For the first time there were more new solar energy systems installed in the U.S. last year than new natural gas plants built.

The stat is according to a new report from the industry group the Solar Energy Industry Association and analysts at GTM Research. The report says solar supplied 29.5% of the new energy sources built in 2015.

The news shows how solar as an energy source continues to grow dramatically in the United States. Last year was a record-breaking year—again— and solar companies installed 7.29 gigawatts of solar panels in 2015, which was 17% more than the industry did in 2014.

These are the economics behind clean energy:

Much of the growth in solar panels over the past few years has been from the large solar panel farms built in rural areas for utilities. Still, at least half of the solar panels installed last year were for utility-scale solar.

To date, there are now 25 gigawatts worth of solar operating in the U.S. That’s enough power for over four million average American homes.

Just over seven gigawatts is enough solar energy to power a little more than one million average American homes. Two out of that seven gigawatts were installed on the rooftops of homeowners. That’s also a record, according to the report, and the U.S. residential solar market grew 66% from 2014.

Yet even though the sector is growing rapidly, solar still only provides a small percentage of the electricity used by Americans. Coal, natural gas, and hydropower (energy from dams) delivers much more electricity.

But energy in the U.S. is undergoing a transformation thanks to state and federal regulations as well as the dropping costs of clean energy.

Solar companies partly rushed to install solar projects to take advantage of a federal incentive that could have expired at the end of 2016. However late last year that incentive, called the Solar Investment Tax Credit, which provides 30% for the cost of a solar system, was extended to 2019 and will be reduced to 10% by 2022.

Get Data Sheet, Fortune’s technology newsletter.

Outside of the U.S., countries around the world are embracing clean energy, both solar and wind. According to a report from Bloomberg earlier this year, investors and governments around the world put $330 billion into clean energy last year. The bulk of that funding (about $200 billion) went to industrial-sized projects that provide clean power to utilities.

About a third of that funding happened in China as the Chinese government has been aggressively committing to clean energy to meet rising electricity demand and reduce pollution problems. Investors in the U.S. and European countries spent about half compared to China’s clean energy funding last year.