU ($0.11 till 8 am, 8am<$0.30<2pm and then 2pm<$0.46<8pm). The main reason for clipping out the DST is I would have an 1 hour of shift between DAT and non DST data.

I'm doing this is a spreadsheet so any more complex rules would be too unwieldy; I would need to pull this into a programming language.

Bottom line, the SE panel peaked somewhere around 10:30AM STD (9:30AM DST) and the SW peaked around 1:00 PM STD (12:00(Noon) DST). The peak so early is owning to the solar roll off pattern more despite the shift in the "peak" to 2:00 PM.

You can get some idea of time shift in harvest from early day to later afternoon. I did a quick numerical TOU average of the data and I got a 16% difference as summarized in the lower left hand comer. Obviously is is still a bit gross and well as un-optimized but it is probably indicative of the penalty the 152 degree az roof.

Sensij, I'm not sure why you are not getting a bigger difference.

TOU_analysis.jpg

EDIT: I noticed I screwed up and used 152 rather than 158 deg. It needs to be done anyway.

EDIT 7/28/2015
I refined the simple spreadsheet analysis to actually bin every hourly KW data point from PV watts for both a SE 158 degree 32.7 panel and a SW 248 degree 28 degree panel. Considering a static TOU for daylight savings of 0.11,0.30 and 0.46 with transitions at 8am, and 2pm as it is just simpler, and excluding the winter as it would require another spreadsheet.

There are some interesting results. Fixed panels are usually optimum sometimes or to say it another way,most the year/day a fixed panel is almost always wrong. So being wrong less if the best you can hope for. However, it is clear that daily timing in favor of capturing higher TOU rates does offer some of the most fertile ground available for $ cost efficiency under Net Metering. Going through this exercise for the OP's situation bear this out.

This comparison is only for Day Light Savings time which extends from Mar 8 to Nov 1. The comparison is to evaluate the feasibility of the SE roof v.s. the SW roof as a location for mounting panels. I assume the OP wants to use the SE facing roof for aesthetic reasons. The west facing roof is closer to the front of the house and much smaller. Regardless this is still a useful exercise. For the SE roof at 158 the tilt angle of 32.7 was selected as the optimum according to http://www.solarpaneltilt.com/.
Sensij also ran some numbers showing that was in fact the best (based on Kw-Hr produced). The SW roof was selected obviously as it is much more west, lowering the tilt angle for the 248 az brings the normal back further to the east and also further emphasises the summer months so I did not lower the tilt any more. If fact if the OP so choose he would likely pick something for easy mounting. That portion of the roof appears to be much more shallow that the SE roof, so the roof slope would probably dictate the mounting angle as much as anything.

TOU_SE_SW_Compare_DLS.jpg

In the figure above we see that the aggregate numbers for DLS show that although the (7Kwatt panels for both) SW panel generated about 5% less energy, it produced about 9% more revenue. The obvious time shift in the harvest, yielded less Kw-Hr but a higher Net Meter credit.
SE SW
Kwatt-Hr 8349 7960 95.3%
$ 2389 2601 108.9%

For me it reinforces the value of tracking, as the placement of a fixed array can only do so much even comparing such extremely different orientations. For the OP perhaps it offers an option to mount panels on the SW roof where it is cheaper to mount and 9% better economically in terms of NM credits. If you can't get all of the panels you want or need on the SW roof, placing the balance on the SE roof will capture more low angle solar energy and outperform the SW roof in winter. Being off of DLS will also tend to increase the value of that roof as well.

[sensij]

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

I've been busy. I tried to follow a lot of the nonsense since my last post, but it makes my head hurt. Sorry, I haven't been able to turn that into a productive response.

With respect to TOU, under SCE's TOU-D-A plan, a 158 deg azimuth, 45 deg tilt system might produce $2951 of credit annually. At 45 deg tilt, the maximum revenue a 7 kW array would produce is $3132 under the same assumptions (azimuth for that case is 195 - 200 deg), so it pushes the annual difference with respect to flush mount from the $70 base case I described earlier to $181. That assumes you can even fit the 7 kW array side tilted on the roof in the unshaded portion, which is not assured given the extra space between rows required. It also assumes the roof itself won't cast any shade on the obliquely mounted panels, which is also not assured, especially at 12:12 pitch.

If custom mounting is on the table, a custom tilt could also be considered, I guess. The max revenue I can get out of this location is at 195 deg azimuth and 32 deg tilt, good for $3206 annually. That same revenue would come from increasing a flush mount array from 7 kW to 7.6 kW... basically, adding two 300 W panels, maybe $1000 of costs, or maybe $2000 installed. There would also not be the roof/self-shading problems to try designing around. Does anyone really think the custom engineering, shade analysis / mitigation, more complicated mounting system would cost less than $1000? I don't.

Also, back to the questions the OP actually asked, I was wrong to say that all panels would be equally good. On a steep roof, sticking with 60 cell panels for ease of handling might have some benefit to the installer.

Edit: There was an error... the baseline run was at 158 az/20 deg tilt instead of 158/45 deg tilt. The numbers have been updated.

[posplayr]

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.

Not that it really matters if you re read what I posted, I did a quick pivot from 202deg to 225deg to achieve a solar production peak at 2 pm. I did describe the 202 degree point but went to 225 for the ease of TOU analysis.

As it works out there are 3 hours between solar peak at 180deg, and so that point is 45 degrees from a 2:00 pm TOU rate transition.

45 degrees = (2+1)hrs*360/24

If I recall 180+45=225 is SW.

Rereading the OP's comments about minimal shading morning or evening, he might be able to go even further to the west. As an example, considering strong afternoon sun to perhaps 6pm and a rate transition at 2; that is a 4 hr spread, splitting the difference would have a desired solar production peak at 4pm to grab the most of the peak TOU.

75 degrees = (4+1) hours*360/24

180+75=255 degrees. That is along ways from 158 deg (97 degrees).

All that would need vetting with something like PV watts, and 255 may be too far, but it sure seems like from a TOU perspective 158 is close to 90 degrees out. Is it serendipity that there is another roof already 90 degrees out?

[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.

[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...

[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.

[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]

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.

[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?

[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.

[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.

[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.