It costs 3.5 cents to save a kilowatt-hour in energy efficiency. That’s really cheap—as long as the low-hanging fruit holds out.

Two things you should know about energy efficiency: it’s still the cheapest “fuel” around, but it’s getting more expensive, according to a new report from the Edison Foundation’s Institute for Electric Efficiency (PDF).

In 2010, utilities spent $4.8 billion on ratepayer-funded energy efficiency programs, and as a result saved about 112 terawatt-hours of electricity, enough to power nearly 10 million homes, the report finds. (That’s about 2.5 percent of the 4,000 terawatt-hours of electricity the U.S. consumed in 2009, according to the Energy Information Administration.)

via Energy Efficiency: Cheapest Power Around, but Getting More Expensive : Greentech Media.