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Varying output between panels - is parallel best?

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  • Varying output between panels - is parallel best?

    I'll be mounting 4-6 100W panels on the roof of my RV. The RV roof is somewhat curved, and space and shade (A/C shadow) limitations prevent long consistent arrays.

    This is a 12v battery system with a 150v max input MPPT controller - so pretty much all options are on the table.

    So given that the panels will likely be of varying orientation and varying inclination, would you recommend running them all in parallel? I was somewhat arbitrarily thinking of running them in 2-3 panel sets of 24-36v... but that was mostly due to wiring size simplification (2 8AWG runs from the roof). But if there was a significant benefit, I could wire them up with each panel in parallel. I would cost me another $60 in fuses.... while an extra 100W panel costs about $125.

    Tradeoffs... Just curious if anyone had any opinions. Solar's tough on the interwebs... lots of conflicting opinions.

  • #2
    Originally posted by chilly View Post
    I'll be mounting 4-6 100W panels on the roof of my RV. The RV roof is somewhat curved, and space and shade (A/C shadow) limitations prevent long consistent arrays.

    This is a 12v battery system with a 150v max input MPPT controller - so pretty much all options are on the table.

    So given that the panels will likely be of varying orientation and varying inclination, would you recommend running them all in parallel? I was somewhat arbitrarily thinking of running them in 2-3 panel sets of 24-36v... but that was mostly due to wiring size simplification (2 8AWG runs from the roof). But if there was a significant benefit, I could wire them up with each panel in parallel. I would cost me another $60 in fuses.... while an extra 100W panel costs about $125.

    Tradeoffs... Just curious if anyone had any opinions. Solar's tough on the interwebs... lots of conflicting opinions.
    To the extent that absolute efficiency is important to you, most if not all CCs work slightly better if the panel voltage is not too far from the battery voltage.
    But I do not think that the difference between 36V and 18V is going to be great enough to justify the extra wiring and complexity.
    The one factor that may still influence your choice is whether you expect there to be shading on some of the panels during peak production hours.
    If so, then there is an additional advantage to putting the panels in parallel, in that shading one panel will not reduce the output of the other panels.
    But by the same token, a partially shaded panel in a string of two or there may still contribute half of its rated power, while on its own the output voltage will be below the operating threshold of the CC.
    But, again, if you have two strings of three and one of them has even one shaded panel, you may not get anything from the whole string if it is paralleled with and unshaded string into the same MPPT input.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      I think I understand some of what your saying. It's dawning on me somewhat how the parallel and series interactions work - and looking at the IV curve.

      I guess each individual panel under any given conditions (sunlight, shade, angle, temp, etc.) will have a different IV curve from a same panel in different conditions. The less ideal the conditions, the lower the curve.

      When you have multiple panels in series, each panel will see the same current but will have differing voltages (even if only slightly different). If one of the panel's IV curve is low enough, it's Isc could be low enough to cause the good panel on the string to be current throttled.

      When in parallel, all panels see the same voltage. So a panel in suboptimal conditions will have the lower IV curve, and should just have a very low current - but shouldn't affect the other panels.

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      • #4
        Originally posted by chilly View Post
        I think I understand some of what your saying. It's dawning on me somewhat how the parallel and series interactions work - and looking at the IV curve.

        I guess each individual panel under any given conditions (sunlight, shade, angle, temp, etc.) will have a different IV curve from a same panel in different conditions. The less ideal the conditions, the lower the curve.

        When you have multiple panels in series, each panel will see the same current but will have differing voltages (even if only slightly different). If one of the panel's IV curve is low enough, it's Isc could be low enough to cause the good panel on the string to be current throttled.

        When in parallel, all panels see the same voltage. So a panel in suboptimal conditions will have the lower IV curve, and should just have a very low current - but shouldn't affect the other panels.
        You are getting closer. But what you need to know is that there are two ways in which light level affects the IV curve.
        1. Lower, but still uniform over the panel, light level reduces the current values Isc and Imp, but has almost no effect on the voltages Voc and Vmp.
        2. Local shade on one cell of the panel (or up to an entire bypass diode section) will not affect Voc, but will cause Vmp to drop by the voltage of the bypassed string plus the voltage drop in the bypass diode. Because of the highly non-linear effect of the bypass diode, the curve between Vmp and Voc will be very strange compared to the normal uniform light curve.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Ah, very interesting distinction! Thanks for the info.

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          • #6
            Hi Chilly, I'm doing the same thing, and have a curved roof Nomad Trailer. I'll be using 3X Kyocera 130W panels all in parallel. The nice thing about these is that they have 6' cables on them which all reach to my refer roof vent, so I can then run the individual wires to a panel/box above the refer which is where my Panel breakers will be and where I will connect them all. Any shading will be dealt with by the controller.

            The only thing I'm on the fence about is whether to run the individual outputs to the MPPT charger to the front cabinet of the trailer, so the run from the controller to the battery is shorter, or put the charger (Midnight Kid) right up there above the refer and run a larger gauge cable to the battery bank. I'll be around 26A, 16' run and not terribly worried about a little loss, but the Kid is designed for 10AWG max conductor size...

            I have a lot of good 10AWG cable, but would have to buy something bigger if I wanted to minimize losses with a longer run and using a larger gauge cable somehow...
            House-Sun Earth Hot Water.
            RV-390W Kyocera, Kid.

            Comment


            • #7
              Originally posted by ZoNiE View Post
              Hi Chilly, I'm doing the same thing, and have a curved roof Nomad Trailer. I'll be using 3X Kyocera 130W panels all in parallel. The nice thing about these is that they have 6' cables on them which all reach to my refer roof vent, so I can then run the individual wires to a panel/box above the refer which is where my Panel breakers will be and where I will connect them all. Any shading will be dealt with by the controller.

              The only thing I'm on the fence about is whether to run the individual outputs to the MPPT charger to the front cabinet of the trailer, so the run from the controller to the battery is shorter, or put the charger (Midnight Kid) right up there above the refer and run a larger gauge cable to the battery bank. I'll be around 26A, 16' run and not terribly worried about a little loss, but the Kid is designed for 10AWG max conductor size...

              I have a lot of good 10AWG cable, but would have to buy something bigger if I wanted to minimize losses with a longer run and using a larger gauge cable somehow...
              Since it is an MPPT type charger the wiring from it to the batteries could see a higher amp load then the wiring from the panels to the charger. I would think you want to keep the wiring with the higher amp load shorter.

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              • #8
                Zonie, if your controller is limited to 10AWG (although you could fairly easily step that up), it probably does make sense to keep it as close to the batteries as is feasible. If you have a lot of extra 10AWG, you could run the parallel panels farther independently instead of joining them up on the roof - just run 3 10 AWG and join them near the controller. You know your panel current is never going to be *less* than the battery current, no matter what your config, so seems like a good way to go. If I'm thinking correctly - you could see more than 25A in ideal conditions too.

                I wish I had more room around that area - mine's on my bedroom wall so I can admire it while I sleep! It want's 6" clearance for heat exchange, and I didn't want to sacrifice the only closet with hangars I have.

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                • #9
                  Originally posted by SunEagle View Post
                  Since it is an MPPT type charger the wiring from it to the batteries could see a higher amp load then the wiring from the panels to the charger. I would think you want to keep the wiring with the higher amp load shorter.
                  Yep.


                  Originally posted by chilly View Post
                  Zonie, if your controller is limited to 10AWG (although you could fairly easily step that up), it probably does make sense to keep it as close to the batteries as is feasible. If you have a lot of extra 10AWG, you could run the parallel panels farther independently instead of joining them up on the roof - just run 3 10 AWG and join them near the controller. You know your panel current is never going to be *less* than the battery current, no matter what your config, so seems like a good way to go. If I'm thinking correctly - you could see more than 25A in ideal conditions too.

                  I wish I had more room around that area - mine's on my bedroom wall so I can admire it while I sleep! It want's 6" clearance for heat exchange, and I didn't want to sacrifice the only closet with hangars I have.
                  Yeah, that is more than likely what I will do, But that will put the thing next to my bed too... on the overhead cabinet above the be d that I bang my head on all the time...
                  House-Sun Earth Hot Water.
                  RV-390W Kyocera, Kid.

                  Comment

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