For the first question, manufacturers define Pnom
= "Nominal power", valid for any temperature conditions
, and Pmax
= "Maximum power", attainable in some conditions (not always well defined, usually an operating period).
To my understanding and after discussion with some manufacturers, this seems to correspond to a device internal temperature limitation:
File comment: Pnom and PMax according to temperature
Inverter_Pmax.png [ 2.94 KiB | Viewed 8512 times ]
Now for the Array temperature increase when power decreases: this is evident from the energy balance used for the evaluation of the module's temperature:
Tarray - Tamb = ( Alpha · Ginc · (1 - Effic)) / U
where Alpha = absorption coeff., Ginc = incident irradiance and Effic = module efficiency.
However with Alpha = 0.9, Ginc = 1000 W/m2, U = 29 W/m²K, the cell temperature increase is 26.4°C for effic = 15%
and 31°C for effic = 0
Yes the array temperature is slightly higher, but at this time the array doesn't have to produce anything !!!
(or works at reduced power).