PVWatts derate bug?

If you set the PVWatts derate factor to 1, I expect that in the Performance Data output, Hourly Energy would exactly match the Hourly DC Output. This is not the case, and there is no clear correlation between input derate factor and the difference between Hourly DC Output and Hourly Energy. Is this a bug, or am I misunderstanding how Hourly Energy is calculated? SAM documentation specifies that Hourly Energy is the "System AC Output" and that DC Output is "DC Output of the array". Is there something besides the derate factor that relates them?

Hello,

PVWatts has some built-in assumptions about the inverter efficiency. It estimates an hourly inverter efficiency (based on an assumption of 92% nominal conversion efficiency) that applies in addition to the derate factor.

The next version of SAM will use the new version of PVWatts (Version 5) that makes more up-to-date assumptions about the photovoltaic system.

Best regards,
Paul.