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.