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Thread: How many batteries do I need?

  1. #1
    Junior Member
    Join Date
    Jul 2012
    Posts
    8

    Default How many batteries do I need?

    I was reading a post in the General section where a poster wanted to build a off-grid system without batteries, and I read that the battery bank has to be matched to the total solar panels being used. I did not know this. So can you answer this question.

    If I use the standard 6V golf cart batteries (Amp Hours), how many batteries would I need for solar panels totaling 1000 watts? And how many batteries for 2000 watts? (I had decided to use one of the two sizes, but I did not know that I had to match it with a certain number of batteries. The number of batteries I will need to buy may effect my decision on the size.)

    Also, I keep reading about 12volt, 24 volt, 48 volt systems. Does it matter which voltage I use assuming I am going to power an DC to AC inverter to run some house appliances. If I understand correctly, the voltage you use only effects the wire size. I could be wrong.

    I really don’t mind buying a book on building a solar system, and I hate posting simple questions like this, but I don’t want to drudge through a 300 page book full of graphs and math. I don’t plan to make a life’s work on the subject. What I need is a 25-50 page book on the subject that will explain it in simple math. I own a volt ohm meter and I know how to read it. I also own a power watt meter which I find handy for checking the load of appliances. Perhaps there is a web site that has the math laid out in a straight forward manner. I did find a few good articles on the Sun and Wind web site.


    Thanks
    Azel

  2. #2
    Solar Fanatic
    Join Date
    Feb 2010
    Posts
    13,434

    Default

    Quote Originally Posted by Azel View Post
    If I use the standard 6V golf cart batteries (Amp Hours), how many batteries would I need for solar panels totaling 1000 watts? And how many batteries for 2000 watts? (I had decided to use one of the two sizes,
    Azel you are asking good questions. OK to answer your question you would not want to use golf cart batteries. But assuming you did the answer depends on what battery voltage and charge controller type you use. At 1000 watts you would only want to use a MPPT type controller. At 12 volts you would not use 6 volt batteries because the Amp Hour Capacity of the 6 volt battery is too low forcing you to use parallel strings. At 12 volts will require a minimum of 640 Amp Hours and a max or 960 Amp Hours. If you tried to do that with 6-volt 225 AH golf cart batteries would require a minimum of 3 parallel strings or 6 batteries. That is a big mistake.
    Quote Originally Posted by Azel View Post
    Also, I keep reading about 12volt, 24 volt, 48 volt systems. Does it matter which voltage I use assuming I am going to power an DC to AC inverter to run some house appliances.
    It makes a huge difference. 12 volt systems are for toys and RV's and should be avoided. As I demonstrated above at 1000 watt panel wattage you would need 6 of the 6 volt batteries. Not only that but one very expensive $800 dollar 80 amp MPPT charge controller. Not only that but some very expensive wire to handle 80 amps of current.

    If you were to say go 48 volt battery at 1000 watt panel power then minimum battery capacity drops to 160 AH and a max of 240 AH. You could now use say 4-12 volt 200 AH batteries which is a perfect fit. It also means you can use a much less expensive 20 amp $250 MPPT charge controller and much less smaller less expensive wire.

    Now with all that said, forget everything I just told you. A off-grid battery system is designed on the maximum amount of power used daily under the worse locale conditions. If you do not do that there is a 99% chance you will fail. The deign dictates what size panels, charge controller, battery, and inverter is sized at. I can just about guarantee you if you go willy nilly design like most that come here it will fail miserably. 99 out of 100 folks who come here going willy nilly discover that it will take 5 to 20 times more equipment than they hoped, or already bought, and find out they wasted a boat load of money and needing to come up with 5 to 20 times more than they already lost. Trust me they do not like to hear that, get mad and leave. All I can say at that point is don't let the door hit you in the butt on the way out.

    So ask the right questions now, or learn the hard way.
    MSEE, PE

  3. #3
    Junior Member
    Join Date
    Aug 2012
    Posts
    1

    Default

    Quote Originally Posted by Azel View Post
    I was reading a post in the General section where a poster wanted to build a off-grid system without batteries, and I read that the battery bank has to be matched to the total solar panels being used. I did not know this. So can you answer this question.

    If I use the standard 6V golf cart batteries (Amp Hours), how many batteries would I need for solar panels totaling 1000 watts? And how many batteries for 2000 watts? (I had decided to use one of the two sizes, but I did not know that I had to match it with a certain number of batteries. The number of batteries I will need to buy may effect my decision on the size.)

    Also, I keep reading about 12volt, 24 volt, 48 volt systems. Does it matter which voltage I use assuming I am going to power an DC to AC inverter to run some house appliances. If I understand correctly, the voltage you use only effects the wire size. I could be wrong.

    I really donít mind buying a book on building a solar system, and I hate posting simple questions like this, but I donít want to drudge through a 300 page book full of graphs and math. I donít plan to make a lifeís work on the subject. What I need is a 25-50 page book on the subject that will explain it in simple math. I own a volt ohm meter and I know how to read it. I also own a power watt meter which I find handy for checking the load of appliances. Perhaps there is a web site that has the math laid out in a straight forward manner. I did find a few good articles on the Sun and Wind web site.


    Thanks
    Azel
    Hello Azel,

    Please drop me an email so I can formally apologize to you for the way you were communicated to by one of my forum members. Martin(at)mjlorton(dot)com.

    Cheers,
    Martin.

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