I am just wondering how pvwatts_dc method is intended to be used. Looking at the test, it sounds like we have to create a custom module_parameters with pdc0 and gamma_pdc. Those two parameters are not present in the standards module objects retrieved from CECMod or SandiaMod.
pdc0 looks like being more a parameter tied to a group of modules rather than a module alone. Could it be calculated from the standard module_parameters entered to the instance in combination with modules_per_string and strings_per_inverter?. It would perhaps be more "system like", and we would have a use of the instance calculating pvwatts_dc.

As well, there is no eta_inv_nom / eta_inv_ref in the inverter_parameters object retrieved from CECMod. Calculating pvwatts_ac is not going to work straight. Maybe it could be more explicit that it has to be manually inserted (raise a custom error if not present ?).