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Solar PV voltage and wattage

Discussion in 'General Electronics Discussion' started by Nvr-2-Old, May 13, 2010.

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  1. Nvr-2-Old


    May 13, 2010
    How does the voltage and wattage output of a solar panel vary with increasing light? That is, when you put up a "30W" / "12V" panel, how do these vary? Does the wattage go up and down or does the voltage or both?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In general terms the amount of light determines the maximum amount of power the panel can deliver.

    You'll often find 3 things quoted:

    1) max power
    2) open circuit voltage
    3) short circuit current

    The maximum power is pretty much never achieved. The panel needs to be pointed directly at the sun in the middle of the day with no cloud or haze. Practically, it's not going to happen too often. The max power also falls as the cells age or as dust settles on the glass over the panel.

    The open circuit voltage (say 17.5 volts for a nominally 12 volt panel) tells you the maximum voltage that the panel will deliver. It's of limited value because this is at zero current, i.e. zero power.

    The short circuit current indicates the maximum current a panel can deliver. It is also of limited value as into a short circuit the panel is also delivering no power.

    Multiplying the open circuit voltage by the short circuit current gives a figure in watts that means nothing and can never be achieved by this panel -- so don't do it :)

    The power you can get from a panel depends on the current you draw from it, and the voltage that appears across the panel when you do so. The power varies with load, and the "best" point (i.e. maximum power) varies with the amount of illumination on the panel.

    There are power tracking regulators that continually hunt to find the optimum point, which can improve efficiency of (say) battery charging by 10 to 15%.

    So the answer, in short, is yes, yes, and both (for a given current)
  3. Resqueline


    Jul 31, 2009
    Everything steve says applies.
    My own observations are as follows:
    At a given illumination the voltage/current graph of a solar panel is quite square.
    The knee point (max power) is found maybe 10-20% down from the open circuit voltage & short circuit current.
    When charging a battery with the ordinary cheap control panels you do so at 14.5V instead of at the point of maximum power - say 17.5V - so the wattage rating of the panel does not apply. Instead, a current applies, being somewhere between the point of maximum power and the short circuit current.
    I'd expect the voltage to be more or less logarithmic in relation to the illumination. That is it quickly rises with illumination but has a terminal voltage of 0.45V per cell.
    However, as soon as the open circuit voltage drops down to 14V (due to low illumination) the panel can not charge the battery at all.
    I'd expect the short circuit current to be more or less linear in relation to the illumination (measured in lux for example).
    Even on a very bright but otherwise overcast day (no shadows) you won't get any more than 0.1A charging current out of a 6A panel.
  4. trobbins


    Jun 15, 2010
    Yes the short circuit current is linear with solar input (Mj/m2 or kW/m2) - you can get higher than the rated Isc if you have very good sun and little bit of hazy cloud. The voltage characteristic changes with temperature (ie. allow them to have free air movement front and back).
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