Elon Musk, the star CEO of the electric car company Tesla Motors and the rocket company SpaceX, staged an elaborate event last month to unveil a new product. Not a sleek new luxury car. Not a rocket that can ferry people to Mars. A home appliance — a battery — that can be mounted on a garage wall and ignored. Here are the 7 biggest questions about Powerwall Tesla’s battery, and the answers brought by the conference.