This thread is a bit of a “Showing my work out loud” thread in the context of a potential homeowner installed rooftop solar system I was talking with a friend about.
He has a good south facing roof exposure in the Boise area, and based on some ballparking estimates of roof area, the design concept is for 12 panels (2 rows of 6) on an upper story roof. This may adjust slightly based on the actual setback requirements, but shouldn’t meaningfully alter any of the system details.
Boise is, lucky for them, under their own AHJ as far as solar - far as I can tell. Solar Energy | City of Boise contains relevant documents.
This is the submission checklist, which is an awful lot easier than what I had to deal with… https://www.cityofboise.org/media/10465/406-residential-photovoltaic-system-submittal-checklist_-feb-2020.pdf
Anyway, various design criteria:
- As a roof mount system, under NEC 2017, rapid shutdown is required. This means per panel electronics, and there’s no way around it. This means either a microinverter based system, or per panel optimizers. The Enphase IQ8 microinverters might do it except they don’t exist. They’re shipping those on Musk-time, apparently.
- Backup power is a desired option, if affordable. Generators are cheap, batteries are expensive, and a Secure Power Supply type outlet is also cheap, if one goes that route.
- Minimum roof penetrations. As a second story install on the rear of the house, ground appearance doesn’t matter (beyond it not looking horrible), so there’s no particular need to match panels and rails. The roof is 15-20 years old, so may be replaced before the project starts, but that’s separate.
So, starting out with the design spitballing, there are two ways to do this - microinverters (with no backup power option), or per panel electronics, a string inverter, and a backup outlet. I’ll spitball prices for both, since it doesn’t really impact anything but the inverter/optimizer cost.
I’m going to spitball this with 320W panels, so 3840W (for $/W calculations).
Panel width varies slightly, but in general, one can assume 1000-1020mm (39.37" to 40.15"). Throw in some UFO mounting widths, and one can assume 40-41" “width per mounted panel.”
For a row of 6 panels, that means 240-246" - or a hair over 20’.
Iron Ridge rail span charts can be found here: XR Rail Family - IronRidge - but for 20psf snow loads, with reasonable winds, the Iron Ridge XR1000 rails (the big boys) can span 8’. And, the Iron Ridge rails support a 40% of allowable span cantilever - so, 3’ 2" or so. On either side.
With an 8’ span and a 20’ panel array length, this can be done with 6 mounts per set of panels - two 8’ spans in the middle, with about 2’ hanging off each side. (2’ - mount - 8’ - mount - 8’ - mount - 2’) However, splice kits can’t go in the center 1/3rd of the spans. So, they have to be offset a bit. A 11’ and 14’ rail gives plenty of wiggle room to mount things without the splice in the dead center.
Iron Ridge flashed feet mount under shingles, the UFOs mount panels to the rails, and some assorted end caps/T bolts/etc are required.
Materials cost, rough estimates:
- Iron Ridge XR1000 11’ rail x2: $60/ea, $120 total
- Iron Ridge XR1000 14’ rail x2: $75/ea, $150 total
- Iron Ridge Flash Foot, x12: $12/ea, $144 total
- Iron Ridge T-bolt, x12: $2/ea, $24 total
- Iron Ridge UFOs, x28: $3/ea, $84 total
- Assorted clamps and end caps: $50
Total cost for mounting: $572, or $0.15/W
For no reasons beyond “They’re available from a local supplier in large quantities at $220,” I’m specing some Mission Solar 320W panels (60 cell) - https://rexel-cdn.com/Products/MissionSolarEnergy/MSE320SR8T.pdf?i=208087FE-4EFD-402A-928C-CFC44B4A46D1
Materials costs, rough estimates:
Mission Solar Energy MSE320SR8T, x12: $220/ea, $2640 total, or $0.69/W.
This can be lowered substantially, if one cares to find cheaper new panels, which can be done. I just can’t get my previous local contacts to respond (at about $0.50/W), and for 12 panels, the freight costs of a pallet don’t really justify the savings unless you get a really good deal. But I can get these all day long at this price.
There’s a bit of handwaving here in pricing, as I’m not calculating wiring costs in great detail yet. There’s a couple hundred dollars of wire either way, and without accurate lengths on the install (which I don’t have), I’ll just ignore those for now.
The first option here is to go with microinverters - which, sadly, means Enphase. I don’t like them, but… they’re about the option.
IQ7+ microinverters (currently backordered) can be found for around $145/ea, but… aren’t actually in stock. Oh well. Maybe they’re tooling up for IQ8, or maybe they’re just suffering supply chain disruptions.
Tigo TS4-O optimizers are around $45/ea.
For a string inverter with backup power, there are two SMA Sunny Boy US options that would work - the 3.0-US, and the 3.8-US. Both will tolerate the specified array, though the 3.0-US has a peak AC output of 3000VA, which means it would likely be clipping a bit more often. The 3.8-US, on split phase, can output 3840VA, which seems about ideal for this system… The 3.8-US can output a maximum AC current of 16.0A, which actually means the entire system could be wired up with 12 gauge wire - a nice savings over having to go with fatter wires. There are two MPPT inputs, though there’s actually no reason to use both with optimizers. Open circuit voltage of the panels is 489V, which leaves a good buffer up to the 600V max voltage of the inverter, and running voltage is around 410V - squarely in the MPPT operating range of 195-480V of the 3.8-US. You could run two strings of 6 panels, but with optimizers, there’s no good reason to do this, and it would simply cost a bit more in wiring for no practical gain.
For backup outlet use with the optimizers, you also need some way to get around 18V into the inverter for backup use: TECHNICAL NOTE: Sunny Boy US-41: Secure Power Supply Operation with TS4-R-O - Stellavolta.com
Materials Cost (Microinverters):
- Enphase IQ7+ x12: $145/ea, $1740
Total cost for microinverters: $1740, or $0.45/W
Materials Cost (String Inverter):
- Tigo TS4-O optimizers x12: $45/ea, $540
- SMA Sunny Boy 3.8-US: $1180
Total cost for string inverter with optimizers: $1720, or $0.45/W
Notice that the total cost here is the same, either way. Only one gets you a backup outlet.
HOWEVER: I am a bit uncertain as to if the monitoring gateway for the panels is required. I don’t believe it is for main functionality, but it would add some cost to the optimizer based system, if you wanted detailed per panel information.
I’m adding in $1000 to the system costs here to cover the random other stuff - permits, wiring, conduit, mounting, etc. Plus or minus a bit, it should be about right. There’s no real difference between the two system types in terms of wiring cost, just what runs where. There will be some ground cable and such as well, but, again, no real difference. This is actually a high estimate, especially for the short runs involved in this system, but there’s a lot of “round out” material I’m not calculating out here.
So, for 3840W of solar, installed by a homeowner, the total cost comes to $5932, or $1.54/W. Take the federal 26% off, and you’re at $4389, or $1.15/W, installed.
Commercial installers will be in the $2.50-$3.50/W range, if you find one that’s not out to screw you over, and north of $4/W otherwise.
Nothing in here is going to require more than 12 gauge. Nothing in here should be done on 14 gauge. Except ground, that probably requires 6AWG bare copper, though I’m less certain about how to ground roof mount systems than ground mount…
One could do them, but I would be quite surprised if anything came out to other than 12AWG/90C wire. As a bonus, this can be run in 1/2" (or, if you hate tight pulls) 3/4" conduit.