We used both PV Watts and SAM (System Advisory Model) to generate results for 25yr average, and it still comes in around 51’000kWh/year on average over 25 years. I have noted the losses used as per SAM defaults, which I believe are in line with standards.
Can you please confirm the Azimuth (deg) you have been using for your calculations? This particular project, we have reviewed roof and it is likely the panels will be able to face north, so our azimuth is 0.
Snow: 0% (Perth)
Light-induced degradation: 1.5%
Total System Losses: 14.59%