As the Obama administration prepares to leave office, it is seeking to underscore just how much has changed in the last eight years in the way we get energy — and to take some credit for it. Since 2008, costs for wind and solar have plunged by 40 and 60 percent, respectively, according to an analysis provided by the Energy Department. That’s even as the United States has installed 100 gigawatts, or billion watts, of generating capacity