The nation’s biggest retail store chains have learned that their massive, flat roofs offer a viable way to power their stores, the New York Times reports.

In recent months, giant retailers including Whole Foods Market, Wal-Mart Stores, Kohl’s, and Safeway have installed solar panels to generate electricity on a large scale. 

“It’s very clear that green energy is now front and center in the minds of the business sector,” said Daniel M. Kammen, an energy expert at the University of California, Berkeley.

Company officials mention as particularly important a federal tax credit for renewable energy that Congress has let expire and then renewed several times.  A Congressional dispute over oil drilling has clogged legislation that would renew the credit for next year.

So far, most chains have outfitted fewer than 10 percent of their stores but will consider a solar construction program to outfit all stores if Congress renews a favorable tax provision and more states offer incentives.

The retailers, in anticipation of America’s renewable energy boom, hope to create a bigger market and position themselves as at the forefront of this national shift.  In addition to solar panels, the companies are exploring other renewable energy technologies by testing wind turbines and reflective white roofs, which keep buildings cooler in hot weather.  “It really helps make it economical for the retailer,” said Kim Saylors-Laster, Wal-Mart’s vice president for energy.