The solar industry in the U.S. is now so large that 2 million separate installations are in service across the country, according to Wood Mackenzie Power & Renewables and the Solar Energy Industries Association (SEIA). Projects can be as big as a few square miles, but most are small, with 96% of the installations on residential rooftops. This type of installation is the specialty of Tesla (NASDAQ:TSLA), Sunrun (NASDAQ:RUN), and Vivint Solar (NYSE:VSLR). The big projects are built by companies like First Solar (NASDAQ:FSLR) and financed by utilities and asset owners like NextEra Energy Partners (NYSE:NEP) and TerraForm Power (NASDAQ:TERP), which have put billions of dollars into their solar projects.

It took the industry over 40 years to reach 2 million installations, but analysts now expect the next 2 million to take just four years to install. As the industry grows in both scale and geographic reach, it creates more opportunities for investors.

Where solar energy is going

According the the SEIA’s data, California accounted for 51% of the first 1 million solar installations in the U.S., so it is certainly the industry’s hub. But among the second million, the state accounted for only 43% of installations, so geographic diversity is improving. In the last few years, states like Florida, Texas, and Minnesota have opened up as large growth markets, and economics, not subsidies, are driving the industry’s growth.

Read more: 2 Million U.S. Solar Installations Are Just the Start