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  • Solar questions on off-grid system: Newb!

    Hello to all... I'm new to the forums, and trying to understand the basics here. I know the word "newb" or "newbie" probably makes you shriek...sorry if it does I've been searching the internet for about 3 days, reading here and there, but have a long ways to go. Don't know much about Solar, but trying to learn what I can. This is a theoretical setup that I've come up with so that I can see if I have at least a little bit of understanding, based on what I've been reading in different places the last three days. This forum seems to know more about solar than any other place I've visited, so I thought I would join, ask questions, and hopefully be able to help others as well. I live in N.W. Arkansas, and read several places about the area making a difference in your calculations. I checked out a few links, but I'm so new at this, I have a migraine from trying to understand it. Anyway, thanks for having me, and thanks for having a place around for peeps to come and learn more about solar.

    12V with 105 Amps (X2)Hooked together would allow for 24V with 105 Amps24 X 105 = 2520 Watts Per Hour

    Now I

  • #2
    First you are making some assumptions that are not true. For one I assume you live in Wallyworld AR and in Winter your Solar insolation is only 3 Sun Hours. So with a 250 watt panel you are only going to generate 500 watt hours per day of usable energy. Well that is if you use it.

    For a off-grid solar system it is designed based on the worse case conditions, and your average 24 hour usage. During that 24 hours the battery is to be discharged no more than 20%, not the 50% you quoted.

    Last thing I do not think you are considering is cost. Using your examples, would yield you 500 watt hours per day of usable electricity. Electricity in Wallyworld is dirt cheap of around 9 to 10 cents per Kwh. So your example only generates 4 to 5 cents worth of electricity per day. To build the system you used in your example is going to cost you roughly $1100 with $350 of that in a battery that needs replaced every 5 years if discharged 20%/day. Based on 5 year battery replacement cost for the first 5 years instead of paying SWEPCO 10-cents per Kwh, you will be paying $1.21/Kwh or 1200% more for electricity. Not a very good decision and use of capital. In reality there is no way to utilize the energy a battery system is capable of generating each day so the cost are actually much higher.

    Read this thread and then come back with Questions.
    MSEE, PE

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    • #3
      Congratulations, you have not been greenwashed. Yep, you are getting it, lots of panels for quick recharge, and big batteries to keep it going all night.

      First thing though, you figure out what your LOADS are, instead of getting batteries first.

      Calculate the loads

      Size the batteries, for daily discharge of only 20% (80% unused)

      Size the charging (generator and solar) to keep the batteries above 50% , or get an account with
      the battery store to buy replacements often.
      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

      Comment


      • #4
        Thank you! I feel better that I wasn't too far off. I don't want to invest or plan for anything until I fully understand how things work so that I don't under prepare for what I want. I'd rather have more than less; especially if one wants to add to as they go along.

        So... Is it possible to have the 2 12V batteries and add one more? Or do I have to do the batteries in increments of 24V since it will be setup as 24V? I guess you can't run two parallel and then one serial?

        Thanks for the suggestions. I will re-evaluate and try again as well

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        • #5
          With batteries when it is time to upgrade you use the old ones for boat anchors and replace them. You do not mix old batteries with new. Battery lifetime depending on the quality of the product, and care you give them need replaced every 1 to 7 years, with 3 years being average. Being a newb you will likely destroy your first set in a year or less.
          MSEE, PE

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          • #6
            Originally posted by Sunking View Post
            With batteries when it is time to upgrade you use the old ones for boat anchors and replace them. You do not mix old batteries with new. Battery lifetime depending on the quality of the product, and care you give them need replaced every 1 to 7 years, with 3 years being average. Being a newb you will likely destroy your first set in a year or less.
            YAY! lol! Knowing me, I'll fry them upon trying to put it all together

            And yes, I was thinking that earlier. So I decided to go with like 4 (6v) 225a = 5400w/hr = 24V system. I'm configuring numbers now, so we'll see how unlearn'ed' I is

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            • #7
              Originally posted by Mike90250 View Post
              Congratulations, you have not been greenwashed. Yep, you are getting it, lots of panels for quick recharge, and big batteries to keep it going all night.

              First thing though, you figure out what your LOADS are, instead of getting batteries first.

              Calculate the loads
              I am doing this as a very expensive learning process. And basically for something in a power outage situation. If have have 1500 watts to play with, then I won't plug in anything that will drain the battery during the outage. I know I'll be able to use some LED lights and have some type of lighting in the house for when we have no power at all. To have just a little electricity when there is no power at all will be a blessing. And I know that using a gas generator would be WAY more economical, but I'm thinking that I might not have the gas available living where I live, so this would be the alternative option for me. I can play with about $4,000, so I won't get too extreme. Thanks for any and all help you guys can provide. I am extremely new to this, but learning fast...I think

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              • #8
                You wouldn't use gasoline for a generator. You would use NG, LPG, or diesel.
                MSEE, PE

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                • #9
                  Okay, here is what I have come up with:


                  4 (6v) 225a = 5400w/hr = 24V system

                  At 30% usage to maintain battery life, it leaves 1620w/hr

                  To charge battery back up to 100% from using 1620w/hr

                  2 (250w) Solar Panels

                  500w

                  Comment


                  • #10
                    Originally posted by Xcalibre View Post
                    So... Is it possible to have the 2 12V batteries and add one more? Or do I have to do the batteries in increments of 24V since it will be setup as 24V?
                    If you want to add batteries add entire series strings. In other words, if you have 2 12V batteries to give you 24V, add 2 more batteries at a time to add series strings. Don't mix old and new batteries in the same string.

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                    • #11
                      Originally posted by billvon View Post
                      If you want to add batteries add entire series strings. In other words, if you have 2 12V batteries to give you 24V, add 2 more batteries at a time to add series strings. Don't mix old and new batteries in the same string.
                      Incorrect Sir. You do not want to add a new string in parallel with a old string. The old string will drag the new string down. Impedance will be too far off. The older string internal resistance will be considerable higher than the new string, forcing the new string to do all the work.
                      MSEE, PE

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