We're using a SAM script (thanks for making that available!) to get hourly distributions for the TMY2 locations at 40 azimuth/tilt combinations. Using the hourly data, we're deriving monthly distributions and total annual kWh/m2/day.
To derive kWh/m2/day for each month, I divide the total production estimate for that month by the system's AC Watts times the days in that month. The derived irradiance data is about 9% off from provided irradiance data for my test location (CA Sacramento).
FYI, I get AC Watts with: STC Watts * (1 - inverter derate) * (1 - total user losses)
What am I doing wrong? Thanks!