The EPA has asked Fortune 500 companies to double their current level of green power purchasing. The goal of the green power challenge is to exceed 5 billion kilowatt hours of green power purchases. Five billion kilowatt hours could power more than 400,000 average American homes or avoid the equivalent CO2 emissions associated with more than 680,000 passenger cars each year, according to the EPA.

Corporations that have already said they will significantly increase their green power purchase levels include Cisco Systems, Hewlett Packard, Staples, Starbucks and Wells Fargo.

“Through the Fortune 500 Green Power Challenge, we are encouraging the largest companies in the country to purchase more green power,” said Bill Wehrum, EPA’s acting assistant administrator for Air and Radiation. “Purchasing green power is good for the environment, good for the economy and good for our energy independence.”

There are more than 40 Fortune 500 companies participating in the Green Power Partnership; their annual green power purchases currently total 2.5 billion kWh. EPA’s current top five Fortune 500 Green Power Partners are Wells Fargo, Whole Foods Markets, Johnson & Johnson, Starbucks and DuPont.