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Thread: 12v and 24v panel

  1. #1
    Join Date
    Jul 2012
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    England Bath
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    Question 12v and 24v panel

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

  2. #2

    Default 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.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

  3. #3
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    Default Re: 12v and 24v panel

    Quote 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

  4. #4

    Default Re: 12v and 24v panel

    Quote 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.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

  5. #5
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    Default Re: 12v and 24v panel

    Quote 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.

  6. #6

    Default Re: 12v and 24v panel

    Quote 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.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

  7. #7
    Join Date
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    19,676

    Default 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.

  8. #8
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    Default 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

  9. #9
    Join Date
    Jul 2012
    Location
    England Bath
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    Default 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

  10. #10
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    central Vermont
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    Default Re: 12v and 24v panel

    Quote 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

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