Retailers are typically obsessed with what to put under their roofs, not on them. Yet the nation’s biggest store chains are coming to see their immense, flat roofs as an untapped resource. In recent months, chains including Wal-Mart Stores, Kohl’s, Safeway and Whole Foods Market have installed solar panels on roofs of their stores to generate electricity on a large scale. One reason they are racing is to beat a Dec. 31 deadline to gain tax advantages for these projects.

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