L.A., San Francisco, Houston are stars of energy efficiency

— -- In a list of America's greenest cities, Los Angeles and Houston wouldn't top most people's lists.

But those two cities, with San Francisco between them, are the U.S. cities that have the highest number of energy-efficient commercial buildings, says the Environmental Protection Agency.

Each building — there are 6,205 of them nationwide — earned the EPA's Energy Star rating. They typically use 35% less energy and emit 35% less heat-trapping "greenhouse" gases than average buildings, said EPA Administrator Lisa Jackson.

Energy Star buildings cost on average about 50 cents less per square foot to operate than average buildings, says Maura Beard of the Energy Star Buildings Program.

There are "phenomenal" savings opportunities in older buildings, says Robert Keller, energy director for JC Penney, which has 52 Energy Star certified buildings. Replacing old heating and cooling systems has saved up to 30% in energy costs a year in some buildings, Keller says. Swapping inefficient fluorescent lights with modern ones "saves millions in kilowatt hours."

The EPA program increased dramatically last year, certifying 3,300 buildings, up 130% from 2007, says Beard. They saved more than $1.1 billion in energy costs and cut 7 million metric tons of greenhouse gas emissions in 2008, according to the EPA.