As a fully integrated transportation and energy provider/supplier, they will simply be called...TESLA. Just like classic brand names simplified; International Time Recording Company and Computing Scale Company to IBM, PepsiCola to Pepsi, Apple Computers to Apple, etc. They have already simplified their internet address from teslamotors.com to tesla.com. Anyone looking for the company batteries will be directed to this site. Elon Musk is already the largest shareholder of Solarcity and Tesla Motors, so their trillion dollar market value/merger will most likely be under this one moniker.
Musk says he's ready to launch new Tesla 'master plan' | Reuters