WASHINGTON -- While attention in the energy sector has focused on fracking and natural gas, behind the scenes another energy source has seen a banner year: solar power.

Installation is up, way up. How up? The U.S. solar energy grid now generates enough energy to power 2.2 million homes. According to the U.S. Solar Energies Installation trade group, more solar cells were installed in the last 18 months than in the previous 30 years combined.

"Photovoltaic (PV) installations continued to proliferate, increasing 41 percent over 2012 to reach 4,751 [megawatts]," the Solar Energies Installation trade group said in a release Wednesday. "Solar was the second-largest source of new electricity generating capacity in the U.S. [at 29 percent], exceeded only by natural gas. And the cost to install solar fell throughout the year, with average system prices ending the year 15 percent below the mark set at the end of 2012."

EcoWatch, an environmental news site, noted the release in an article Wednesday. The site's Rhone Resch drew attention to states where solar has seen the greatest inroads.

"California continues to lead the U.S. market and installed more than half of all new U.S. solar in 2013," Resch wrote. "In fact, the Golden State installed more solar last year than the entire United States did in 2011. North Carolina, Massachusetts and Georgia also had major growth years in 2013, installing 663 megawatts (MW)—more than doubling their combined total from the year before. On the whole, the top five states (California, Arizona, North Carolina, Massachusetts and New Jersey) accounted for 81 percent of all U.S. PV installations in 2013."

A graphic image of the top ten states by installation in 2013 is available here (pdf).

The cost of installing photovoltaic modules -- the panels that produce translate sunlight into power -- increased slightly from 2012 to 2013, the energy trade group said. But the price of components dropped.

"Prices fell substantially for other components such as inverters (which decreased by 15 percent to 18 percent) and racking systems (19 percent to 24 percent)," the trade group reported. "Other factors including downstream innovations drove down overall system prices throughout the year in all market segments. By the end of the year, system prices had fallen 9 percent in the residential market, 16 percent in the non-residential market and 14 percent in the utility market."

Solar has long faced the issues of price and energy storage. With the introduction of better batteries and lower prices, solar power has become increasingly viable. The demand for electric cars -- such as Tesla's Model S -- has driven new efforts to produce batteries that can power vehicles for longer distances at lower cost.

Coal power, the number one source of U.S. electricity, is also under threat from renewable sources and natural gas. According to U.S. Energy Information forecasts, a spate of coal power plants will face closure by 2016. New emissions standards and lower gas prices are encouraging U.S. utilities to shutter coal plants. The EPA recently proposed regulations that will impose higher standards on the release of climate change gases, mercury and toxic chemicals.

Key figures, according to to the solar trade group:

* The U.S. installed 4,751 MW of solar PV in 2013, up 41 percent over 2012 and nearly fifteen times the amount installed in 2008

* There is now a total of 12.1 GW of PV and 918 MW of CSP operating in the U.S.

* There were 140,000 individual solar installations in the U.S. in 2013, and a total of over 445,000 systems operating today

* Q4 2013 was by far the largest quarter ever for PV installations in the U.S., with 2,106 MW energized, up 60 percent over the second-largest quarter (Q4 2012)

* More solar has been installed in the U.S. in the last eighteen months than in the 30 years prior

* The market value of all PV installations completed in 2013 was $13.7 billion

* Solar accounted for 29 percent of all new electricity generation capacity in 2013, up from 10 percent in 2012. This made solar the second-largest source of new generating capacity behind natural gas.

* Weighted average PV system prices fell 15 percent in 2013, reaching a new low of $2.59/W in the fourth quarter

* We forecast 26 precent PV installation growth in 2014, with installations reaching nearly 6 GW. Growth will occur in all segments, but will be most rapid in the residential market.

* The U.S. also installed 410 MW of concentrating solar (CSP) in 2013, increasing total CSP capacity in the U.S. by more than 80 percent

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