Correct me if I'm wrong, but quick calculation indicates that Tesla using only energy from Cholla (one of the dirtiest plants in the US) would produce about the same amount of CO2 (13.6kg per 100km) as 5L/100km car (Ford Fiesta) would produce (12kg per 100km).
As you can probably improve your electricity mix quite easily, it sounds like that from fuel CO2 emission perspective, electric car is always at least as good choice as gasoline car.