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  • DIY Solar Setup Voltages

    Hi everyone, I have got 100 4.4 watts solar cells (making 444 watts in total) that I am going to make into 2 solar panels (45 cells each, 27v open circuit voltage). My Grid Tie Inverter accepts voltages from 14-28v, so I would like the incoming panel voltage to be somewhere in between (around 22v operating voltage) which is possible by making 2 200w solar panels, 45 cells each. However, I have a premade 100w solar panel which has 36 cells but only puts out 21.3v open circuit voltage. When the 100w solar panel is connected to the GTI, it drops to 17v, which is abit low for my liking. So my question is, shall i make a 10 cell panel, which will put out 6v and run that in series with the 100w panel, which will give me 27.3v and then run that in parallel with the 2 200w diy panels to create an overall voltage of 27v oc, or shall i create my diy panels to the same specification as the 100w panel? I prefer higher voltage due to better efficiency, but value all of your opinions. Thanks!

  • #2
    welcome.

    First, understand, that advice here is for educational purposes only, as DIY panels and plug in GT inverters are not allowed by Code or Insurance companies in North America.

    A) Bare cells (wafers) are nearly all factory rejects, and seldom live up to spec for extended periods of time.

    B) I don't know why your purchased panel's voltage sags so much, what does the VPmax spec on it's sticker indicate?Sharp208_c.jpg

    C) I'd aim for a open circuit voltage, just shy of the Vmax input of the inverter, exceed that, and you cook the inverter. The inverter should have a efficiency curve - Vin vs Power out, that's the voltage to target your PV panels to. If the inverter pulls the panels output voltage lower than you expect, well, it's non approved junk, and is not likely to be the "best of class".
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      If you are going to parallel mismatched solar panels, you want to keep the Vmp value of the panels within ~5% of each other to allow one MPPT controller to find the maximum power point for the combination and for that combination to be close to the sum of the maximum of the two panels.

      If the ratio of VMP to VOC of the two kinds of panels is very different (it should be close to .83 for mono- or poly-crystalline silicon) then it is a good indication that the cells involved are a different technology or are defective.

      Given the choice of making the VOCvalues match or the VMP values match, definitely try to match VMP.

      The 17 volt output when connected to the GTI could either be the result of a poor MPPT impementation or unusual cells in the panel.
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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      • #4
        That voltage sag sounds about right for that panel and is probably right about Vmp
        NABCEP certified Technical Sales Professional

        [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

        [URL]http://www.calculator.net/voltage-drop-calculator.html[/URL] (Voltage drop Calculator among others)

        [URL="http://www.gaisma.com"]www.gaisma.com[/URL]

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        • #5
          Thanks for all the help, I think i will just make 2 36 cell panels to match the premade panel and then wire them all up in parallel. Thanks, mcdaydavies.

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