Lyndon Rive, CEO of the dominant U.S. solar installer SolarCity, likens the trajectory of solar panels to the development of computers over the past 20 years. Decades ago business computers were made by a variety of small companies. Today the PC industry has consolidated around big names like HP HPQ -0.90% , Dell, Lenovo, and Apple AAPL -2.02% . The same trend is afoot for the companies that are installing and financing solar panels. A handful of companies—SolarCity, SunEdison SUNE -7.62% , and Sunrun among them—have been scooping up smaller firms in their quest to survive in a market where scale is everything.