This next week will be a big litmus test for many supporters of green energy and electric cars as Tesla Motors, the electric car startup scheduled to debut in public trading on Tuesday, could deliver the biggest IPO for a U.S. car company since Ford Motor’s initial public offering more than half a century ago. But while the day of the IPO itself will be a fleeting event, Tesla’s performance on the stock market will deliver something more lasting: a signal about the degree of public confidence in electric vehicles at this early stage of the market, and the role that IPOs will play in financing next-gen cars.