Rule of thumb for derating PV output based on ambient temp. Is there such a thing?

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Fe-Wood
Fe-Wood Solar Expert Posts: 96 ✭✭
in Off Grid Solar & Battery Systems #1
I finally have my PV panels up and running. I'm wondering if there is a rule of thumb for de-rating PV output based on ambient temp.

As always- Thanks for all your help!
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
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    Re: Rule of thumb for derating PV output based on ambient temp. Is there such a thing?
    Fe-Wood wrote: »
    I finally have my PV panels up and running. I'm wondering if there is a rule of thumb for de-rating PV output based on ambient temp.

    As always- Thanks for all your help!

    Not so much a rule of thumb as a specification. The panels should list temperature coefficients for Vmp, Imp and POWERmp as a function of temperature.

    The STC output assumes a panel temp of 25C, so it is an easy calculation from there once you figure out how hot the panels will get in the sun. That heating can be large compared to changes in ambient temperature. And it will depend on how well ventilated the area under the panels is.
    SMA SB 3000, old BP panels.
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #3
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    Re: Rule of thumb for derating PV output based on ambient temp. Is there such a thing?

    PV modules usually have exact specifications for voltage loss and current gain based on cell temperature. Converting ambient temperature to cell temperature is more complicated as ambient temperature is one of several factors contributing to cell temperature.

    If you have one of those infrared thermometers, you can semi-accurately measure cell temperature (will be actually measuring glass temperature) and you can compute from their from the panel specs.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #4
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    Re: Rule of thumb for derating PV output based on ambient temp. Is there such a thing?

    So, looking at the Data Sheet:

    Cs6p - Canadian Solar
    Temperature Coefficient CS6P-235/240/245/250/255P Electrical Data
    • Pmax -0.43 %/℃
    • Voc -0.34 %/℃
    • Isc 0.065 %/℃
    • NOCT 45±2℃ (Normal Operating Cell Temperature

    So, if you your air temperature is 30C and you have a 15C rise for a cell temperature of 45C, and you are using an MPPT charge controller, the calculation should look something like this:
    • Power = 245 watts [1 + (-0.43 %/℃ power derate) * (Tcell - Tstd)]
    • Power = 245 watts [1+ (-0.0043 /℃ power derate) * (45C - 25C)] = 222.95 watts (note % shifted decimal point removed)

    And assume 95% efficient controller:
    • 222.95 watts * 0.95 = 212 Watt output to battery (ignoring wiring losses)

    Our standard rule of thumb for relatively warm locations with typically dirty/aging panels, wiring losses, and less than clear blue sky would be:
    • 245 watts * 0.77 panel+controller derating = 189 Watts typical maximum power from array+controller

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Fe-Wood
    Fe-Wood Solar Expert Posts: 96 ✭✭
    #5
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    Re: Rule of thumb for derating PV output based on ambient temp. Is there such a thing?

    Thanks guys- I knew it was to vague a question to ask. Ambient temp. being the veriable and all. I guess I'll have to get a infered thermometer someday.
    PS
    I love the midnight solar Classic lite. The way it fluctuates the voltage to support the amperage needed to support a load... Awesome!
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