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Deciding Between Roof- and Ground-Mounted Solar Panels

paul_miltenburg | Posted in General Questions on

I am looking at installing a solar PV system; the payback is reasonable and I want my kids to become familiar with it.

Does roof mount make sense if you have a good site for ground mount?  I have a 24×40 workshop with 4:12 steel clad roof.  The trusses were specified for solar when I built the shop in 2012.  I’m not worried about having to re-roof and a possible small leak from all the penetrations is not as detrimental in a shop.  It faces about SE which makes less than 10% difference according to PVwatts.

Ground mount questions:
-where I would put it is about 250′ from the shop, would you put the inverter at the array or the building? Inverter inside or outside the shop?
-any preference to installing posts in the ground vs mounting it on concrete blocks sitting on the ground?

I did some PVwatts analysis on seasonal tilt for ground mounting: 10kW system facing south, tilt angles from 15 to 70 degrees, electricity value of 0.17 $/kWh (this is our current all-in bill)  I was surprised how little tilting affected the system.  The best fixed case was 35 degrees, with an income of $2100.  If I cherry picked the best month from each tilt, the total income was about $2180.  Alternatively, an extra 500W on the system at 35 degrees generated about $2200.  The extra racking cost for tilting would never pay, not to mention adjusting it almost every month (may june and july would be the same).

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  1. user-723121 | | #1


    This blog on a solar installation may help answer some of your questions.

  2. Expert Member
    NICK KEENAN | | #2

    Agree that tilting rarely makes sense.

    I would put the inverters as close as possible to the panels, the general rule with electricity is that you want to travel distances at the highest voltage possible to minimize losses. That said, 250' is a long way to run electricity at any kind of load. With a 10KW system @ 240V you're looking at 40 amps. To run 250' I think you'd need 4/0 aluminum which would cost $1000 just for the wire. You'd probably want to bury it which can get expensive. You probably have to price out both installations but at that distance I wouldn't be surprised if roof is cheaper.

    1. paul_miltenburg | | #4

      Wouldn't it make more sense to run 4 or 5 hundred vdc from the array to the inverter at the shop then? I still have to look up what the latest code requirements for solar are in terms of disconnects and whatnot.

      1. Expert Member
        Akos | | #5

        High voltage DC is a different beast than AC. It is not something you want to generally mess around with, always minimize the amount of HV DC in your install. If you do want to run long distance, generally the best is to step up the AC at the source to 600Vac and step it back down to 240Vac at the panel. With the power level and distance you are talking about, this doesn't seem worth it though.

        For smaller solar, go with micro inverters. Makes the install significantly simpler and the material cost is about the same. With micro inverters you also see much less power loss from shading from snow or trees plus it makes the install much more flexible as you don't have to worry about balancing strings.

        If your shop has standing seam panels, take a look at S5 clamps. These minimize or can even eliminate racking, makes the install much cheaper.

      2. charlie_sullivan | | #6

        If you were going to run a 250 foot cable, in principle, 400-500 Vdc would be more efficient and/or require a smaller aluminum cable than running 240 Vac. As Akos correctly notes, it can be more dangerous. That's a disadvantage both because you have that danger present in your yard, and because the code requirements are rightfully more onerous. But I think you can do it and make it safe if you want to. I'm just not sure it will be cheaper than running the 240 Vac, by the time you are done. And I agree that microinverters have a lot of advantages.

        If your hope is to get a little more power out by using a ground mount, spending the money you'd spend on the cable on a few extra panels makes more sense.

        A possible reason I would see to go with ground mount would be if you like the results better aesthetically, but usually people like like roof mount better in that regard. Another possible reason is that it allows DIY without working on the roof.

    2. woodguyatl | | #7

      I know nothing about solar but a little about electricity. 4/0 wire is rated for about 200 amps. Why would that be required? Wouldn't #8 aluminum be enough? It allows a 20% de-rating from 50 to 40 amps to allows for the continuous load.

      1. Expert Member
        NICK KEENAN | | #8

        Due to the length of the run you need a bigger cable to avoid voltage drop. The smaller the cable the higher resistance. At that length and current the resistance of an 8ga cable is significant.

      2. charlie_sullivan | | #9

        For example, you lose about 2% voltage (and 2% power) with a 250 ft run of AWG 2 aluminum at 40 A and 240 V. You are spending maybe $150 on wire and losing 200 W, which you could make up by buying an extra panel. You could spend less on wire, lose more, and buy two extra panels, or spend more on wire and lose less, and not buy any extra panels. Exactly what's optimum depends on the panel cost and the wire cost.

        1. this_page_left_blank | | #11

          Don't forget that even 4/0 wire has a 0.65% drop. So you're only losing 130W, and that's only when the array is at full power, which is not most of the time. I'm not sure if you're saying $150 for the 2AWG wire, or $150 difference. But I am pretty sure the difference in cost between 4/0 and 2AWG is going to be a lot more than $150.

          1. Expert Member
            Akos | | #12

            With longer runs the issue you might run into is too much voltage droop or increase. I've run into this with a shorter AC run for the PV install at my cottage and had to add in an auto-transformer to keep the inverters from curtailing at full sun due to high grid voltage. This becomes a bigger issue if you are at the end of the local feeder.

    3. this_page_left_blank | | #10

      4/0 aluminum for 250' and 40A is overkill. I have a 300' run from my service entrance to my house. I calculated that 4/0 aluminum was sufficient for 125A service (confirmed by the ESA inspector). For 40A and 250', 4AWG copper results in only a 2% drop, which is pretty reasonable.

  3. joenorm | | #3

    Roof mount all the way. Structure is already there and facing the right direction. You'll grow tired of adjusting the tilt on a ground mount. After year two you will not do it.

  4. dfvellone | | #13

    My input regarding ground vs roof mount would depend on where you live and your expected reliance on solar. I live in a farily cold and snowy climate, and being off-grid rely completely on my solar array. The lightest skim of snow on it blocks 100% of electricity production, and if it's cold enough that skim can stay for a long time. I have the ability to adjust the panels vertical during the winter when the sun is already low on the horizon and the reflection off the snow on the ground offers a bump in gain. No need to have to adjust more than a couple times a year: prior to snow season, and again in the Spring. I drive by a lot of homes with roof mounts that are often covered with snow, but they're also grid-tied. If you're not in a similar climate then roof mount would be better than a 250' dc run or inverter outside the home.

    Can you mast-mount your rack to gain some height if that's the issue with the distance and snow is a problem?

    1. paul_miltenburg | | #14

      Good points Daniel. I am in SW Ontario close to Lake Huron, we get a fair bit of lake effect snow; my shop has had a good blanket on it for at least a month now. The system would be grid tied, so it's production is not critical.

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