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  • 24 volt back up system inverter

    Ive learned a lot from this forum. Im hoping to get a quick opinion from people. I am installing a very small back up system at my very small weekend cabin. The power goes out somewhat frequently. I want to run my small water pump and a few other very small things during short power outages. The cabin is surrounded by trees. However, I have a shed that gets full sun all day. Its in the Colorado mountains so I get a lot of very strong sun. I will be installing 2x100 watt panels on the shed that charge two 12 volt 50 amp hour batteries. I have a 12/24 amp 30 amp PWM controller (Renogy Adventurer). The problem is the batteries and inverter need to be about a 55' run from the panels. All I have left to buy is my inverter and the wire from the panels to the charge controller. My problem is do I buy a 12 volt or 24 volt inverter?

    -If I run the two panels in parallel and keep the system at 12 volts I have about a 10% voltage drop at peak power over that 55' run (10 gauge wire, 12 volts, 12 amps). But, it is very easy to find cheap, reliable, 12-volt inverters. I could run 8 gauge but its still 7%.
    -If I run the two panels in series and make it 24 volt I have about a 3% voltage drop over that run (10 gauge wire, 24 volts, 6 amps). But, Im having a harder time finding a 24-volt inverter. They seem to be a lot more expensive and dont get very good reviews.

    1. Am I thinking about this correctly? This is my first time working with solar.
    2. How big of a deal is the 7%-10% voltage drop in a very small back up system like mine?
    3. Do you have any suggestions for an affordable 24 inverter? Probably 1500 watt continuous.

    Thanks! -Will

  • #2
    You size the inverter by
    1) size of batteries and PV array that can support it
    2) size of expected loads, plus 25%. If your Lights, fan and TV come up to 500w, you need a 625W inverter (roughly).
    The larger the inverter is, the more power it draws from the battery when idle. If you go too large (1500w) the inverter idle loss becomes more than your regular loads.
    3) Pumps. AC pumps need : Pure Sine Wave and Large enough inverter to manage starting surge and PF losses. Motors over 1/10 hp often start to require a larger system. Maybe a DC pump designed for RV use might be best.
    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 have grid power you wouldn't need solar power to keep a back up battery bank charged. Get an AC battery charger/maintainer. Your batteries will remain fully charged and when the power goes out for those short outages turn on the inverter. Don't connect it directly to your mains power. Keep the inverter separate.
      2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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      • #4
        The 55 feet might be a problem for saving money. Hundred-Foot is the longest mc4 cables come in ,I've heard. Also, the appliances you seem to want to run all come in DC versions to save on the conversion. And not need an inverter except for special purposes.

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        • #5
          If you have AC power, you have no use for solar being a huge waste of money and time.

          But I will answer your questions and ask you a couple of questions you will not like but intended to make you think.

          1. Am I thinking about this correctly? This is my first time working with solar.
          2. How big of a deal is the 7%-10% voltage drop in a very small back up system like mine?
          3. Do you have any suggestions for an affordable 24 inverter? Probably 1500 watt continuous.
          A1. No because you know nothing about solar or energy and leading you to the wrong conclusion. Example if you have AC, solar is the last thing you want to use to do what you want. You would just buy an inexpensive properly sized AC battery charger, battery, and Ineverter to do what you want without all that expense and wasted space for equipment you never use. Once your batteries arre charged, you panels collect dust and take up space.

          A2. In you application absolutely no value what so ever. It only becomes important if you can utilize most of the power the panels can generate in a day. All it means to you is how long it take to recharge and how much are you willing to pay for a fast recharge with solar. Why on earth would you care if it takes a 1 day,or 2 days to recharge? So why would you pay up for it?. Save that money and send it to me. I will put it to good use.

          A3. Certainly would not be 1500 watts at any voltage. Your toy size batteries can only supply a toy sized Inverter. Makes no difference what voltage Inverter you select because your batteries can only handle 150 to 200 watts maximum . Have no clue how you came up with 1500 watts on a 12 volt 100 AH battery, or 24 volts @ 50 AH battery. Must of learned that from a communist because that is how you start fires and burn your cabin down.

          Stick around, I will save you $400 to $800 with a system that exceeds your expectations.
          Last edited by Sunking; 09-30-2018, 04:01 PM.
          MSEE, PE

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