Derate factor?

[Jon Fisher]

Hi, we live in Scottsdale, AZ and have just finished installation of a SunPower PV grid tied system, 24 panels E20, 327 Watt each, installed on flat cool roof, south facing, no obstructions, ~20 degree tilt.We see a difference between the theoretical output in DC and the actual output of AC (which I think is called a derate factor) of around 20% (factor 0.80). When asking around how to rate this output we got a variety of answer ranging from 'exceeding expecations' (installer) to 'average' (independant solar engineer). Personally we had expected to see a better factor based on some literature searches.
I would be interested in your views and experiences. Thanks!

[Ian S]

Hi, we live in Scottsdale, AZ and have just finished installation of a SunPower PV grid tied system, 24 panels E20, 327 Watt each, installed on flat cool roof, south facing, no obstructions, ~20 degree tilt.We see a difference between the theoretical output in DC and the actual output of AC (which I think is called a derate factor) of around 20% (factor 0.80). When asking around how to rate this output we got a variety of answer ranging from 'exceeding expecations' (installer) to 'average' (independant solar engineer). Personally we had expected to see a better factor based on some literature searches.
I would be interested in your views and experiences. Thanks!

If you're considering your theoretical output to be 24x327=7848 kW DC, don't forget that is measured at around 70F. Here in the Valley of the Sun, the high temperatures will reduce that significantly.

[russ]

PV Watts from NREL uses a default derating factor of 77% - that is conservative but better safe than sorry and in most cases not that far off.

The temp effect is a loss of between 0.3% and 0.5% for most panels per degree C. It will be listed in the panel spec.