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  • New to solar, easy question for most

    I would like to run continuous 20 watt led lights outside during the night, probably 4. could someone tell me what i need for total watt panels and batteries. I have very good sun exposure

    Thanks Deano

  • #2
    Hello dinows and welcome to Solar Panel Talk.

    Running 4 x 20watt lights at night for say 8 hours will consume at least 730 watt hours (20w x 4 x 8hours x 1.15 = 736wh)

    A 12volt battery system that can safely deliver 750 wh a day would be around 250Ah to 300Ah.

    To understand a little more I would suggest you read some of the stickee posts in the off grid area of this forum.

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    • #3
      Thanks, I have been reading up on solar from the posts but a little bit tough at first. I am making this my first project so if i got a little help on it I think I could move along a bit faster. So could you tell me how many watt panel I need and how that fits into the equation ? Thanks in advance

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      • #4
        Originally posted by dinows View Post
        Thanks, I have been reading up on solar from the posts but a little bit tough at first. I am making this my first project so if i got a little help on it I think I could move along a bit faster. So could you tell me how many watt panel I need and how that fits into the equation ? Thanks in advance
        Another quick calculation to determine the amount of panels watts would be to start with using 1/10th the Ah rating of a FLA battery system as your required charging amps from the pv.

        So if you have a 300Ah battery system you would need about 30amps for charging. Using an MPPT type charge controller you would then multiple the amount of amps times the battery voltage to determine a starting point for panel wattage. ie. 30A x 12v = 360watts.

        Now that is just a starting point because depending on where you live. The shortest time is usually in the Winter months where more time is available to receive good sunlight in the Summer months. The farther North you live will also reduce the number of useful sunlight hours for charging.

        You need to generate more Watt hours from the panels then you use from the battery system to make sure you put back what you take out minus any losses in the charging circuit.

        The biggest issue we find with people sizing their batteries is underestimating the amount of daily watt hour usage or over estimating the number of "usefull" sunlight hours to recharge the batteries. Usually either mistake will kill off your batteries quickly.

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        • #5
          https://www.solarpaneltalk.com/forum...battery-design

          go through that link. Keep with 12 volt.

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          • #6
            Originally posted by cebury View Post
            https://www.solarpaneltalk.com/forum...battery-design go through that link. Keep with 12 volt.
            Ah yes. The 'learning sticky' where it is proposed that the correct way to charge FLA batteries is at a 500 Ah / 30 A = C/16.7 rate. I remember it well.

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            • #7
              Originally posted by AzRoute66 View Post
              Ah yes. The 'learning sticky' where it is proposed that the correct way to charge FLA batteries is at a 500 Ah / 30 A = C/16.7 rate. I remember it well.
              Well IMO a C/16.7 rate is too slow to do the job. Especially if the battery is 500Ah which is huge and you need to kick it in the side or it will sulfate the plates. Even a C/12 is at the low end of the charging rate for a standard FLA.

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              • #8
                Originally posted by SunEagle View Post

                Well IMO a C/16.7 rate is too slow to do the job. Especially if the battery is 500Ah which is huge and you need to kick it in the side or it will sulfate the plates. Even a C/12 is at the low end of the charging rate for a standard FLA.
                Tell the author of the sticky.

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                • #9
                  Originally posted by AzRoute66 View Post

                  Tell the author of the sticky.
                  I went through the link to the sticky that cebury posted and did not see anywhere it mentioned a C/16.7 rate.

                  Also per the author of that sticky who has stated many times, an FLA system should be charged somewhere between C/8 and C/12 so a C/16.7 rate would be way too slow.

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                  • #10
                    I think he's referring to Sunking's suggestion of a 30A charge controller, which with a 500AH battery would give the C/16.7 rate. But it might be an oversight because as you said he doesn't suggest less than C/12. We'll have to ask him about that.

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                    • #11
                      Originally posted by sdold View Post
                      I think he's referring to Sunking's suggestion of a 30A charge controller, which with a 500AH battery would give the C/16.7 rate. But it might be an oversight because as you said he doesn't suggest less than C/12. We'll have to ask him about that.
                      Ok I can see where there might have been a misunderstanding by AzRoute66 concerning what type of charge rate is needed.

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                      • #12
                        Originally posted by SunEagle View Post
                        Ok I can see where there might have been a misunderstanding by AzRoute66 concerning what type of charge rate is needed.
                        What was the misunderstanding? I didn't advocate a C/16.7 charge rate, SunKing did. Right there in the sticky where beginners are sent to get the basics.

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                        • #13
                          Originally posted by AzRoute66 View Post
                          What was the misunderstanding? I didn't advocate a C/16.7 charge rate, SunKing did. Right there in the sticky where beginners are sent to get the basics.
                          Maybe we are looking at 2 different sticky posts because I do not see anywhere it mentions "C/16.7" charge rate.

                          I do see where Sunking states that you can use a 30Amp CC but that is in the part determining the charging amps for a 1400watt pv & 48v battery system. It does not mention using that 30amp for a 500Ah system but I can see how you presume that is what SK was working toward.

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