The White House just got a little bit greener. Last week government workers started installing solar panels at 1600 Pennsylvania Avenue, fulfilling a promise President Obama made nearly three years ago. The solar panels will help heat the building's hot water and provide electricity.

A White House spokesperson told the Washington Post that the American-made solar panels are "part of an energy retrofit that will improve the overall energy efficiency of the building." The spokesperson said the broader retrofit "will help demonstrate that historic buildings can incorporate solar energy and energy efficiency upgrades." Although the cost was not disclosed, the project is expected to pay for itself in about eight years.

This is actually the third time that solar panels have been installed at the White House. President Jimmy Carter famously installed 32 solar panels above the West Wing in 1979, only to have them removed a few years later by President Ronald Reagan. Interestingly, President George W. Bush installed 167 solar panels on a White House landscape maintenance shed in 2002 or 2003 (reports vary as to the exact date), but he never told the public. Those panels are still producing electricity today.

Environmental groups praised the action, even though it took three years to come to fruition. Daniel Kessler, spokesperson, told the Christian Science Monitor that this month's installation is "a real teachable moment on the capabilities of solar, the economic benefits of it, and on the climate crisis." Back in 2010, founder Bill McKibben carried one of the Carter-era solar panels to Washington from Unity College (where about half of the panels are still in use heating hot water) to urge Obama to install new solar panels on the White House. McKibben said that the Carter panels should never have been taken down. "It's very good to know that once again the country's most powerful address will be drawing some of that power from the sun," he said.

Other energy-saving technologies being installed as part of the retrofit will include variable-speed fans and updated building controls. Obama has set goals that require the U.S. government to reduce its overall energy usage and get 20 percent of its power from renewable resources by the year 2010.

As the finance site The Motley Fool points out, the U.S. installed 3.3 gigawatts of solar capacity in 2012 and is on track to double that in 2013. Costs for solar systems, meanwhile, have dropped 37 percent over the past two years, a trend that is expected to continue.

Obama has not publicly commented on the new solar panels. He and the first family have just returned from vacation at Martha's Vineyard.

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