Looking for advice: is this a fair price?

[fraser]

So there are two answers. 1) I have a flat roof . I wanted to go with 10 degrees east west tilt and regular panels but have been unable to find a residential provider. So panels will be tilted south’ish (196degrees) at 20 and 30 degrees. Tilting will be just enough to use the backside space which would have been wasted on inter row spacing without bifacial panels. 2) the panels are rated for the front side and they were the best price to size ratio that I found just for the front. It would be even more important with microinverters (fewer per watt) but they will clip a lot on 540w panels. Now I’ll just have to paint the roof white or something.

[fraser]

Sorry forgot to answer planned layout. 6 rows of landscape 540 panels (4 or 3 to a row), middle and last rows at 30degrees (one for required 3 foot walkway) and rest at 20 degrees.

[J.P.M.]

So there are two answers. 1) I have a flat roof . I wanted to go with 10 degrees east west tilt and regular panels but have been unable to find a residential provider. So panels will be tilted south’ish (196degrees) at 20 and 30 degrees. Tilting will be just enough to use the backside space which would have been wasted on inter row spacing without bifacial panels. 2) the panels are rated for the front side and they were the best price to size ratio that I found just for the front. It would be even more important with microinverters (fewer per watt) but they will clip a lot on 540w panels. Now I’ll just have to paint the roof white or something.

Interrow spacing in necessary to avoid array self-shading.
If the row pitch (the distance between the same point on any two rows) is too small, the more southerly panels will shade the more northerly panels.
That will more than kill any small advantage of bifacials which need a high(er) tilt, high albedo and a lot of open space behind them to add anything to array production.

If a sawtooth array is tightly packed (small row pitch), painting the roof white will gain little as a high proportion of the roof will be shaded and so will have nothing to reflect.

So as not to rehash a lot of the same material, see the thread you started on 05/13/22 that asked about row spacing calculators and such. My suggestions at that time were to get familiar with SAM. There may be other algorithms available that I'm unaware of that are openly available and there are some that are proprietary.
And, as I wrote in that other thread, also to know that, to a first approximation the highest annual output of any sawtooth array on a (mostly) horizontal roof is pretty much limited to the output of a flat, continuous array that's parallel to that roof.
Bottom line why: The higher tilt angle of a row (up to about the location latitude) will increase the annual output per square area of panel, but that is offset by the increased row pitch that is required because the increased panel tilts will require increased row pitch which will decrease the available panel area. It's one of those situations where there ain't no free lunch.

I worked on this for a couple of years back in the early ,80's and the early days of PC's. The horizontal array approximation, while not exact, is a semi fact of life in a practical sense and is not much affected by using bifacial panels which don't add much to most residential arrays. They might be worth a few cents more per PTC watt, but that's usually more than offset by the increased cost of racking and loss of area/output caused by the tilting requirements as described above.

Also, and in spite of what those who peddle bifacials would like us all to believe, bifacials for common residential horizontal roof applications (and most other residential applications s for that matter) are of little benefit.

Take what you want of the above. Scrap the rest.

[fraser]

Yep the higher tilt angle causes a loss of usable roof space. I think I also mentioned that as a reason the east west 10 degree racking is preferred, but there are a lack of options for that for residential flat roofs in seismic areas. The gain from 20 to 30 degree Tilt at the 37 parallel is negligible about 1.5% or so. It’s more about roof space optimization while maintaining the minimum row spacing. There is a calculator online if you need it. That’s what I used. However you might as well have the most northern row at the optimum tilt as shading isn’t a factor. One more row of 30 degree tilt since I need a 36 inch walkway for permitting. Also all tilted racking is in landscape as it fits my roof better. Racking cost differences are inconsequential compared to the roof utilization losses. Finally I’m pretty sure I mentioned that the bifacial cost is cheaper per watt, for just the front side. The back is just extra. The key for solar installation is optimizing your roof space and minimizing your.cost per installed watt. Not trying to get the most from every panel. Unless of course you have no roof space limitations. Everyone should be over sizing not right sizing like the sales people try to tell you.