Elon Musk is what I call a "systems realitator". In order to bring his major dream, going to Mars, into reality the next big field (besides propulsion & rocket technologies which he is doing with SpaceX) is energy, and how to generate this in space.
Abundant energy in space is solar light, so SolarCity is the next puzzle block.
However this still not grabs people's attention really. What shakes emotions right now (and that hasn't gone lower in the last years) is cars in particular.
TESLA with all its connectivity, and "gadgets" around electric energy use on the road is the practical use (business) case on earth, that is pulling interest, competition, and as a consequence of that ever improving parameters of solar, and energy storage technologies.
TESLA is sort of a magnet for future technologies to emerge, and pulling present technologies to their very best.