The number of U.S. solar power installations is small compared to other renewable sources of power such as wind, but solar is growing.
The U.S. has doubled the number of solar installations to 2 million in the past three years, according to the Scotland-based research firm Wood Mackenzie and Washington, D.C. -based trade group Solar Energy Industries Association.
Wood Mackenzie forecasts that solar power is on track to double again to 4 million installations in another four years.
California accounted for nearly half of the nation's solar installations and is expected to continue to retain that lead, reflecting ambitious state goals for renewable energy generation and high power costs that encourage residents to look for cheaper sources.
Solar energy is catching on in other states, including South Carolina, which went from 1,160 solar projects three years ago to 18,000 today and is on track to add 22,000 more systems over the next five years, according to the report.
Another state expected to add significantly more solar is Illinois, which has 4,000 installations today but is expected to reach 100,000 in five years.