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  • Using 2 panels on one micro inverter (numbers not adding up)

    I am looking at using two 205watt 18.2v panels in series per IQ7 Plus micro inverter. (I already have the panels)

    However, in looking at the max continuous output some #s don't add up.
    IQ7+


    (Assuming peak performance of my panels)
    Panels
    (205W * 2) *.97 efficiency for inverter = 389.5W

    Inverters IQ7+
    recommended module pairing 235-440W
    max continuous output - 1.21A for 240V
    1.21A * 240V = 290.4W

    Will loose out on about 100W of power during peek output if I do this configuration?

    I wish the iQ8's were out, anyone know what the output for those will be?


  • #2
    The specs for the IQ7 should tell you how much panel wattage it can handle or if you can wire in more than one panel.

    The IQ7+ may be limited to just under 300watts so using 2 x 200 watt panels would be a waste.

    Second, how do you plan on wiring that IQ7 to? If you try to go with a grid tie system you really need to first get approval from your POCO.
    Last edited by SunEagle; 07-30-2020, 06:47 PM.

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    • #3
      Yes, the IQ7+ specs say 235-440W panels.

      Wiring is not an issue, 2 panels in series into one inverter. I am working on the design for electrical approval right now, that is why I am asking.

      Comment


      • #4
        You're pretty much correct. But it may not be exactly like you're thinking.

        First, the IQ7+ is not 97% efficient at maximum power. It may be 97% efficient at some moderate power but less at full power. They specify it as 96.5% EN 50530 (EU) weighted efficiency or 97% CEC weighted efficiency, which means sometimes higher and sometimes lower. 95% at full power may be more realistic.

        The panels you mention only put out 205 watts when new and at standard test conditions. At hotter temperatures, lower light or other conditions, output will be less. With time, they will reduce output, roughly 1% per year, give or take.

        So what happens in real life? Let's assume a sunny day, no clouds, no trees and nice, cool temperatures. In the morning, the sun begins to rise and you get a little power. As time goes on, you get more power and more power, until high noon, when the panels are capable of their full output. Then, it reverses and the output starts to decay.

        Between 10AM and 2PM, give or take, the panels are capable of putting out more than 160 watts, and you're not using them to their potential. Earlier and later, panel capability is less and your inverter is not limiting you. You'll see a curve something like this (note that this is not a curve of your panels and inverter, just a typical plot of someone's system):

        lost-energy.jpg

        The area in the blue bars represent the power coming out of the inverter. The area of the top red "plate" represents energy that you could have had, if the inverter were larger. Judge for yourself with your particular graph, but in many cases, it isn't a lot of energy lost.
        7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

        Comment


        • #5
          Originally posted by runfastman View Post
          I am looking at using two 205watt 18.2v panels in series per IQ7 Plus micro inverter. (I already have the panels)

          However, in looking at the max continuous output some #s don't add up.
          IQ7+


          (Assuming peak performance of my panels)
          Panels
          (205W * 2) *.97 efficiency for inverter = 389.5W

          Inverters IQ7+
          recommended module pairing 235-440W
          max continuous output - 1.21A for 240V
          1.21A * 240V = 290.4W

          Will loose out on about 100W of power during peek output if I do this configuration?

          I wish the iQ8's were out, anyone know what the output for those will be?
          What size system are you installing? What factor pushed you toward micros?

          Comment


          • #6
            Originally posted by runfastman View Post
            Yes, the IQ7+ specs say 235-440W panels.

            Wiring is not an issue, 2 panels in series into one inverter. I am working on the design for electrical approval right now, that is why I am asking.
            Are you sure you want to wire in series not parallel. Is the Voc temp corrected for max low temp within the specs fo the IQ7?

            ? Why no use 2 micros

            Andy

            Comment


            • #7
              You have some very good comments ... let me add another. ASSUMING your layout supports it, you can get greater harvest by pointing one of the 2 panels somewhat east of south, and one somewhat west of south. You'll have to do the research based on your conditions and location.

              Comment


              • #8
                Originally posted by gbynum View Post
                ......., you can get greater harvest by pointing one of the 2 panels somewhat east of south, and one somewhat west of south. .......
                Despite some viewpoints that suggest the optimal orientation is always due south I tend to agree with your observation. Recently I saw several examples of that while driving the I5 in Central California. These were all ground mounts that had no physical constraint on orientation. I don't know if these were powering some agricultural pumps or nearby commercial building because there were a variety of both.

                Most of the panels in these installations were facing south but up to 25% were facing west. Perhaps it was more favorable TOU rates that was driving this orientation or the owners own use. These all appeared to be grid tied and none were larger that 100 kiloWatts based on my rough count of panels and/or sq. footage.
                9 kW solar. Driving EVs since 2012

                Comment


                • #9
                  Originally posted by gbynum View Post
                  You have some very good comments ... let me add another. ASSUMING your layout supports it, you can get greater harvest by pointing one of the 2 panels somewhat east of south, and one somewhat west of south. You'll have to do the research based on your conditions and location.
                  The "optimal" array orientation and layout depends on the application and also on the climate - including cloud patterns and clearness index of the location.

                  For most, but not all residential applications with grid tied net metering and without a T.O.U. type tariff in effect, if the goal is to minimize the annual electric bill in the most cost effective way (most bang for the buck), that's usually accomplished by maximizing the annual array output per installed STC kW. In such cases there will be one array orientation (array azimuth and tilt) that that per STC kW annual output. For such situations, other orientations will produce less output per installed STC kW. That will decrease the output/installed STC kW and so reduce the array cost effectiveness and so not meet the goal of being the most cost effective design.

                  For most residential net metering applications subject to a T.O.U. type tariff, there will still be one array orientation that minimizes the array cost for a certain level of annual electric bill reduction and that orientation will in all probability be different than the orientation that maximizes array production. That "optimum" orientation will be the one that produces the greatest amount of annual electric bill $ offset per installed STC kW.

                  For most applications, splitting array orientations will reduce annual total array output per installed STC kW and so require a larger total STC array size.

                  In other words, if the goal is to get the most bang for your buck (assuming array cost is generally proportional to array electrical size) splitting an array orientation with the idea that doing so will produce more annual output per installed STC kW will almost always produce the opposite effect.

                  That's not to say that application goals and site constraints may mean splitting an array is necessary or even desirable, but intentionally splitting an array with the idea that it will increase output per installed STC W is most always incorrect.

                  Do this: Do a PVWatts run on 1 STC kW equator facing array of some reasonably optimal tilt (say, at latitude) for any location. Then, do the same for any two 500 STC W arrays of different orientations and combine their outputs.

                  Comment


                  • #10
                    Originally posted by J.P.M. View Post

                    The "optimal" array orientation and layout depends on the application and also on the climate - including cloud patterns and clearness index of the location.
                    You are unquestionably correct. What I meant, and said poorly, was that IF CLIPPING WAS EXPERIENCED from too much panel, directing the panels at other than absolute maximum production to distributed production would give more Wh out of the inverter. Kinda like this COVID "flattening the curve".

                    Comment


                    • #11
                      Originally posted by gbynum View Post

                      You are unquestionably correct. What I meant, and said poorly, was that IF CLIPPING WAS EXPERIENCED from too much panel, directing the panels at other than absolute maximum production to distributed production would give more Wh out of the inverter. Kinda like this COVID "flattening the curve".
                      Unless your system is clipping a lot, more than likely as it ages you will tend to lose production and go below clipping. IMO the best reason to point the panels West instead of South is because a TOU rate makes it better for the homeowner to generate later in the day when the rate is higher.

                      Comment


                      • #12
                        Originally posted by SunEagle View Post

                        Unless your system is clipping a lot, more than likely as it ages you will tend to lose production and go below clipping. IMO the best reason to point the panels West instead of South is because a TOU rate makes it better for the homeowner to generate later in the day when the rate is higher.
                        On orientation, it's most usually not an all one or the other thing. As an example, around here, a 270 deg. azimuth at a 20 deg. tilt array will produce ~ 1,530 kWh/yr. per installed STC kW while a 180 azimuth, 20 deg. tilt will produce ~ 1,740 kWh/yr.

                        BUT, (the less informed often counter, with I'd note usually with some indignation) "because of T.O.U. pricing and timing, it's more cost effective to face an array west @ 270 deg. azimuth."

                        Well, that's probably not true for most applications. I got curious and ran the current and common SDG & E T.O.U. tariff for residential PV customers by hour over a year and combined that with PVWatts hourly outputs in 10 deg. increments for azimuths from 90 to 270 deg. and tilts of 20, 30 and 40 deg. FOR UNSHADED ARRAYS sized for <= 100 % annual usage offset.

                        For my example above, again, provided the array is not oversized or shaded, using PVWatts hourly outputs and current hourly rates for SDG & E tariff DR-SES, every installed STC kW of PV at a 180 deg. azimuth and 20 deg. tilt will produce ~ $479/yr. of "income" that can be used to offset an electric bill, either as a direct offset against usage or banked at the T.O.U. rate for future use. That's a per kWh rate of ~ ($479/yr.)/(1,740 kWh/yr) = $0.27530/kWh.

                        The similar numbers for the 270 deg. azimuth, 20 deg. tilt are 1,530 kWh/installed STC kW and $0.28105/kWh value/kWh generated for a $430/yr. bill offset per installed STC kW.

                        So, for folks in my area on T.O.U, while an unshaded west facing array will give a slightly higher value for the per kWh of generated electricity, the product of (output*offset value/kWh) of the unshaded 180 azimuth, 20 deg. tilt array = that $479/yr. bill offset per installed STC kW, is greater than the $430/yr. bill offset value per installed STC kW for the unshaded 270 deg azimuth, 20 deg. tilt array.

                        As it turns out, south facing is usually better than west facing with respect to annual bill offset.

                        As it works out for my location, the most bill offset on T.O.U happens when the array is at ~ a 205 deg. azimuth and about 30 deg. tilt which gives about a $490/STC kW bill offset/yr., but the $ offset #'s are not real sensitive to change for maybe +/- ~ 20 deg azimuth and maybe +/- 10 deg on tilt on either side of the optimums.

                        Bottom line: For many, if not most orientations of arrays operating under a T.O.U type tariff, unless proved otherwise by some number crunching, an azimuth orientation closer to equator facing will probably be more cost effective than one farther off of equator facing.

                        As for max. output to an inverter going below inverter clipping output as the system ages, at an array degradation loss of, say, 0.3 - 0.5 %/yr., unless an inverter, string or micro, is sized really tight to the array STC, I'd think it would take array degradation quite a while - something like 10 yrs. or so, maybe longer, to knock an array's (or panel's) max. output below an inverter's capacity, especially given how seldom that happens in a reasonably well designed system.
                        Last edited by J.P.M.; 08-07-2020, 09:54 AM.

                        Comment


                        • #13
                          Originally posted by J.P.M. View Post
                          ...........
                          Bottom line: For many, if not most orientations of arrays operating under a T.O.U type tariff, unless proved otherwise by some number crunching, an azimuth orientation closer to equator facing will probably be more cost effective than one farther off of equator facing.
                          ........
                          The array orientations I observed illustrate that point. We will never know what assumptions went into the number crunching of those installations.

                          It all depends on where you are standing.
                          9 kW solar. Driving EVs since 2012

                          Comment


                          • #14
                            Originally posted by Ampster View Post
                            The array orientations I observed illustrate that point. We will never know what assumptions went into the number crunching of those installations.

                            It all depends on where you are standing.
                            I aim to stand on the firmament of knowledge and experience verified by observation, measurement and experiment.

                            I'm writing about using models as design aids that seem to have - at least to my experience - reasonable and verifiable agreement with measurements to be used as a GUIDE in design of residential grid tied PV systems and T.O.U. billing tariffs.

                            If some such application negates an optimum orientation or winds up requiring a split array, such as for limited area or shading, etc., that's one thing. But to go into a design with the idea that splitting an array is the best way to go when it's not necessary or that it'll somehow be more cost effective is starting off on the wrong foot. That, BTW, applies to just about any application, residential, commercial, on/off grid, whatever.

                            The numbers and methods I used in my analysis and the precis I used to describe the method are examples only and meant to illustrate the idea that, contrary to SunEagle's point, the idea that a 270 deg. azimuth is mostly or always a good first choice for use with a T.O.U. tariff in a residential application, a 180 deg. azimuth for those on T.O.U. rates will often and probably yields a greater annual bill reduction per installed STC kW, and so yield a more cost effective installation. AND, the usually greatest annual bill offset is with an orientation that's often relatively close to equator facing. Point is, it ain't a slam dunk and closer to equator facing orientations will probably be more cost effective more often than less equator facing orientations.

                            The same applies to your idea of splitting an array. First off, I'm referring to residential applications, not large ground mounts or ag/commercial installations you seem to be referring to. There may well be good reasons for splitting an array. As I and others harp on, every application is different. I can sure find or think of good design reasons to split an array - such as for ag water pumping and to minimize storage requirements (for both water and power) and associated costs for example.

                            Second, I've got some numbers behind my examples. You point at non residential installations you know nothing about except where they're located and call it confirmation of your unverifiable opinion.

                            Can you tell me how and in what ways any observation of yours that you are referring to illustrates any point(s) relative to what I have discussed in this thread relative to residential applications ?

                            Comment


                            • #15
                              Originally posted by J.P.M. View Post
                              .............
                              Can you tell me how and in what ways any observation of yours that you are referring to illustrates any point(s) relative to what I have discussed in this thread relative to residential applications ?
                              No problem. You specifically said, " unless proved otherwise by some number crunching,"
                              For example if all the available southern facing roof space is taken, then a west facing array may be worth some investigation. In that situation if the cost of an installation divided by the kWh production over a reasonable period of time is less than the cost of energy that would otherwise be purchased it would make economic sense. Simple economics supersedes optimal engineering theory. Do you want a real life example?

                              I made my original comment to support the comment that @gbynum made about considering some other orientations to mitigate clipping before you made any comments on this thread. My analysis does not depend on mitigating clipping but that is a whole other topic worth discussing
                              Last edited by Ampster; 08-08-2020, 12:37 AM.
                              9 kW solar. Driving EVs since 2012

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