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Last year there was a record-breaking 3.3 gigawatts worth of solar panels — or 16 million individual solar panels — installed in the U.S., making solar power the fastest-growing energy source domestically. That’s according to a new report from the Solar Energy Industries Association and GTM Research.

In comparison, a large nuclear or coal plant can generate about a gigawatt, so there was the equivalent of three of these types of large power plants installed across rooftops in the U.S. in 2012. The 3.3GW worth of solar panels was more than the three previous years combined, said the report, and showed a 76 percent growth over 2011.

A gigawatt of those solar panels were installed in California, while Arizona and New Jersey also installed hundreds of megawatts. The fourth quarter in particular in the U.S. saw 1.3 GW worth of solar panels installed, which was a record-breaking quarter, said the report.

The reason that solar panels were the fastest growing type of energy in the U.S. last year was due to the fact that the price of solar panels has dropped dramatically. Companies are also offering financing deals that cover the upfront costs of the systems, and states are offering strong incentives. It’s not a coincidence that states like California with the best subsidies for solar panels had the most installations.

The growth of solar panels in the U.S. is supposed to continue, says the report. For 2013, the researchers have estimated that there will be 4.3GW of solar panels installed, which would be an increase of 29 percent over 2012.

There are now 300,000 solar panel systems operating in the U.S. and a total of 7.2 GW of cumulative solar panel power in the U.S. Concentrating solar systems — which use the sun’s heat instead of light to produce electricity — deliver 546 MW worth of power in the U.S.

The report is note worthy because solar is becoming a game changer. It, along with wind power, are just starting to deliver real volumes and change the way electricity is created in the U.S.

In comparison, 13 GW of new wind power were installed in the U.S. in 2012. Last year natural gas rose to 27.2 percent of total energy consumption (including electricity, heat and transportation) from 23.4 percent in 2007, according to Bloomberg New Energy Finance. Over the same time period, clean power, which includes wind, solar, biomass and hydropower, rose to 9.4 percent from 6.4 percent. Coal declined as part of the U.S. energy mix to 18.1 percent from 22.5 percent, as did oil to 36.7 percent from 39.3 percent.