Wall Street has fallen in love with solar power. "SolarCity has captured investors' imaginations and become a potent symbol of a stock market ascent that makes the vertigo-inducing heights of Twitter seem tame. SolarCity's share price, which closed at $59.27 on Friday, has soared more than sevenfold since it went public, and the company, which did not exist eight years ago, is valued at roughly $4.9 billion. Depending on whom you talk to, the rise of SolarCity and similar companies is either a sure sign that solar power is finally having its day or that yet another mania has gripped the markets. Two other companies, SunPower and SunEdison, have also exploded in value. In all, an estimated $13 billion was invested in solar projects in 2013, a tenfold increase since 2007." Diane Cardwell and Julie Creswell in The New York Times.