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Solar power capacity in the U.S. in on a continuous upward trajectory, and according to new research from GTM Research and the Solar Energy Industries Association, the second quarter of 2015 marked a milestone in this growth: 20 gigawatts of operational capacity, or enough to power around 4.6 million homes.

According to the researchers, the U.S. installed 2.7 gigawatts of photovoltaic solar in the first half of 2016, with 1.4 gigawatts coming in the second quarter. The authors expect installations to accelerate for the rest of the year, ending up at the record-breaking number of 7.7 gigawatts by the end of 2015. In 2013 and 2014 solar installations skyrocketed in the final quarter of the year as projects rushed to meet deadlines.

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Earlier this summer, China released figures showing that the country had installed 7.73 gigawatts of solar in the first half of this year. China is considering implementing a solar target 10 times bigger than the current U.S. capacity—200 gigawatts—by 2020. Overall global solar capacity is expected to his 200 gigawatts in the near future, with upwards of 50 gigawatts of installation expected this year.

In 2014, China installed 10.6 gigawatts of new solar capacity while the U.S. installed around 7 gigawatts.

Out of all new electricity installations in the U.S. in the first half of 2015, 40% were solar.

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There are two main branches of solar power: residential, or rooftop, and utility-scale, which comes in the form of solar farms or parks. According to the report, residential solar in the U.S. grew 70% year-over-year in the second quarter with 473 megawatts installed. At the same time, 729 megawatts of utility-scale solar came online in the second quarter with another five gigawatts of capacity currently under construction.

“The utility PV market continues to be the bedrock driver of new installation growth," said Shayle Kann, senior vice president at GTM Research. "And in the second half of this year through 2016, growth will reach new heights as a higher share of what comes on-line stems from projects procured purely based on centralized solar’s cost-competitiveness."

What this means is that solar is becoming cost competitive with other forms of electricity such as natural gas, and less reliant on government subsidies. Which is a good thing, because the federal investment tax credit (ITC), which offers solar farm owners a 30% tax credit, could drop to 10% at the end of 2016. This impending deadline also partially explains the rush to build large solar installations. The (ITC) was first enacted in 2006, and there is concern that its curtailment could lead to a lull in solar installations for at least a few years.

The trajectory of solar's growth in the near future depends a lot on state and federal policies that either stifle or encourage the use of the renewable energy source. With its forward-looking energy and climate policies, as well as abundant sunshine and large population, California easily leads all other states in solar capacity with over 11 gigawatts.

Even with all this growth, solar power still represents just about half a percent of all U.S. electricity supply while coal and natural gas combine to make up around two-thirds of all demand.