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  • #16
    Originally posted by monicaj View Post
    "The wire between the panels and charge controller need to be rated about 1.25% greater than the Imp rating of one of those panels, and sized not to exceed a 3% voltage drop."

    Sorry for the way I'm about to mangle terms...

    So, let me see if I understand the first part of that (Imp). I have 4 panels, all have 79.5A as the Imp rating, and since they'll be connected in series, that Imp figure doesn't change. If I split them and connect 2 in parallel, and then connect those together in series, am I right that the Imp rating of the system would be 79.5 x 2? And the Voc of the system would be 44.8 x 2?

    I'm going to connect them all in series and make it a 24v system as recommended but wanted to ask that only to see if I'm understanding how things work.

    Imp "of the system", and Voc "of the system"... is that the right term to use?
    Ah no. The Imp of each panels is 7.95 amps. 2 of those panels wired in series still have an amp rating of 7.95amps but now have a voltage of 2 x the Voc or ~ 89.6v or 2 x the Vmp ~ 70.4v.

    Now if you wire 2 of those panels in parallel you will have 2 x the Imp or 15.9amps but the Voc = 44.8volts.

    If you wire 2 sets of 2 panels in parallel you will have 4 panels with a Vmp of ~ 70.4volts and and Imp ~ 15.9amps at peak production.

    The wire from the 4 panels will need to be sized to handle 15.9amps x 1.25 ~ 20 amps as long as it isn't too long and drops that 70.4v more than 2%. If it does the wire will need to be bigger.
    Last edited by SunEagle; 03-29-2017, 04:00 PM. Reason: corrected Vmp value

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    • #17
      Originally posted by jflorey2 View Post
      Yes, you need a DC/DC converter. They are readily available on the net. Search "24V to 12V converter." I would recommend getting an isolated one.
      Thanks!

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      • #18
        Originally posted by SunEagle View Post

        Ah no. The Imp of each panels is 7.95 amps. 2 of those panels wired in series still have an amp rating of 7.95amps but now have a voltage of 2 x the Voc or ~ 89.6v or 2 x the Vmp ~ 70.4v.

        Now if you wire 2 of those panels in parallel you will have 2 x the Imp or 15.9amps but the Voc = 44.8volts.

        If you wire 2 sets of 2 panels in parallel you will have 4 panels with a Vmp of ~ 70.4volts and and Imp ~ 15.9amps at peak production.

        The wire from the 4 panels will need to be sized to handle 15.9amps x 1.25 ~ 20 amps as long as it isn't too long and drops that 70.4v more than 2%. If it does the wire will need to be bigger.
        Huge thanks. I feel cleared up there. So for example, 6 panels... Imps = 5 amps each, Vmp = 10v each... If I made 2 sets of 3 panels in parallel, and then hooked the 2 sets together in series, I'd end up with 6 panels with an Imp of 15amps and Vmp of 20v, right? (Hope so cause I'll be excited if I am)


        I feel fuzzy these things below... For a 1000 watt ( 4x 250watt) solar panel 24volt system, using an MPPT CC rated 45Amps...

        - The wire between the charge controller and batteries (depending on the length) should be rated 50Amps and have a 50amp fuse. In every case, do you use wire that has a rating of 5Amps higher than the Amps of the CC, or is there a formula?

        - The wire between the batteries and inverter needs to be rated about 100amps with a 100 amp fuse. Here's where I'm drawing a complete blank. What is the calculation that determines size of the wire?

        I'm staying away from voltage drops and wire length until I'm clear on the other stuff.
        Last edited by SunEagle; 03-29-2017, 04:01 PM. Reason: corrected Vmp value in my post

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        • #19
          Originally posted by monicaj View Post

          Huge thanks. I feel cleared up there. So for example, 6 panels... Imps = 5 amps each, Vmp = 10v each... If I made 2 sets of 3 panels in parallel, and then hooked the 2 sets together in series, I'd end up with 6 panels with an Imp of 15amps and Vmp of 20v, right? (Hope so cause I'll be excited if I am)


          I feel fuzzy these things below... For a 1000 watt ( 4x 250watt) solar panel 24volt system, using an MPPT CC rated 45Amps...

          - The wire between the charge controller and batteries (depending on the length) should be rated 50Amps and have a 50amp fuse. In every case, do you use wire that has a rating of 5Amps higher than the Amps of the CC, or is there a formula?

          - The wire between the batteries and inverter needs to be rated about 100amps with a 100 amp fuse. Here's where I'm drawing a complete blank. What is the calculation that determines size of the wire?

          I'm staying away from voltage drops and wire length until I'm clear on the other stuff.
          On the first part you are basically correct. Usually you would wire 2 panels in series first so if they had an Imp = 5amp and Vmp = 10v you would have a set of 2 at Vmp = 20v and Imp = 5amp.

          Then if you wired 3 sets of those in parallel you would end up with 6 panels with a Vmp = 20v and Imp = 15amps. So you were correct with the Vmp and Imp values.

          Now the wire between the charge controller and the batteries is usually sized for the maximum charging amps of the CC. So if the CC is rated for 60amps then the wire should be rated for at least 60amp or larger. The fuse protecting that wire should be rated at 60amp or lower because you want the fuse to open before the wire gets too hot and causes a fire. Going bigger on the wire size will always help as long as you do not make it too big which makes it hard to connect to the terminals of the battery, charger or inverter.

          The wire between the battery and inverter would be based on the wattage rating of the inverter divided by the battery voltage. If you had a 600watt inverter and a 12volt battery the wire could see as much as (600watt / 12v = 50 amps). So the wire would need to be rated for more than 50 amps to be safe and the fuse would be rated 50 amps or less.

          This is where we warn people that want to use a 3000 watt inverter on a 12volt battery because that inverter could draw as much as (3000w / 12v = 250amps) which is a lot and requires a very big wire.
          Last edited by SunEagle; 03-29-2017, 04:03 PM.

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          • #20
            Originally posted by SunEagle View Post

            On the first part you are basically correct. Usually you would wire 2 panels in series first so if they had an Imp = 5amp and Vmp = 10v you would have a set of 2 at Vmp = 20v and Imp = 5amp.

            Then if you wired 3 sets of those in parallel you would end up with 6 panels with a Vmp = 20v and Imp = 15amps. So you were correct with the Vmp and Imp values.

            Now the wire between the charge controller and the batteries is usually sized for the maximum charging amps of the CC. So if the CC is rated for 60amps then the wire should be rated for at least 60amp or larger. The fuse protecting that wire should be rated at 60amp or lower because you want the fuse to open before the wire gets too hot and causes a fire. Going bigger on the wire size will always help as long as you do not make it too big which makes it hard to connect to the terminals of the battery, charger or inverter.

            The wire between the battery and inverter would be based on the wattage rating of the inverter divided by the battery voltage. If you had a 600watt inverter and a 12volt battery the wire could see as much as (600watt / 12v = 50 amps). So the wire would need to be rated for more than 50 amps to be safe and the fuse would be rated 50 amps or less.

            This is where we warn people that want to use a 3000 watt inverter on a 12volt battery because that inverter could draw as much as (3000w / 12v = 250amps) which is a lot and requires a very big wire.
            Awesome!! I know this is basic stuff to you, and there's a huge mountain of knowledge that I will never know, but I feel some of the path has just been lighted, light bulbs turning on! THANKS!!

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            • #21
              Originally posted by monicaj View Post

              Awesome!! I know this is basic stuff to you, and there's a huge mountain of knowledge that I will never know, but I feel some of the path has just been lighted, light bulbs turning on! THANKS!!
              You are welcome.

              Let us know what your estimated watt hours loads are going to be and we can help you determine the best battery system or at least come up with a battery system that will match up to your solar panel wattage.

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              • #22
                Originally posted by SunEagle View Post

                You are welcome.

                Let us know what your estimated watt hours loads are going to be and we can help you determine the best battery system or at least come up with a battery system that will match up to your solar panel wattage.
                I have the sloppy plan of just matching up the battery system to the panel wattage and then seeing what I can and can't do. I haven't the funds to add more panels.

                The 3 big things I'd like to run are: washing machine, freezer, and water pump.

                I don't have any of those things yet so I don't know the watts, etc.

                But just to see if I have a handle on calculating kWh... (not counting surges)

                If:

                freezer is 200 watts, then 200 x 12 hours usage = 2,400 watt hours
                washer is 500 watts, 500 x 1/2 hour usage = 150 watt hours
                well pump is 750 watts, 750 x 2 hours = 1,500 watt hours

                2,400 + 150 + 1,500 = 4,050 total watt hours. 4,050 / 1,000 = 4.05 kWh

                Am I on the right track?

                I have to get off here for now, but I was going to try to muddle through calculating how much power my solar system would produce and see if it would meet the needs of all 3 appliances... I think I would have stumbled.

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                • #23
                  Originally posted by SunEagle View Post
                  Welcome back monicaj.

                  So if you want to use all 4 of those 280 watt panels I would suggest getting at least a 60 amp CC and build a 24volt battery system. You would get about 47 charging amps out of the 1120 watts which would be enough for a 470Ah battery system.
                  Holy cow. So... I woke up this morning thinking I'd go try to work through figuring out how much power my system would produce and compare it to the other calculation of what I'd like to be able to run.

                  That is not even going to be slightly within my reach. In fact, it's so far out of reach that I need a space ship. So difficult,, yet I have this burning compulsion to master this stuff. Some day...

                  ok. So, batteries. I guess lead-acid battery is most commonly used, but how about AGM? I read that they take a really long time to charge and so aren't efficient for a solar setup?

                  If AGM is a good choice for the battery bank, how does this one look...
                  https://www.apexbattery.com/12v-250a...w2gaAtdV8P8HAQ

                  ...12v 255ah... I would need to get 4, hook 2 sets in series, then hook them together in parallel for a 510ah, 24v battery system?

                  Should I not go up to 510ah, but go a little lower instead?

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                  • #24
                    Originally posted by monicaj View Post
                    ok. So, batteries. I guess lead-acid battery is most commonly used, but how about AGM? I read that they take a really long time to charge and so aren't efficient for a solar setup?
                    AGM's are lead acid; you just can't add water to them.

                    Flooded batteries are cheaper, last longer and are pretty easy to maintain. And it's harder to kill them by abusing them.
                    ...12v 255ah... I would need to get 4, hook 2 sets in series, then hook them together in parallel for a 510ah, 24v battery system? Should I not go up to 510ah, but go a little lower instead?
                    Do as many series batteries, and as few parallel batteries, as possible. 6V in your case would probably be the best bet.

                    Comment


                    • #25
                      Originally posted by monicaj View Post

                      I have the sloppy plan of just matching up the battery system to the panel wattage and then seeing what I can and can't do. I haven't the funds to add more panels.

                      The 3 big things I'd like to run are: washing machine, freezer, and water pump.

                      I don't have any of those things yet so I don't know the watts, etc.

                      But just to see if I have a handle on calculating kWh... (not counting surges)

                      If:

                      freezer is 200 watts, then 200 x 12 hours usage = 2,400 watt hours
                      washer is 500 watts, 500 x 1/2 hour usage = 150 watt hours
                      well pump is 750 watts, 750 x 2 hours = 1,500 watt hours

                      2,400 + 150 + 1,500 = 4,050 total watt hours. 4,050 / 1,000 = 4.05 kWh

                      Am I on the right track?

                      I have to get off here for now, but I was going to try to muddle through calculating how much power my solar system would produce and see if it would meet the needs of all 3 appliances... I think I would have stumbled.
                      I hate to be the bearer of bad news. But the cost just to power those 3 loads will be very high. Based on previous estimates the cost for a solar / battery system to safely generate 1 kWh a day is around $3000. You are looking at more than 4 kWh and that does not include lights, fans, computers or any other electronics.

                      To help you get a better idea of what you are getting into. There is a number of Sticky Posts in our off grid section that helps you understand how to size a battery system and pv solar panel wattage for your loads. IMO trying to run any type of large load is going to cost a lot up front and then every few years you will need to get another set of batteries.

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                      • #26
                        Thanks jflorey.

                        ... https://www.batteriesplus.com/produc...ZG4aAnPF8P8HAQ ...

                        Golf cart batteries, 235Ah, 6v...

                        So then it would be a total of 8 batteries. 2 sets of 4 batteries hooked in series for 24v, 235 Ah... Then hook the 2 sets to each other in parallel to get 470Ah. Good?

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                        • #27
                          Originally posted by monicaj View Post
                          So then it would be a total of 8 batteries. 2 sets of 4 batteries hooked in series for 24v, 235 Ah... Then hook the 2 sets to each other in parallel to get 470Ah. Good?
                          Not ideal but workable. A single string would be ideal, but you can't beat the cost on T105 flooded cells (i.e. GC2's, golf cart batteries.) Two strings isn't too bad. One warning - make sure that these batteries start together and stay together.

                          Question - why not 48 volts? That would let you do one string.

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                          • #28
                            Originally posted by jflorey2 View Post
                            Not ideal but workable. A single string would be ideal, but you can't beat the cost on T105 flooded cells (i.e. GC2's, golf cart batteries.) Two strings isn't too bad. One warning - make sure that these batteries start together and stay together.

                            Question - why not 48 volts? That would let you do one string.
                            And then get a step down converter that will go from 48v to 12v?

                            So with 48 volts, then would this charge controller be the size I'm looking for? MPPT 12v/24/v/48v, 60Amp, 150V pv input...
                            https://www.amazon.com/Hxengy-Solar-...rge+controller

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                            • #29
                              Originally posted by SunEagle View Post

                              I hate to be the bearer of bad news. But the cost just to power those 3 loads will be very high. Based on previous estimates the cost for a solar / battery system to safely generate 1 kWh a day is around $3000. You are looking at more than 4 kWh and that does not include lights, fans, computers or any other electronics.

                              To help you get a better idea of what you are getting into. There is a number of Sticky Posts in our off grid section that helps you understand how to size a battery system and pv solar panel wattage for your loads. IMO trying to run any type of large load is going to cost a lot up front and then every few years you will need to get another set of batteries.
                              Don't feel bad about being the bearer. I already figured that my wants were much bigger than my means.

                              The well pump... I've been picturing using a 12v pump with a small system like I did before, but when I did it before, it was with an artisan well and the water was right up there at the surface, the pump didn't have to suck it up from a depth. I had no idea an AC pump would be so much watts. I have to rethink the whole water system. It wouldn't be good to spend a bunch of money for the well to get dug and not have enough solar power to run the pump to bring the water up. Sigh. Maybe use the money I was going to spend to dig a well to make a rain catchment system instead. And have $$ left over

                              At first I was only focused on trying to crunch the washer and freezer into the system. Thought maybe by unplugging the freezer when I ran the washer, and unplugging it at night, that I could make it work. But I didn't get my hopes up.

                              Is ok. I'll still be able to do more than I did before.

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                              • #30
                                You are going to need to add a small generator to the mix of things, both to charge batteries in bad weather, and to handle large loads (washer, well pump....)

                                Unless you get weekly rain, you need to think of a way to keep clean, stored water, from going stagnant.

                                If you can install a 500 gal tank, you can fill that weekly and use a RV water pump to move it to the faucets
                                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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