can you do a check if gen exists before adding the prod timeseries? I'm pretty sure the way it's run in the GUI calls simulate on the system first so gen should definitely exist, but technically this equation doesn't require simulate to be called beforehand.

Good thinking. What should the code do if gen doesn't exist? Throw an error, or revert to the lat/lon behavior? If the latter, is there a good losses assumption?

Yes I think using the lat/lon, which basically has REopt call PVWatts. Could losses be an optional input?