I want to run the PVWatts model (concretely to get pvwatts_dc) on an Amerisolar 315 module which doesn't seem to appear. What I am trying to do is to replicate the steps in the manual, which only requires system DC size. 
When I go into the power model, the formula says g_poa_effective must be already angle-of-incidence-loss corrected. How do I do this correction? I've thought about using the physical correction formula pvlib.pvsystem.physicaliam(aoi), but is this the right track?
Yes, use an incident angle modifier function such as physicaliam to calculate the AOI loss, apply the AOI loss to the in-plane direct component, then add the in-plane diffuse component.
running PVWatts for module system not in Sandia DB (python library)