At Tesla, Popple could rely on early adopters eager to pay a premium for an electric car. As the new chief executive officer of Proterra, which makes an $850,000 electric bus, he’s got a tougher audience: municipal governments that are used to paying as little as $300,000 for a diesel-guzzler. They’re reluctant to invest in the promise of energy savings down the line. Proterra argues that the wait isn’t long. “We’ve seen paybacks against diesel and hybrids in as little as two years and as long as six years,” says Popple. He’s persuaded some powerful backers. On June 18 he announced a $40 million round of investment led by Kleiner Perkins Caufield & Byers (where he remains a partner), GM Ventures, and the Pritzker family’s Tao Invest, bringing Greenville (S.C.)-based Proterra’s total outside funding to $100 million.