Tesla Motors Inc. shares fell in after-market trading Wednesday after the Silicon Valley electric-car maker reported a net loss of $49.8 million for the first quarter, reflecting a surge in costs associated with expanding its product lineup and customer-service operations and launching a huge new battery factory.

Tesla Chief Executive Elon Musk disclosed during a conference call after the close of New York stock trading Wednesday that Panasonic Corp., manufacturer of the batteries used in his company's luxury electric cars,...