The Tesla Motors CEO this week announced plans to purchase solar power-provider Solar City. Critics and a dropping stock price say Tesla's push beyond the auto industry, which it's hardly mastered, is a gamble too far. But  Tesla is playing the long game.

"The world does not lack for automotive companies," Musk said Tuesday. "The world lacks for sustainable energy companies."

In the two and a half months since introducing the Model 3, Tesla Motors has taken more than 400,000 deposits for the long-awaited $30,000 sedan, due to start production late next year. The rest of the industry is also slowly ramping up its electric offerings, which shouldn't cost any more than a gas-powered car by 2022. That has potential environment benefits, but it won't help much if the aging, fossil-fuel reliant American energy grid can't keep up without piling coal into the furnace.