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  • Panel Wattage Recommendation

    Location: North Georgia, USA

    Current Equipment
    114 Ahr Everstart maxx battery
    400w Wagan inverter
    20A cheap pwm charge controller (on order)
    * EPRC-ST model

    I use this equipment in a barn and currently use a generator to charge the battery. Load includes low wattage lighting (12v and 120), phone charging, small battery charging, radio... A full charge typically lasts three to seven days, depending on use. What wattage 12v panel do you guys recommend to cut down on the generator use? I'm was thinking of starting with one 100 or 120w panel but am unsure after reading some posts.

    Am I correct in thinking that a ~17.5v higher current panel would be better with the cheap charge contrloller than a simularly priced 18+ volt panel?

    Thanks for any sugestions.
    Last edited by abc123; 01-23-2015, 11:50 AM. Reason: added controller info

  • #2
    Originally posted by abc123 View Post
    Location: North Georgia, USA

    Current Equipment
    114 Ahr Everstart maxx battery
    400w Wagan inverter
    20A cheap pwm charge controller (on order)
    * EPRC-ST model

    I use this equipment in a barn and currently use a generator to charge the battery. Load includes low wattage lighting (12v and 120), phone charging, small battery charging, radio... A full charge typically lasts three to seven days, depending on use. What wattage 12v panel do you guys recommend to cut down on the generator use? I'm was thinking of starting with one 100 or 120w panel but am unsure after reading some posts.

    Am I correct in thinking that a ~17.5v higher current panel would be better with the cheap charge contrloller than a simularly priced 18+ volt panel?

    Thanks for any sugestions.
    Easy, Since you are running a 12 volt system and you bought only a 20 AMP Charge Controller, you are stuck with a max of 240 Watts of panel ( 20AMP x 12 Volts = 240 watts). You should return that PWM and get a MPPT charge controller if you can but not really that big of a deal with your system. IF you go with an MPPT you can use a higher voltage panel.

    Comment


    • #3
      Originally posted by mschulz View Post
      Easy, Since you are running a 12 volt system and you bought only a 20 AMP Charge Controller, you are stuck with a max of 240 Watts of panel ( 20AMP x 12 Volts = 240 watts). You should return that PWM and get a MPPT charge controller if you can but not really that big of a deal with your system. IF you go with an MPPT you can use a higher voltage panel.
      Actually his battery limits it even more. It should be charged between C/8 and C/12 with C/10 being about right. 115/8 = 14.375A, 115/10 = 11.5, 115/12 = 9.58

      Find a panel or parralleled panels that the IMP would be in that range. 2 - 100watt panels should be about right.

      WWW

      Comment


      • #4
        Without knowing the watts and hours used of the equipment, we can't do a loads list to size the panel needed. VERY rough math with a lot of assumptions following, take with a grain of salt (preferably in a margarita).

        If you say your 114ah battery lasts 3-7 days, lets assume after 3 days you are at 50% DoD, which means you used 114ah / 2 =57ah in 3 days, or /3 =19ah a day x 12V = 228 watt hours a day. Georgia in Dec, I'll use 3.5 sun hours. 228wh / 3.5 sun hours / .67 system inefficencies = 97 watts of solar needed. So, theoretically, with rough numbers, a 100W 12V nominal (~18Vmp, 22Voc) may work for you, more would be better to help make up for days without sun. But you currently have a genny, so that may still be needed occasionally. Short circuit current would be about 6.06Isc x 1.25 NEC = at least an 8A PWM charge controller needed.

        I wouldn't bother with an MPPT for this size system, PWM is fine as long as you use a nominal 12V panel. However, if you are using a cheap PWM near ham radio equipment (not sure if your radio is ham or music), you may experience noise interference. Just something to keep an eye out for.

        73
        Solar Queen
        altE Store

        Comment


        • #5
          Originally posted by mschulz View Post
          Easy, Since you are running a 12 volt system and you bought only a 20 AMP Charge Controller, you are stuck with a max of 240 Watts of panel ( 20AMP x 12 Volts = 240 watts). You should return that PWM and get a MPPT charge controller if you can but not really that big of a deal with your system. IF you go with an MPPT you can use a higher voltage panel.
          With a PWM controller, amps in = amps out. The OP is correct to suggest that a panel with slightly lower operating voltage, and therefore slightly higher current (Imp) would produce slightly more charging power in his system. The difference in charging power of a panel operating at 17.5 V vs 18.0 V, if rated power is the same, would be a couple percent. The voltage on the 12 V panel needs to be high enough the handle all stages of charging, which is why a panel that operates at 14 V wouldn't work, even though the amount of current for the same power is higher.

          Maximizing the power would be whatever set of panels is capable of producing closest to 20 A. Four 85 W panels (340 W) would probably make something like 19.12 A, which is about the most you could get with same sized panels. Some combination that adds up to 355 W is probably the most you could get and stay under 20 A, with typical panel specs.

          Whether the $ / W of charging power is less for a cheaper PWM controller with more panels (taking up more space) or a more expensive MPPT controller with fewer panels is a question worth asking early in the design, but since the components have already been ordered here, it may not matter.

          Edit: WWW's response is probably more useful than mine, in the context of this system.
          Last edited by sensij; 01-23-2015, 12:44 PM. Reason: Edit
          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

          Comment


          • #6
            Originally posted by Wy_White_Wolf View Post
            Actually his battery limits it even more. It should be charged between C/8 and C/12 with C/10 being about right. 115/8 = 14.375A, 115/10 = 11.5, 115/12 = 9.58

            Find a panel or parralleled panels that the IMP would be in that range. 2 - 100watt panels should be about right.

            WWW
            Right on, misread battery and thought he was using AGM battery.

            Comment


            • #7
              ty

              Originally posted by Wy_White_Wolf View Post
              Actually his battery limits it even more. It should be charged between C/8 and C/12 with C/10 being about right. 115/8 = 14.375A, 115/10 = 11.5, 115/12

              Find a panel or parralleled panels that the IMP would be in that range. 2 - 100watt panels should be about right.

              WWW
              Thanks all.

              The c/? term is what was puzzling. Not familiar with that one. Is c/10 suggested because that is the preferred charging rate for lead acid batteries or just my particular battery?

              Comment


              • #8
                since you already have a pwm controller, i would just get a 120 watt panel. Thats what I use to have, it got me about 6 amps when the sun was overhead. Getting a larger panel wont get you more amps with the pwm controller.

                As an example using a pwm controller with a 240 watt panel also only got me 6 amps, whatever the ip power it saids on the back of the panel thats the max you get with pwm.

                The only thing I concern myself with is how many amps is the panel going to give me to charge my battery.

                Even though 120 watt sounds small, I use to use that connected to a 75ah battery and ran a 12 volt fridge 24/7 for months at a time plus occasional use of my 400 watt inverter to charge my laptop and also ran my swamp cooler in the hot weather and never had to use a generator, 100 percent solar. For your battery to last 3 days between charges, means your not running too many power hungry items.

                Comment


                • #9
                  Originally posted by abc123 View Post
                  Thanks all.

                  The c/? term is what was puzzling. Not familiar with that one. Is c/10 suggested because that is the preferred charging rate for lead acid batteries or just my particular battery?
                  C represents the battery amp hour capacity at the 20 hour rate.
                  So for a 100AH battery, C/10 would be 10A.
                  For floating lead acid the chemistry and internal resistance limit the charging current that can be used to about C/8. And similarly a current lower than C/12 may not provide enough gassing to mix the electrolyte properly.
                  For batteries with a lower internal resistance, like AGM, the maximum allowed charging current is higher, but should be read from the manufacturer's specifications for each particular model.
                  SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                  • #10
                    Appreciate the info.

                    Looks like 100w is the sweet spot for w/$. One would think the production cost of a 120 or 140 would not be that much greater than a 100.

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