Calculating Pmax for various temps

JJones
JJones Registered Users Posts: 6 ✭
in Advanced Solar Electric Technical Forum #1
Have a 230 watt panel that is 209.5 at PTC, has a pmax of -0.38 and a tolerance of +/-5. Looking for a way to calculate performance loss in terms of wattage decrease at various intervals such as 25, 30, 40 and 45 degrees celsius, all based off of PTC. If anyone could help, I'd very much appreciate it.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Calculating Pmax for various temps

    The +/-5% is manf. make tolerance, so what you have is what you get for your particular panel. There will be some degradation over the years but that is not too predicable, other then the more sunlight it is exposed to the greater the yearly degradation. Don't worry about this too much.

    As to voltage and power over temp, it follows the manf spec. Usually about -0.33% per deg C for Vmpp, about -0.48% per deg C. Current temp coefficient is very small, about 0.017% per deg C.

    Beside temp, the angle of sun resulting is more or less atmosphere the radiation has to go through along with moisture and polution content. Don't forget dirt build up on panels.

    The most important thing is your coldest temp Voc and making sure it does not exceed your controller max voltage limit spec.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #3
    Re: Calculating Pmax for various temps

    From this thread, I setup this equation (just based on standard panel specifications):
    • Power = STC Watts * (1 + (25ºC-Tamb)*(panel Power Coeff in %/ºC)) * (measured solar irradiation /1,000 W/sq.m.)
    • Power = 230 Watts * (1 + (25ºC-Tamb)*(-0.38 %/ºC)) * (Si/1,000 W/sq.m.)
    That should get you close.

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