commercial buildings account for almost 20 percent of the nation’s energy use and more than one-third of its greenhouse gas emissions. in this animated energy 101 video, energynow! correspondent lee patrick sullivan looks at how one of new york city’s most famous structures, the empire state building, is becoming one of its greenest.

the building uses 10 megawatts of power per year, with an annual electric bill of about $11 million. but after a 5-year, $500 million retrofit is complete, its energy use will drop by 38 percent and the bill will be cut almost in half. the makeover includes almost $20 million work of energy efficiency improvements.

the building’s heating and cooling will come from a combination of steam power and wind energy credits. also, each of its 3 million lights will be switched from incandescent to fluorescent, reducing lighting costs by 75 percent. every one of the building’s more than 6,500 windows will be upgraded with a film coating and argon and krypton gas between the panes. this allows the building to retain more heat in the winter while keeping it out in the summer, while making better use of natural light. the building is also letting in more light by moving air-conditioning units from the ceilings of each floor to the ground.