solar panel question, understanding the specs.

bmet
bmet Solar Expert Posts: 630 ✭✭
in Solar Beginners Corner #1
How is it that some high wattage panels have a 22 Voc / 10 Imp, while others have a 95 Voc / 2 Imp? In the latter, 2 Amps doesn't seem enough to charge my power pack, but the voltage potential (Voc) is certainly high enough.

If the panel is high voltage / low current, what sort of controller is used with these? Thank you.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #2
    Re: solar panel question, understanding the specs.

    The higher voltage panels are usually intended for grid connected use where grid tied inverters typically operate in the 300-600Vdc range. For battery charging you could use an MPPT charge controller, such as the Outback FM series, Morningstar Tristar MPPT or Xantrex MPPT contollers. They have input voltage limits of about 140Vdc, so the _temperature adjusted_ Voc on the coldest day of the year must not exceed 140V.
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  • ChevyNo1
    ChevyNo1 Solar Expert Posts: 30 ✭
    #3
    Re: solar panel question, understanding the specs.

    Another advantage of the higher-V panels is you can have longer runs between your panels and charge controller, and be able to keep the wiring small. As mentioned good MPPT controllers can handle higher Voc (Morningstar 15A MPPT for example can handle up to 75; their other higher end versions can handle up to 150).

    And of course the biggest benefit of MPPT is the ability to drop the incoming high voltage, but raise the outgoing current, such that it squeezes the most out of your panels.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: solar panel question, understanding the specs.

    Volts X Amps=Watts

    Watts are watts are watts regardless of voltage.

    12 volts X 12 amps= 144 watts

    120 volts X 1.2 amps = 144 watts

    An mppt controller can convert higher voltages and lower amperages to lower voltages and higher amperages. (minus controller loses however.)

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #5
    Re: solar panel question, understanding the specs.

    You are familar with batteries... Each solar cell is basically a little 1/2 volt power source... Mix them in series/parallel connections to get the operating voltage and current you will need.

    One thing that is very different with solar cells--They are (more or less) current mode power sources.... Their output current is proportional to the amount of sunlight hitting the face the cell.

    Voc is the Open Circuit voltage of the cell (a little bit of sunlight, no current). As you start loading down the cell (batteries, power supply, gt inverter, etc.) the output voltage falls a little bit. To the point of Vmp and Imp (voltage and current maximum power). If you draw down the voltage even more, the solar panel current (more or less) remains the same. Unlike a true battery which would try to supply more and more current until it hits it chemical conversion limit or its internal resistance limit.

    Remembering that Power = V*I -- You are looking for a load that will operate at ~Vmp and draw Imp or a bit less. Normally, a battery is a nice load for a solar panel. The battery can take as much (or as little) current as the panel can supply (based on sunlight). And, when the solar power available is less than that required by your loads--the battery makes up the difference to keep your loads alive.

    To a large degree--An off-grid solar power system is very similar to your car's electrical system. The battery supplies power all of the time, and the car's alternator supplies current to the battery only when the engine is running, and just enough current to keep the battery charged and to keep up with the loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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