PVWatts System derates and Inverter model

Hi,
I have been using your PVWatts v1 with great success for some time (I know there are newer versions too). What I want now is to understand the "maths" behind it.

I started reading the "PVWatts Version 1 Technical Reference.pdf":
www.nrel.gov/docs/fy14osti/60272.pdf

But since it does not give all data on used formulas I combined it with your "PVPerformance modeling steps":
pvpmc.sandia.gov/modeling-steps/1-weather-design-inputs
I got stuck at 10) System Derates and 11) Inverter Model and have three questions on these two:




10) System Derates:

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P'dc - derated DC power
Pdc - DC power
?sys - overall system derate
?inv - inverter efficiency at maximum power

P'dc = Pdc * ?sys/?inv

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QUESTION 1:
So by default: ?sys is 0.77 and ?inv is 0.92 ??



11)Inverter Model

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Pinv,dc0 - inverter DC rating
Pac0 - inverter AC nameplate rating
?op - inverter conversion efficiency at operating conditions

Pinv,dc0 = Pac0/?inv
f = P'dc/Pinv,dc0

if (f larger and equal than 0.1) and (f smaller and equal than 1):
....?op = 0.774 + 0.663 * f - 0.952 * (f**2) + 0.426 * (f**3)
....Pac = Pdc' * ?op * ?inv/0.91
elif (f larger than 0) and (f smaller than 0.1):
....?op = -0.015 + 8.46 * f
....Pac = Pdc' * ?op * ?inv/0.91
elif (f larger than 1):
....Pac = Pac0

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QUESTION 2:
So does Pac0 = Pdc0 (Pdc0 by default is 4000 Watts (4kW) ??)

QUESTION 3:
In case f larger than 1, Pac = Pac0 ??
The "PVWatts Version 1 Technical Reference.pdf" says:
"For f larger than 1, the power output is clipped to the maximum AC power rating Pac0."
Does 'clipped' mean 'equal to' ?


Thank you for the help.

Please note that these questions are about PVWatts V1, which is out of date. I've provide answers for both PVWatts V1 and the current version. For more details, see the PVWatts Version 5 manual ( PDF 117 KB ) for the current version, and PVWatts Version 1 Technical manual ( PDF 487 KB ).
Question 1:
Yes. The system derate factor is a user input, and the inverter efficiency is a constant. (The current version of PVWatts uses a different approach, described in its documentation: The inverter is modeled using a part-load efficiency curve with a user-specified nominal efficiency.)
Question 2:
Yes. In PVWatts V1, the inverter's nameplate capacity in AC Watts is equal to the array's nameplate capacity in DC Watts, which is equivalent to a nameplate DC-to-AC ratio of 1. (In PVWatts V5, the DC-to-AC ratio is a user input.)
Question 3:
Yes. Both versions of PVWatts limit the inverter output to its rated AC capacity.
Best regards,

Paul.

Thank you for the reply Paul.