Compared to successfully launching a private rocketry business and an electric vehicle brand, Elon Musk's quest to line your roof with solar cells is a less flashy endeavor. He pitched it last August as Tesla's preamble in its eventually successful attempt to acquire SolarCity, a part of his greater goal to wean society off fossil fuel dependence. We haven't heard much else about it since. But in typical fashion for his announcements -- that is, in a tweet responding to a random question -- Musk told the world that folks can start placing orders for solar roofs in April.