NEW YORK (CNNMoney.com) -- Some of America's largest corporations pledged Monday to support green power, responding to a challenge posed by the Environmental Protection Agency.

So-called green power is generated from renewable energy sources like the sun, wind and biomass such as plants or garbage and helps reduce greenhouse gas emissions that contribute to global warming.

The EPA is calling for Fortune 500 companies to double their green power purchases to more than 5 billion kilowatt hours by the end of next year - or enough electricity to run more than 400,000 homes in the U.S. a year.

Besides helping companies reduce their carbon footprint, purchasing green power is a smart business decision, Kathleen Hogan of the EPA said during a press conference.

Some 40 companies are taking part in the Fortune 500 campaign. Among the participants, Wells Fargo (Charts), Whole Foods (Charts) and Johnson & Johnson (Charts) are the biggest buyers of green power, while Cisco Systems (Charts) and Hewlett-Packard (Charts) both announced sharp increases in their purchase levels.

Notably absent from the list of partners are Exxon Mobil (Charts) and Wal-Mart (Charts), No. 1 and No. 2 on the Fortune 500 list.

Exxon has long come under fire from environmentalists. Wal-Mart, meanwhile, has announced several eco-friendly initiatives, but critics have questioned the seriousness of its intentions.

The latest push by big businesses to "go green" has come amid a run-up in energy prices and growing concern over the future of the environment.