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Community and Q&A

Estimating Solar Power Viability

acrobaticnurse | Posted in General Questions on

Most online apps I’ve seen mostly focus on looking at install cost vs my power bill rather than telling me how many square feet of my roof are good for solar and how much more would be good depending on how much I trim trees. I’m wondering if there is a way to see how viable it would be to place 1-2 kw of solar panels on my roof before actually installing them.

I want to charge an off-grid battery that would mostly be to help power lights, modem/router, and our fridge. I effectively live in a forest but at least part of my roof gets direct sun for several hours per day. When I had local solar contractors contact me I was told solar wouldn’t make sense unless I was trimming/removing trees.

I can understand it not making sense for a contractor to deal with me since I may not have space for 10kw of panels and my power bill is already low. I mostly see installing 2-4 panels myself, wiring them to something like an ecoflow delta pro or another off grid all in one inverter, and maybe having a couple home circuits permanently powered by a lithium phosphate battery to limit generator needs and maybe make a small dent in my relatively lower power bill.

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  1. jadziedzic | | #1

    You might find some helpful folks on this DSL Reports forum:

    A fair bit of DIY stuff for small systems covered in that thread.

  2. Expert Member
    DCcontrarian | | #2

    There's really two questions here:
    #1 is how do you estimate the economics. The standard model is you look up the hours of insolation for your location, multiply by the output of your panels and multiply that by your local electric rate. You've got a couple of compounding factors here -- the impact of shading, and the fact that you're off-grid means you can't take advantage of net metering and have to either align your consumption to when the sun is out or figure out a way to store some, probably a little of each. In terms of estimating the shading, the number you will end up with is a percentage of your annual potential that will be lost to shading. There's a gadget, I don't know the name of it, that's basically a spherical mirror that allows you to take a picture of your skyline and estimate how much of it is obscured. You stand on the roof with the gadget in different places and get your estimate.

    With a DIY system you won't be eligible for the federal tax credit, which is a big factor.

    #2 is about DIY. It's not difficult if you're off-grid, especially if you're OK working on the roof. I'd look at package systems designed for off-grid.

    1. aunsafe2015 | | #7

      "With a DIY system you won't be eligible for the federal tax credit, which is a big factor."

      DC, do you have a source for that? I've read the opposite. I think the exact language is something like an "installed" system. So I had read speculation that it does not matter whether it is "installed" by the homeowner or by a contractor. I've also read that the language "installed" would probably exclude portable systems such as an Ecoflow Delta Pro.

      1. Expert Member
        DCcontrarian | | #8

        I may be wrong about that. I remember when I installed my system it had to be done by a licensed contractor to get all of the incentives, but I just checked the IRS guidelines and it doesn't contain that language. It may have been for the DC incentives.

        1. Tim_O | | #10

          Yes, DIY is eligible, you do have to use new components though. For example, Santan solar used panels are not eligible. I also believe with the old bill, you could replace your roof as part of the solar install. That has since been removed as well I believe. Batteries have been added as an eligible component though.

  3. Expert Member


    No sunnier spots nearby you could have a ground mounted array?

  4. maine_tyler | | #4

    There are apps that offer augmented reality to show you the position of the sun as you point your phone at the sky. One such app that I've used for a situation similar to yours is SunOnTrack. It has the advantage of letting you see the track of the sun for any time of year. It has other shade predicting features as well. I'm sure there are other more robust apps, and certainly professional tools like DC alludes to.

    There are also diy solar specific forums, which will probably offer more info than you'll find here. If you're doing an off grid system, you really want to put some thought into sizing the array and the battery bank. If you are going to be in partial shade conditions, you may also want to consider something like a half cut cell panel and possibly micro inverters or at least a wiring scheme that won't kill the entire array output when one part of one panel is shaded.

  5. maine_tyler | | #5

    Looking at your post again, I see you're not actually off grid. Why run an off grid system then? You can still have battery backup with grid connection.

    And in case it wasn't clear, shade is solar killer. Like even a little bit of part of a panel. So if you really have more shade then sun amd you'reon the grid, one might ask if philosophically why you need the physical panels with you in the forest when they could be out in full sun at a community solar project...
    You could still get batteries if you're really trying to get off the gennie.

  6. Expert Member
    BILL WICHERS | | #6

    You basically need a way to monitor the amount of sunlight you get over a day. When I was making some measurements like that at my house, I put a light sensor (a simple photocell) inside a pingpong ball and put that on a wooden dowel. I would stick the dowel in a spot I suspected might get some sun, then left it in place for a while, with the pingpong ball about a foot or two above the ground. The pingpong ball acts as a diffuser to make the photocell less directional. Record the output of the sensor over the day and do that for a week or month and you get some good data.

    You can do the same thing without anything fancy by just checking the "sensor" periodically with your eye, similar to how a sundial works. Your goal is to find the spot with the highest average amount of sunlight over the course of the day, and try to estimate how much that will vary with seasonal changes (you can also work out the angles to get a better estimate here). If you only get an hour or so of sunlight, with the area in shadow the rest of the time, then a solar system probably isn't going to work too well for you. I wouldn't limit yourself to only your roof, either, since it's easy to build a simple frame to allow a ground mounted system to be installed wherever there is sufficient sunlight.

    If you have a connection to the grid, skip the batteries and just do a grid tied system. Batteries introduce a lot of additional costs and risks, as well as maintenance, that you really don't benefit from with a grid tied system.


  7. gusfhb | | #9

    My neighbor has a system that is surrounded by trees. Almost no totally clear shot except mid summer. He is reasonably happy with its performance.
    I think it comes down to expectations.

    1. maine_tyler | | #11

      It's not just about expectations, it's about efficiency of resource use.

      Shading of panels disproportionately kills the output. If you are not off grid and if you have lots of shading, I don't understand the rationale for locating the physical panels in that location. Use the grid for what it was designed for-- distributing power from generation facilities to consumers. This can include nearby solar farms that will be much more optimized.

      Backup power is a whole nother discussion.

      That said, people have the freedom to do what they want. I get the emotional appeal of having the solar panels on your own roof. Also, it depends on just how much shade we're talking, as people will have different definitions of what "a lot" of shade means, until it's quantified.

      1. gusfhb | | #12

        I do not believe my neighbor has solar panels for their emotional appeal. He saves real money on his electric bill, just not as much as he would if he cut down every one of his trees and his neighbors trees

        1. maine_tyler | | #14

          I think you're missing or misconstruing my point.

          If you are "in a forest" with very minimal hours of full sun, solar output will be abysmal. Look up what even partial shade does to output.

          If off grid, there's little choice but to optimize what you have. If on the grid, I'm simply wondering what the benefit is to installing personal solar in such poor conditions. There's lots of solar going in around the country, and both utility scale and community scale will be constructed where there is much better irradiance and panels will have much better output.

          Oftentimes, installing rooftop panels IS about the emotion. Just my opinion. I'm not saying not to do it, especially when the conditions are right. When the conditions are right, I think it can make a lot of sense.

        2. Expert Member
          BILL WICHERS | | #15

          Keep in mind that "saving money" can sometimes be an illusion. If, for example, you save $10/month on your eletric bill, but you spent $3,000 to install your system, that's a 25 year payoff, just to break even -- and without including interest charges (which make the numbers even worse). I've seen people make that kind of mistake before, and I tell them that is the economics of the system trying to tell you you'd get more for your money if you spent your money someplace else (more insulation, etc.).

          I try to have solar break even around 3-5 years, which is often doable for peak-shave systems if you have an area that gets a good amount of sunlight. If you are situated such that you only get sun part of the day, that will make the payoff period longer for a similarly sized system. I've often found that about 3kw seems to be a pretty good peak shave system in terms of cost/benefit, assuming good sun. If you use microinverters, shaded panels is less of an issue, so you could build a bigger system and "derate" it to get about the same amount of kwh worth of power with your reduced hours of sunlight. The tradeoff in this case is more cost for the same (compared to a "full sun" location) useful amount of power production. Note also that peak shave systems are MUCH less effective in "reduced hours" of sun, since they won't be shaving off your peak load during as many hours of the day, meaning less energy and cost savings over time. Net metering would be better here, if you have it available, or battery storage -- but I really, strongly, advise against battery storage if you have a grid tied system.


      2. Tim_O | | #13

        I don't think community solar projects are very widespread. Ideally it seems like a decent idea, but you'd probably be subject to the utility only paying back a small portion of what the panels you purchase are producing.

        1. StephenSheehy | | #17

          How community solar works varies. In Maine, owning panels as part of a community solar installation is treated by the utility the same as having the panels on your roof. An extra benefit is that if you move, your panels effectively go along, if you're in the same utility area. If you move out of the area, you can sell the panels.

          1. Tim_O | | #18

            That's a great way to do it! I just saw a post about how our local utility is cutting down forest to install solar and at the same time, as a home owner, they only give you about 50% back on solar you feed to the grid.

  8. Tim_O | | #16

    I like PVwatts for calculating output by month. But I'm not sure if it gives you a good idea of shading issues. Google's Project Sunroof seems to estimate sun hours, I think it estimates shading better?

    I wonder if bifacial panels would be good here. They are often times the same price as normal panels, since they weren't subject to the same tariff. I don't know for sure, but my thinking is they would pick up some of the filtered light through the trees. This is for a ground mount only though, they have no benefit on a roof top.

  9. acrobaticnurse | | #19

    Thank you all for the resources. I'd been seeking a way to get a more nuanced explanation than "contact us if/when you've had trees trimmed/removed". When I asked a solar contractor that told me that if they knew someone that could provide guidance on how much trees would need to be trimmed/removed and how much that would improve solar output their response was simply "assuming the image is accurate your shading would prevent any real productivity from occurring."

    To clarify living in a forest, we're nearly surrounded by trees on the edge of a forest but at least half of the roof seems to have direct sun for most daylight hours, so it seems that I could place at least two to four 400 watt panels and have them remain unobstructed most of the time, but maybe there's some level of shading my eyes don't perceive but the solar contractors do when they look at satellite images. Of course this is June so the shade could be more significant the rest of the year, so I look forward to testing out the sunontrack app. The only space in our yard that isn't shaded is used for gardening. Our roof gets more sun than most of our yard.

    I switched my focus to the idea of a simple off grid solar panel setup charging batteries because it would just be for a small amount of power and the Duke net metering agreement in NC is about to get worse with it requiring switching to time of use energy payments for all electricity, lower reimbursement, and higher fees for being grid tied. Energy is cheap here at 8 cents per kwh with an all electric home without solar. Being grid tied may actually raise my electrical bill substantially. My main use for solar would be for when the grid is down so I would need batteries in order to utilize solar even if it was grid tied, and I would likely need a more complex system to support being tied into the grid. 

    It probably makes the most financial sense to skip the solar aspect and focus on having enough batteries to support essentials when wanting to not run a generator but I wanted to see how readily I could put numbers to that assumption. Given how brief power outages usually are here it would likely be cheapest to just be ok running the generator and not even worry about batteries other than a UPS for our modem/router.

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