If you are an investor thinking in those terms, Tesla will probably seem like a terrible company. Overvalued and unlikely to succeed once traditional car companies start taking EV seriously. Like you say, there are no fundamentals to support current valuation and there will never be.
The only way to look at Tesla is to think about it on more broad terms. It is a car manufacturer, yes, but it also is a company that provides a long range EV support infrastructure (superchargers), the (soon-to-be) largest Li-ion battery manufacturer, a domestic energy storage provider, etc...
With that perspective in mind, SolarCity fits right within the catalog of product and services. Tesla is an engineering company focusing the different applications of renewable energy and batteries. Cars just happen to be one that offers an attractive market to address now and a personal interest for Musk.
And that, I think, it's more than just faith in Elon coming through. It is a measurable market, albeit uncertain and dependent on a lot of social transformation and full of regulatory hurdles.