The number of solar installations grew strongly in the nation’s residential, commercial and utility sectors in the third quarter, largely as a result of falling costs, a federal investment tax credit and state programs that support renewable energies, the solar industry’s main trade group reported on Tuesday.

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The Solar Energy Industries Association said that 684 megawatts of solar photovoltaic capacity was installed in the quarter, 44 percent more than in the third quarter of 2011. For the first three quarters of the year, the industry added 1,992 megawatts, compared with 885 megawatts for all of 2011.

The latest increase brings installed photovoltaic capacity to 5.9 gigawatts. When combined with concentrated solar power installations, which involves focusing solar energy on a single point to begin a heating process that will turn turbines, the figure rises to 6.4 gigawatts, or enough to power around one million average American homes, the organization said.

The cost of system installation dropped in all three sectors, the trade group said. The biggest decline was seen by utilities, for which system prices, now $2.40 per watt, dropped 30 percent from a year earlier. Residential prices dropped to $5.21 per watt from $5.45, while commercial prices fell 13 cents to $4.18 a watt.



Solar system costs have fallen because of an influx of low-cost Chinese panels, a development that has forced some American solar manufacturers to close or downsize and prompted the federal government to impose tariffs on imports of Chinese solar panels.

“The cost of solar panels and systems continue to decline,” Rhone Resch, the industry association’s president, said in a conference call with reporters. “More Americans are seeing a value proposition in solar than ever before.”

John Dean, president of JD Energy, a forecasting firm based in Frederick, Md., said the industry’s reported 44 percent growth in solar installations was a “plausible” number, given the sharp drop in costs, the federal investment tax credit and state renewable portfolio standards that require increasing use of renewable fuels. The 30 percent investment tax credit is due to expire in 2017.

“We are quite bullish on solar, having seen the remarkable decrease in cost and the aggressive position taken by the Obama administration,” Mr. Dean said.

Further growth in the solar industry might be aided by a carbon tax or a cap and trade system for carbon emissions, Mr. Dean said. The political prospects for a nationwide system remain slim, although California will usher in a groundbreaking cap-and-trade system on Jan. 1 and some Eastern states have forged a modest cap-and-trade program for electric power generation.

Further growth is anticipated in the fourth quarter, which has historically been the strongest of the year for photovoltaic installations, according to the industry association and GTM Research, a clean-energy consulting firm that took part in the call.