Sure, you've heard about Big Government. But have you seen its energy bill? With $25 billion in annual power and fuel costs, the U.S. government is the largest single energy consumer in the nation's economy, and among the largest in the world. Of course, the 500,000 buildings the government leases or owns include not only office space, but supercomputers, hospitals, and aviation safety radar facilities. And the 600,000 vehicles that Uncle Sam has to tank up include those conveying troops engaged in active combat. For years, it has been clear that there's a big opportunity in the sheer size of this energy footprint. The U.S. Congress has been setting federal efficiency goals since 1978—in hope not only of cutting costs and foreign oil dependence, but also of leading the way for energy savings in the private sector. The Obama administration now is seeking to ramp up that effort dramatically—with the help of an unprecedented $4.5 billion in stimulus funds to be spent by next September entirely for federal green building and renovation projects. By executive order, the federal government is aiming to reduce its greenhouse gas emissions by 28 percent by 2020.