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  • 1200W 24V Off Grid System Design Questions.

    Hi Everyone. I've been browsing these forums for a few days now. Collected a lot of information but nothing that gives me definite answers. So hence my creating a new topic.

    I am in the planing stages of an off grid solar system. I've obtained 4 300watt panels. VMP 36.1v IMP 8.30a VOC 44.6v ISC 8.87. This is all I have obtained regarding my system so i'm looking for advice on size of everything else to purchase. Oh and I'll be running about 1000w AC max through an inverter for now. 13 led lights a tv (50 inch plasma 400 watt - i know it will be replaced with more efficient technology one day) and satellite with the odd phone charger and cordless tool charger.

    How do i wire my panels for the most efficient system while keeping controller costs down? 2 series 2 parallel? How can I combine these parallels? combiner box or MC4 connectors? I've come to learn that i'll likely use an MPPT 60a controller?

    Currently my batteries are small. 2 12v 50ah in series for total 24v. I know this will not power my requirements for long. ( I have a generator after solar runs out with a 24v charger) This is basically to just make sure the system will run and keep my costs down the first year. Next year i plan to purchase 4 6v 428ah batteries to run in 24v.

    Heck I don't even know what size cables to run from the panels to the controller and from the controller to batteries and then from batteries to inverter.

    I know i've asked a lot and don't know a lot but any help with any of my questions would be kindly appreciated.

    Shane Solar Panel.png

  • #2
    1200w pv = 960w harvest under optimum conditions and about 38A output @ 25V So you are in the big boys game. You can reliably charge 380ah of battery, the tall case 428ah batteries will suffer from insufficient bubbling in the final stages of charge and will be prone to stratify.

    Your existing 12V 50ah batteries can only be charged at 5A, for longer life. But I know of no 50ah deep cycle battery, so they are on the way out. You need a charge contoller that can have a current limit set to not cook your batteries. I suggest a Morningstar Tristar MPPT 60. It has a handy web server in it, so you hook it to your home network, and can monitor with any browser
    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
      Thank you for the reply. I was worried about cooking my current batteries, but was hoping i could get a system running that would allow me to change some parameters as i upgrade batteries. Unfortunately This is a remote off grid setup so remotely monitoring isn't an option. With the Morningstar controller am i able to program the charge rate and basically leave it set? or does it need to be constantly monitored? Thanks for the info regarding my future battery setup. A local solar company suggested that amount of amp hours, so i will have to conference with them on that.

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      • #4
        Also, would i maybe be better suited to just buy a cheap PWM charger for these size batteries? and buy say the morning star when i Purchase the larger aH batteries?

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        • #5
          Until you get new batteries, wiring up a single panel is simple. Forget that power hog of a TV because because those batteries you have now will give out in a hour or so. Your current batteries are made for a 150 to 200 watt panel and inverter. When you get some real batteries of 400 to 500 AH wire the panels 2S2P but for now just 1 panel is a bit too large for your batteries.
          Last edited by Sunking; 04-15-2018, 11:02 AM.
          MSEE, PE

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          • #6
            Originally posted by Shane.R13 View Post
            Also, would i maybe be better suited to just buy a cheap PWM charger for these size batteries?
            Not a terrible idea when use with just one panel. Waste of time and money with 2 or more panels. At 8 amps will knock that panel wattage down to just under 200 watts and be a good fit for a 50 AH battery.
            MSEE, PE

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            • #7
              Thank you. So I guess it seems like for now it will be one panel into a pwm controller until I get real batteries. What amp pwm should be I get? Should I still be running batteries in 24v? The reason I ask is that I have a couple 400w 12v inverters already and would rather use them than buy a 1000w 24v and waste power converting that I won

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              • #8
                That I won

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                • #9
                  Originally posted by Shane.R13 View Post
                  Thank you. So I guess it seems like for now it will be one panel into a pwm controller until I get real batteries. What amp pwm should be I get? Should I still be running batteries in 24v? The reason I ask is that I have a couple 400w 12v inverters already and would rather use them than buy a 1000w 24v and waste power converting that I won
                  You are welcome. Each panel is rated for 8 amps.

                  PWM Output Current = Input Current.

                  8 amps per panel is the magic answer. OK I just gave you a trick answer. Note the equation above. 8 amps of current is perfect for a 50 AH battery. So if you used 24 volt battery only one panel. But if you wire the batteries in parallel makes a 12 volts @ 100 AH battery and can easily handle 16 amps of current and to do that you would wire two panels in parallel.

                  Here is the trick and what is confusing you right now asking WTF is going on. Watts = Volts x Amps.

                  8 amps x 12 volts = 96 watts.
                  8 amps x 24 volts = 192 watts

                  You have 300 watt panels and each panel max output is 8 amps. So two panels in PARALLEL is 16 amps

                  16 amps x 12 volts = 192 watts from 600 watts of panels

                  MPPT is different. MPPT Output Current = Panel Wattage / Battery Voltage.

                  Try running numbers like 300 watts / 12 volts = ??

                  or

                  300 watts / 24 volts = ??

                  What is in common? Either way you get 300 watts with MPPT. Know the math, and you have the power and rule.







                  MSEE, PE

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                  • #10
                    Ok, so lets say i have a 60amp MPPT controller, if i hook one 300w 8amp panel into it. That means my output amperage to the batteries is going to be 300/24 = 12.5 amps or 300/12 = 25amps? What i'm gathering from your previous post is that no matter the size of my controller amperage wise it will only output panel watts/battery voltage? less panels less output more panels more output.

                    on another note, if i did one panel at 8amps with a 10amp controller (25% safety margin) then i wont cook my batteries?

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