900kWh per month is 30kWh per day, generally speaking a 6kW solar array would be sufficient. To more precise you should go to
https://sam.nrel.gov/ and use the solar calculator for your location and roof direction to determine the capacity of the solar array needed and then compare to the actual roof square footage you have available minus plumbing vents Etc.
You didn't mention if you want to charge the Tesla during the day when the sun is out or store the entire required charge in batteries for use at night. This consideration would have a major impact on price.
Overall based on limited info provided, it does seem reasonable to set up solar for charging your Tesla. Batteries are the deciding factor in my opinion.