As a rule, in retail every square inch of inside floor space is maximized for profitability and efficiency, while outside, the rooftop pretty much sits vacant.

Over the past few years, chain retailers from Target to Costco to Staples have looked at this underutilized space as an opportunity to shave a few dollars off their energy bills—and improve their eco-friendly credibility in the process—by installing solar panels.

“Solar power makes sense from an environmental standpoint,” says Ashley Hawkins, a spokeswoman for Whole Foods Market. “We are able to help reduce peak demand on local power grids to avoid the need to build traditional power plants.”

Even older, more conservative retailers are taking part. Macy's currently has 27 stores with photovoltaic (PV) panels on their roofs and plans to add panels to 33 more by year's end. Kohl's has 60, with plans to add PVs to 130 more.

Most of those stores are purchasing solar-generated electricity under a power-purchase agreement, which allows them to buy power at a fixed rate without investing in the solar systems. Under this scenario, all maintenance, which may include cleaning and maintaining or reviewing PV system components, is handled by the solar energy provider. (A smaller percentage of stores buy the systems outright and perform their own maintenance.) The panels produce about 20 percent to 25 percent of a store's energy requirements.

Fears that Congress would not renew federal solar tax credits for 2009 had caused many stores to rush to complete projects before the end of the year so they could take advantage of the benefits. Others scrapped 2009 projects altogether. Congress, however, relieved worries on Oct. 3 by extending the Commercial Buildings Tax Deduction through Dec. 13, 2013. Originally part of the Energy Policy Act of 2005, the extension passed this time as part of the Economic Stabilization Act, whose main purpose was to stabilize the financial markets.