@rathod-b do we need to use a dummy default tilt for PV in validators.py here? Could we not set null=True in models.py and update in validators as we do for other inputs (e.g. operating reserves required)?

@rathod-b do we need to provide this message in msg_dict["info"] every time off_grid_flag is true?:

            if "ElectricTariff" in self.models.keys():
                msg_dict["ignored inputs"] = ("ElectricTariff inputs are not applicable when off_grid_flag is true, and will be ignored. "
                                "Provided ElectricTariff can be removed from inputs")
            msg_dict["info"] = ("When off_grid_flag is true, only PV, ElectricStorage, Generator technologies can be modeled.")```

Could we just show it if a disallowed key is supplied, as we do in the Julia Pkg [here](https://github.com/NREL/REopt.jl/blob/8d7beb8a916d1cdf8571c268d44a6ef86dc07a7b/src/core/scenario.jl#L88)? 

@rathod-b This is mostly for my own edification, but do we (and why?) need null=True in models.py if we are also providing a default value, such as for the offgrid_other_capital_costs here?

I've finished reviewing and left some comments. The PR description also needs to be filled out.

@rathod-b This is mostly for my own edification, but do we (and why?) need null=True in models.py if we are also providing a default value, such as for the offgrid_other_capital_costs here?

Technically we can remove the null=True from fields like this because per my understanding, setting a default value means this field is not going to be null (unless the code fails). If we were to update the default value downstream in cross_clean() then we would remove the default here so the field can be set correctly in cross_clean (as we do with PV tilt now).

My understanding is also that null=True isn't necessary if a default is provided, though I haven't been able to find this said explicitly in the Django docs so had been holding off on the issue. Removing all unnecessary null=True is something that I've been keeping in mind as a low priority to-do for a while and was going to include in a general validator.py/models.py clean up after finishing more urgent capabilities.

@rathod-b This is mostly for my own edification, but do we (and why?) need null=True in models.py if we are also providing a default value, such as for the offgrid_other_capital_costs here?

Technically we can remove the null=True from fields like this because per my understanding, setting a default value means this field is not going to be null (unless the code fails). If we were to update the default value downstream in cross_clean() then we would remove the default here so the field can be set correctly in cross_clean (as we do with PV tilt now).

My understanding is also that null=True isn't necessary if a default is provided, though I haven't been able to find this said explicitly in the Django docs so had been holding off on the issue. Removing all unnecessary null=True is something that I've been keeping in mind as a low priority to-do for a while and was going to include in a general validator.py/models.py clean up after finishing more urgent capabilities.

Good idea, once we merge everything into API v3 over the next few weeks it would be nice to clean up the code and remove redundancies/make functions for repetitive validations in model cross_clean