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  • 12v and 24v panel

    hello
    can anyone tell me what the difference between a 12v solar panel and a 24 v solar panel

  • #2

    Re: 12v and 24v panel

    Re: 12v and 24v panel

    Yes indeed.

    They are "nominal" Voltage ratings for panels: a "12 Volt" panel is meant to work with a 12 Volt battery system, and a "24 Volt" panel with a 24 Volt system utilizing a PWM type charge controller. As such, the panel Voltage at maximum power (Vmp) is in a range suitable for "direct connection" to the battery. For 12 Volt systems this is 17 to 18 Volts, for 24 Volt systems it is 2X that.

    Unfortunately there are panels label as 12 or 24 which are not suitable. Some "12 Volt" panels have a Vmp of 16, which can lead to not having enough Voltage to actually charge the battery once the panel gets hot (reduces output Voltage) and also has to overcome wiring resistance (hence the need for Vmp several Volts above the actual charging point). Many "24 Volt" panels have unsuitable Vmp because they are actually designed for grid-tie systems which have a different goal (string panels in series to come up with a high Voltage array for a GT inverter; no batteries involved). These "oddball" Vmp panels can be used with battery systems, but you either have to buy a (more expensive) MPPT type controller or lose a significant amount of power.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

    Comment


    • #3

      Re: 12v and 24v panel

      Re: 12v and 24v panel

      Originally posted by Cariboocoot View Post
      Yes indeed.

      They are "nominal" Voltage ratings for panels: a "12 Volt" panel is meant to work with a 12 Volt battery system, and a "24 Volt" panel with a 24 Volt system utilizing a PWM type charge controller. As such, the panel Voltage at maximum power (Vmp) is in a range suitable for "direct connection" to the battery. For 12 Volt systems this is 17 to 18 Volts, for 24 Volt systems it is 2X that.

      Unfortunately there are panels label as 12 or 24 which are not suitable. Some "12 Volt" panels have a Vmp of 16, which can lead to not having enough Voltage to actually charge the battery once the panel gets hot (reduces output Voltage) and also has to overcome wiring resistance (hence the need for Vmp several Volts above the actual charging point). Many "24 Volt" panels have unsuitable Vmp because they are actually designed for grid-tie systems which have a different goal (string panels in series to come up with a high Voltage array for a GT inverter; no batteries involved). These "oddball" Vmp panels can be used with battery systems, but you either have to buy a (more expensive) MPPT type controller or lose a significant amount of power.
      so how do you know what type of solar panel you are buying. i mean how can i tell if a solar panel is truly 12 or 24v and how can i tell a solar panel is not for a grid tyed only system

      Comment


      • #4

        Re: 12v and 24v panel

        Re: 12v and 24v panel

        Originally posted by shella View Post
        so how do you know what type of solar panel you are buying. i mean how can i tell if a solar panel is truly 12 or 24v and how can i tell a solar panel is not for a grid tyed only system
        Very good question.
        Get the full specifications for the panel: Voc, Vmp, Imp, and Isc.
        If they can't/won't supply that, don't buy it.

        The 12/24 issue is mainly one of the Vmp. Many of the grid-tie panels have a Vmp of about 30 and are promoted as "24 Volt panels". But since a 24 Volt battery system charges at 28.8 Volts (+/-) 30 Volts isn't enough to do the job. When panels get hot (as they do) the Voltage goes down. More Voltage is lost in the wiring. So by the time you get to the battery that 30 Vmp can be less than the charging Voltage: no current flows, no charging occurs. A "real" 24 Volt panel will have a Vmp of 35 to 36.

        Beware also of high Voltage panels; there are some with Vmp around 60 which can only be used with MPPT controllers on battery systems.
        1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

        Ohm's Law: Amps = Volts / Ohms
        Power Formula: Watts = Volts * Amps

        Comment


        • #5

          Re: 12v and 24v panel

          Re: 12v and 24v panel

          Originally posted by Cariboocoot View Post
          Very good question.
          More Voltage is lost in the wiring. So by the time you get to the battery that 30 Vmp can be less than the charging Voltage: no current flows,
          Strictly speaking, voltage is only lost in the wiring when current is flowing. So the temperature factor could prevent charging completely while the wire resistance will only reduce the amount of current available for charging. Both are bad and both are reasons for needing a higher Vmp from the panels.
          You can reduce the wiring losses by using better terminals and larger, shorter wires. You cannot do much about the temperature factor.
          Sunny Boy 3000US, 18 x BP Solar 175b panels, installed 2009.

          Comment


          • #6

            Re: 12v and 24v panel

            Re: 12v and 24v panel

            Originally posted by inetdog View Post
            Strictly speaking, voltage is only lost in the wiring when current is flowing. So the temperature factor could prevent charging completely while the wire resistance will only reduce the amount of current available for charging. Both are bad and both are reasons for needing a higher Vmp from the panels.
            You can reduce the wiring losses by using better terminals and larger, shorter wires. You cannot do much about the temperature factor.
            Strictly speaking, photovoltic panels are a current source and allow the Voltage to go to whatever point. As such they always put out current, so the Voltage drop is always a factor and a concern.
            1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

            Ohm's Law: Amps = Volts / Ohms
            Power Formula: Watts = Volts * Amps

            Comment


            • #7

              Re: 12v and 24v panel

              Re: 12v and 24v panel

              Vmp=Voltage Maximum Power
              Voc=Voltage Open Circuit (no current flowing)
              Imp=Voltage Maximum Power
              Isc=Short Circuit Current (if a panel's wiring got shorted

              For the most part, we are looking at Voc-cold (very cold panels output higher voltage--need to make sure solar charge controller can handle the maximum input voltage).

              Vmp needs to be a few volts higher than battery charging voltage... Panels as they get hot, have Vmp fall. So Vmp~17.5 to 18.5 volts is "optimum" for use with "simple" PWM charge controllers.

              If Vmp is >> Vbatt-charging, then you need a MPPT type charge controller (seriously more expensive) to use the panel Vmp*Imp "efficiently" (it is easy to lose 1/2 of the efficiency or more with wrong Vmp to battery bank voltage selection).

              Imp determines the current "rating" of the charge controller used... Isc is used to size the wiring/fuse/breaker to protect the solar panels in larger configurations to insure no overheating of wiring during normal operation or if there is a short circuit somewhere.

              -Bill
              20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

              Comment


              • #8

                Re: 12v and 24v panel

                Re: 12v and 24v panel

                Shella,
                depending on what you want to do with your solar power system, you may not need to think too much about all the info you have been provided. There are a number of factors that may mean you should be using an MPPT controller.

                If you use an MPPT controller you don't have to worry about whether they are "real" 12 or 24 volt panels. Just buy the cheapest panels (which are often not "real" 12 or 24 volt).
                If you use an MPPT controller you will simplify wiring and fusing at the combiner box (saves money). Depending on the size of your system you may not even need a combiner.
                If you use an MPPT controller you will get more usable power from any panels of any voltage.
                If you use an MPPT controller you can use thinner wire (saves money) between the combiner and the controller.

                --vtMaps
                4 x 235w Samsung, Outback fm60 & vfx3524 & mate, Midnite E-panel, four Interstate L16, Trimetric monitor, Honda eu2000

                Comment


                • #9

                  Re: 12v and 24v panel

                  Re: 12v and 24v panel

                  hi
                  this is great info
                  i like to know as much as ik believe if you only know some info (like just learning about 12v systems only) you are heading for a big disapointment jmho.
                  so on a 12v system could i get the panels that say 24v and mppt controller would that work and what of the higher imp is that a typo or i havent understood its the same as vmp ?
                  so how would i wire the panels does the same rules apply

                  Comment


                  • #10

                    Re: 12v and 24v panel

                    Re: 12v and 24v panel

                    Originally posted by shella View Post
                    so how would i wire the panels does the same rules apply
                    First, does it make sense to use an MPPT controller? MPPT controllers cost more. In a small system it often doesn't pay to spend the extra on MPPT.

                    For example, MPPT controllers can simplify the wiring and fusing of a combiner. If your system is only 1 or 2 panels, you may not even have a combiner, so no gain there by using MPPT.

                    I assume your system is small because you ask about a 12 volt system. How small?

                    --vtMaps
                    4 x 235w Samsung, Outback fm60 & vfx3524 & mate, Midnite E-panel, four Interstate L16, Trimetric monitor, Honda eu2000

                    Comment


                    • #11

                      Re: 12v and 24v panel

                      Re: 12v and 24v panel

                      Originally posted by shella View Post
                      hi
                      this is great info
                      i like to know as much as ik believe if you only know some info (like just learning about 12v systems only) you are heading for a big disapointment jmho.
                      so on a 12v system could i get the panels that say 24v and mppt controller would that work and what of the higher imp is that a typo or i havent understood its the same as vmp ?
                      so how would i wire the panels does the same rules apply
                      Basic difference between a PWM type controller and an MPPT type:
                      The PWM will pass the current rating of the panel only, with the Voltage at the battery level. Any extra Voltage (and thus power) above that is lost.
                      The MPPT will convert the input Watts to output Watts, switching higher input Voltage and converting it to greater output current at the battery level.

                      If the panel is designed for the system Voltage and all other factors are equal (no long wire lengths or cold temperatures) there really is not much difference between the two controllers.
                      The MPPT type will show better performance when you need to "down convert" high Voltage panels (with a PWM controller the difference between the panel "ideal" and the panel "actual" Vmp is power lost). This is useful for adapting "odd" panels to battery systems. It is also useful for overcoming the extra resistance in long wire runs from panels to controller (higher Voltage array = less power loss from Voltage drop in the wiring). This is actually the main benefit of MPPT controllers. The "cold temperature" advantage (cold panels output higher than rated Voltage) is not available to everyone, and can cause headaches for those who can use it (Voc also increases and may exceed the controller's input maximum).

                      It is easier to understand this with a specific scenario, rather than in generalized terms.

                      Let's say you want to run panels with a Vmp of 30 and a Imp of 7.8 on a 12 Volt system (235 Watts). The "ideal" Vmp for 12 Volt systems is about 17.5. On a PWM controller these panels would yield 7.8 Amps @ system Voltage or about 112 Watts each or about 47% of the rated output. The Wattage missing is due to the batteries pulling the Voltage of the panel down to their level. Using an MPPT type controller you would see about 77% of the panel power actually reaching the batteries because it can turn the difference between 17.5 "ideal" Vmp and 20 "real" Vmp into current. In fact you might see 15 Amps @ 12 Volts or 180 Watts. The actual output from an MPPT controller is less predictable, as it will select what it feels is the best "loading" to achieve maximum charge current for the prevalent conditions.
                      1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

                      Ohm's Law: Amps = Volts / Ohms
                      Power Formula: Watts = Volts * Amps

                      Comment


                      • #12

                        Re: 12v and 24v panel

                        Re: 12v and 24v panel

                        Originally posted by shella View Post
                        ... and what of the higher imp is that a typo or i havent understood its the same as vmp ?
                        In case this has not been cleared up for you yet, here are the four main parameters of a panel:

                        Voc (open circuit voltage) is the voltage that you would measure with no load on the panels. It does not vary strongly with the amount of light hitting the panels.
                        Isc (short circuit current) is the current that the panel can send through a short circuit. It is the maximum current that the panel can produce and is roughly proportional to the amount of light hitting the panel.
                        Those are the easy ones.
                        Vmp (maximum power voltage). Since the power from a panel is volts times amps, clearly at Voc and no current you get no power. At Isc and no voltage you also get no power. Somewhere in between those two is the maximum power point (MPP) where the product of the two is as large as possible.
                        For an ideal battery that will be at exactly 1/2 of Voc and 1/2 of Isc. But for a solar panel instead of a battery, it is reached closer to Voc and also fairly close to Isc.
                        Vmp is that voltage, at the sweet spot.
                        Imp (maximum power current) is the current measurement at exactly the same operating point.
                        So the highest power you get from the panel (at the standard light conditions) will be Vmp time Imp.

                        A controller which tracks the MPP (MPPTracking or MPPT) keeps adjusting the load it puts on the panel to keep the panel working at the current value of Vmp, which will vary with temperature, etc. and at the constantly changing Imp which depends on the incoming sunlight.
                        Sunny Boy 3000US, 18 x BP Solar 175b panels, installed 2009.

                        Comment


                        • #13

                          Re: 12v and 24v panel

                          Re: 12v and 24v panel

                          Another analogy... A single speed bike vs the automatic transmission in a car...

                          The single speed bike is very efficient when operated when geared to the average speed (up hills, vs flat road, etc.)... PWM controller. Matched solar panels (Vmp~17.5 volts to "12 volt" battery).

                          An MPPT controller is sort of the automatic transmission... The vehicle speed is the "battery bank", and the engine/gas peddle is the solar array. The transmission "shifts" to convert the high speed/low torque of the engine to the low speed/high torque at the wheels. The automatic shifting to match the available engine speed to the current vehicle speed/wind loading/grade/etc... MPPT controller from Vmp~17.5 volts to Vmp~100 volts or more (or less), depending on controller.

                          Note that the typical MPPT controller has limits... Maximum input voltage/current. The input voltage always needs to be a volt or so higher than output voltage to the battery (the typical buck converter used in MPPT controllers can only "down convert" from high voltage/low current to low voltage/high current--there are other types--but that is for another discussion).

                          -Bill
                          20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

                          Comment


                          • #14

                            Re: 12v and 24v panel

                            Re: 12v and 24v panel

                            thanks to you all for your replys
                            ok i am trying to digest this info
                            i have been reading that if you have a hotch botch of solar panels the mppt would be the way to go
                            i still having trouble understanding the voc vmp and such will have to divert my attentions to these
                            i knew i would have trouble with the maths lol

                            Comment


                            • #15

                              Re: 12v and 24v panel

                              Re: 12v and 24v panel

                              Originally posted by shella View Post
                              thanks to you all for your replys
                              ok i am trying to digest this info
                              i have been reading that if you have a hotch botch of solar panels the mppt would be the way to go
                              i still having trouble understanding the voc vmp and such will have to divert my attentions to these
                              i knew i would have trouble with the maths lol
                              If you have a "hotch botch" or "hodge podge" of solar panels an MPPT type controller will not necessarily help.
                              For parallel connections, the Vmp of the panels needs to be close, preferably within 5% to 10%. The greater the difference the greater the power loss.
                              For serial connections it's the Imp that matters, same rule-of-thumb. In a series string of panels the current will be reduced to the lowest Imp in the string. No controller can make up for these differences.

                              We actually have a glossary that may help you understand all the terms used here on the forum: http://forum.solar-electric.com/show...?6136-Glossary

                              Everybody has trouble with the math, especially as there's quite a lot of it!
                              1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

                              Ohm's Law: Amps = Volts / Ohms
                              Power Formula: Watts = Volts * Amps

                              Comment

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