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  • Redesign small off grid/grid system

    Hi guys. I m looking for advise to redesign my off grid and probably include grid system.

    Currently running on 12V 6panel total 180w. With PWM and 2x12V 65AH battery. All connect in serial to run on 24V.

    Uasage is around 80-70% of the battery. So half a day, the battery will fully charge up. The rest of the day, the panel is sitting duck.

    I try to redesign the system to include the cheap china made Grid tie inverter 300W. So I can pump the remaining power to the grid after the battery full. Will add in another 2x 33V, 144W panel.

    I already have all the parts in the diagram. Will this design work ?



  • #2
    Will it work - somewhat.
    is it efficient - Not at all.

    The 3 big problems are
    PV panel mis-match. the Lowest voltage panels (144w @ 33V) will pull the higher voltage panels ( 60w @ 36.4v) to the lower voltage. These are actually pretty darn close, but you loose a bit.
    Parallel Arrays You really should have a combiner box when there are more than 2 strings in parallel. You have 4 and are planning a 5th. Combined with different Imp on the strings, this is the DANGEROUS part, of several strings being able to cause a fire on a faulty string, Each panel usually has a series fuse spec. Since these strings vary from 1.6 to 4.4 amps, fuses will likely be required, getting DC breakers in 2A and 6A sizes will be pretty tough You may want some 10A DC breakers to act as "switches" if there would ever need to be any troubleshooting done on the array. (I would install them)
    2 MPPT controllers on 1 array. The 2 controllers (grid inverter and charge controller) will fight each other, trying to determine the arrays' Max Power Point. Neither will be successful and your harvest will suffer.

    Additionally, your GT inverter is very likely not UL certified, and if there is ever a grease fire in your kitchen, and the insurance inspector sees an unapproved device, you will loose your fire insurance coverage, Woe be if there are any injuries from an electrical fire,

    Minor - your small inverter will eventually backfeed the grid at some point, and trigger an alarm via the PoCo Meter, Then you are busted for an unapproved backfeed device and your Grid power is cut off.

    Yes, I'm harsh, loosing your grid is harsh, and fire is harsher. Just the facts
    Last edited by Mike90250; 09-21-2016, 10:37 PM.
    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
      Thanks for your advise Mike90250.

      I will take note on the safety to include the fuse for the each panel. Currently the 3 string in parallel have a 30A DC breaker on the main wire. (as a switch).

      I did notice the panel mismatch and some inefficiency on the design system. But since I combine with grid tie, the small loose is nothing. If I design mainly for off grid, I will combine the 3 string parallel

      I did run the uncertified grid tie inverter for about a year. Before reconfigure to off grid. It is kind of boring without somekind of off grid project. But soon realize half a day, the panel is sitting duck. That's why come out such an idea of combining both together.

      Since both 2 MPPT will be fighting each other, I shouldn't combine both and takeout the inverter. But it comeback to the square. What should I do with the excess/remain power/day that panel can generate ?

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      • #4
        I just hope you are not using that cheap china grid tie inverter in the US because it is both dangerous and illegal. More than likely your POCO will find out about it and cause you problems.

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        • #5
          Originally posted by SunEagle View Post
          I just hope you are not using that cheap china grid tie inverter in the US because it is both dangerous and illegal. More than likely your POCO will find out about it and cause you problems.
          No. Not in US.

          I m from Malaysia.

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          • #6
            Originally posted by cheamc View Post

            No. Not in US.

            I m from Malaysia.
            Well you still have your local POCO to worry about but the bigger issue I see is the mismatch between the panel Imp ratings.

            The Vmp are pretty close between the 2 series 30 watt (36.4v) and the single 144w panel (33v) but the Imp for the 2 x 30w is 1.65a compared to 4.4a for the 144w. That is a big difference and may overload that cheap grid tie inverter when your batteries are totally charged.

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            • #7
              That system could possibly send ~40 amps to the batteries. They won't live long at that high of a charge rate. You need about a 400AH battery bank.

              No telling how the inverter and charge controller will act being paralleled like that. A better solution would be to install the inverter as a dump load on the batteries with the proper dump load controller. The inverter should also be large enough to handle all the arrays wattage too.

              WWW

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