Third Party Owned PV System

Hi
I am using SAM to model a PV system installed on a multi-apartment building. I want to evaluate the business case from the perspective of a external third party owner (i.e. invests and operates system, selling generated electricity onsite to residents).

I will use the electricity load of the building as an input, however any excess electricity not consumed on site will be sold at a different rate (i.e. wholesale or feed-in rate).

If I understand correctly, the third party ownership model evaluate the case from the perspective of the electricity end-user/ off taker. Can you suggest what financial model I can use to evaluate a system installed and operated by a PV company on a third party building?

regards,