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  • Solar panel estimates / pay back period

    We've gotten several estimates. First company (Paradise Energy Solutions) offers a 10 year guarantee on production, so I'm inclined to believe them. Here are the specs on the proposed system:

    Proposal #1:
    system size 11.31 kW
    39 Pelmar 290 panels
    1 SolarEdge 7600H inverter
    39 SolarEdge P370
    Total cost - $33k (before credit)

    Proposal #2:
    System size 7.7 kW
    23 LG 335w panels
    Enphase micro inverters
    Total Cost - $22k (before credit)

    The first system is estimated to produce 9489 kWh a year. This is factoring in 36% shade reduction due to trees that can't be removed. The cost per watt of the system is 2.98 cents (before credit). But the payback on the system is almost 20 years.

    The second system is estimated to produce 9600 kWh a year. The cost per watt of the system is 2.9 cents (before credit). But I'm skeptical of the production numbers because they don't seem to account for any shade reduction. And they don't have any kind of guarantee, so he has no stake in the game. He said ("summer would be no problem, winter would be some shading...."), so that's why he said to go with micro inverters to improve output. The payback would be just under 10 years.

    I don't think anybody would suggest investing in something that has a 20 year payback.

    Any advice?

  • #2
    You can use PVWatts to make your own assessment of production potential, and adjust the output as you think it would be appropriate for shade. As a starting point, select "premium" panels and 10% loss.

    A SolarEdge system and microinverters will handle shade equally, so neither system has an advantage there.

    11.31 kW of panels on a 7.6 kW inverter is an unusual combination. Are the panels split between east and west facing slopes, or is there some other reason that the mid-day peak is expected to be low?
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #3
      Originally posted by johnlantz View Post
      The second system is estimated to produce 9600 kWh a year. The cost per watt of the system is 2.9 cents (before credit). But I'm skeptical of the production numbers because they don't seem to account for any shade reduction. And they don't have any kind of guarantee, so he has no stake in the game. He said ("summer would be no problem, winter would be some shading...."), so that's why he said to go with micro inverters to improve output.
      A few points.

      1) Microinverters do indeed help with shading issues. If 10% of your array is shaded you'll lose a similar (or slightly greater) percentage of output, as opposed to losing an entire string with a conventional string inverter. So in your case they may well help. (Note that both the proposed systems will accomplish this.)

      2) When he emphasizes summer he is alluding to the fact that most of your production will come in the summer. So losing 50% of 3000kwhrs means a lot less than losing 50% of 6600kwhrs. And in most net-metering schemes, it is yearly production that matters.

      All that being said, the best way to figure out shading issues is with a tool like the solar pathfinder, which will tell you how much shading you will see during what times of the year. It will identify what's going to be blocking sun when, and that will allow you to make better decisions on what (if any) trees to fell and how much energy you will get overall.

      As to payback period that's a complex question and depends on not only yearly production but also on how your power bill is structured and how you use power. Will a small system move you down one tier if you are on a tiered plan? Then it's going to generally be money well spent. Are you on a TOU plan? If so generation during peak times will help even more. Will you or your family decide "woohoo! our power is free now!" and use a lot more power? Then you're not going to see a fast payback.

      Comment


      • #4
        Originally posted by jflorey2 View Post
        A few points.

        1) Microinverters do indeed help with shading issues. If 10% of your array is shaded you'll lose a similar (or slightly greater) percentage of output, as opposed to losing an entire string with a conventional string inverter. So in your case they may well help. (Note that both the proposed systems will accomplish this.)

        2) When he emphasizes summer he is alluding to the fact that most of your production will come in the summer. So losing 50% of 3000kwhrs means a lot less than losing 50% of 6600kwhrs. And in most net-metering schemes, it is yearly production that matters.

        All that being said, the best way to figure out shading issues is with a tool like the solar pathfinder, which will tell you how much shading you will see during what times of the year. It will identify what's going to be blocking sun when, and that will allow you to make better decisions on what (if any) trees to fell and how much energy you will get overall.

        As to payback period that's a complex question and depends on not only yearly production but also on how your power bill is structured and how you use power. Will a small system move you down one tier if you are on a tiered plan? Then it's going to generally be money well spent. Are you on a TOU plan? If so generation during peak times will help even more. Will you or your family decide "woohoo! our power is free now!" and use a lot more power? Then you're not going to see a fast payback.
        I'd find a better way to express that shading penalty. 10% of what ? Array area? And at/over what time(s) - a day ? - a month ? - a year ? Without getting too far into particulars, maybe describing the shaded output loss as 10% of unshaded output or a 10 % reduction in POA irradiance over some time period, and including over what time period those collective losses occur might be more useful and descriptive. 10% array shading around midday at low zenith angles (summer) is going to produce more loss than 10% shading in late afternoon at high zenith angles (winter).

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        • #5
          OP: You need more information before you go running off and probably getting yourself more confused by running software without understanding the input, much less the output and what it means or what to do with it, or by paying attention to what others write before you understand what they are talking about.

          Buy and read a copy of "Solar Power Your Home for Dummies". 20 bucks at bookstores or amazon. Read the book and, for starters, you'll quickly see that those two vendors are not on the same page.

          Doing so will also allow you to discover that PV system selection, including system sizing and equally important vendor selection, while not terribly complicated, are areas you need more and basic information to get the basics and beyond to help you in making better informed and therefore hopefully wiser decisions.

          Comment


          • #6
            I have found the follow resources in addition to PVWatts good for sanity checks especially for non-experts:

            * https://www.google.com/get/sunroof - very simple tool to get an idea of what kind of sunlight your roof can expect
            * pvoutput.org - good way to check real system outputs in your area

            Comment


            • #7
              Originally posted by solardreamer View Post
              I have found the follow resources in addition to PVWatts good for sanity checks especially for non-experts:

              * https://www.google.com/get/sunroof - very simple tool to get an idea of what kind of sunlight your roof can expect
              * pvoutput.org - good way to check real system outputs in your area
              Suit yourself. The first source is a sales pitch and has little useful or helpful information. The second is useful for comparison and many posters are aware of it and are on it, but it only gives system outputs results and provides little in the way of basic and necessary information about how to design a system or size it.

              Suggest you get and read the book. Your solar ignorance is making you vulnerable to a poor outcome.

              Comment


              • #8
                Originally posted by J.P.M. View Post

                Suit yourself. The first source is a sales pitch and has little useful or helpful information. The second is useful for comparison and many posters are aware of it and are on it, but it only gives system outputs results and provides little in the way of basic and necessary information about how to design a system or size it.

                Suggest you get and read the book. Your solar ignorance is making you vulnerable to a poor outcome.
                I didn't suggest skipping the book just adding a couple simpler tools that the OP may not be familiar with. I have read the free PDF download of the book and it's good but there are simpler (not perfect) but still useful tools for beginners than a 400 page book or SAM type of tools.

                Comment


                • #9
                  Originally posted by solardreamer View Post

                  I didn't suggest skipping the book just adding a couple simpler tools that the OP may not be familiar with. I have read the free PDF download of the book and it's good but there are simpler (not perfect) but still useful tools for beginners than a 400 page book or SAM type of tools.
                  Get the right tool for the right job. Most of what's in your first source is already available from NREL without the pitch or giving someone your info to sell off as a lead for the peddlers.. I'd skip that entirely and go to the source of the data, and a lot more useful info, that is, NREL, but I'd start with the book for an overview of Residential PV.

                  As for the book and all the information in it, it doesn't need to be absorbed in its entirety at one sitting. One can read a bit at a time. You eat the elephant one bite at a time.

                  The book, BTW, is no longer available as a free download. This isn't a pitch, but it was a good free source, even if the PDF was an outdated version. 20 bucks for the updated ed. is still a bargain.

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