PVWatts System derates and Inverter model

[bernard]

Dear Solar Panel talkers,
For the last couple of days I was trying to understand the "math" behind PVWatts v1 using their PVWatts Version 1 Technical Reference.pdf.

For some reason the System Derates and Inverter Model are parts where some uncertainties arise.
I tried using their PVPerformance modeling steps but didn't get the answers.
I would be very grateful if some of the experts around here could give me some hand.


10) System Derates:
P'dc - derated DC power
Pdc - DC power
ξsys - overall system derate
ηinv - inverter efficiency at maximum power (suggested to be 0.90 < ηinv < 0.96 in this topic)

P'dc = Pdc * ξsys/ηinv

question 1:
Does PVWatts team by default offer value of 0.77 for ξsys?



11)Inverter Model
Pinv,dc0 - inverter DC rating
Pac0 - inverter AC nameplate rating
ηop - inverter conversion efficiency at operating conditions

Pinv,dc0 = Pac0/ηinv
f = P'dc/Pinv,dc0

if (f >= 0.1) and (f <= 1):
    ηop = 0.774 + 0.663 * f - 0.952 * (f^2) + 0.426 * (f^3)
    Pac = Pdc' * ηop * ηinv/0.91

else if (f > 0) and (f < 0.1):
    ηop = -0.015 + 8.46 * f
    Pac = Pdc' * ηop * ηinv/0.91

else if (f > 1):
    Pac = Pac0

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


I aslo contacted the PVWatts team, and got response that they do not support the v1 anymore.
Thank you for the help.