The Solar Energy Industries Association's study of private sector solar installations places Walmart solidly at the top. Other retailers such as IKEA, Macy's, and Kohl's are included, but Walmart and Costco account for almost 104MW of installed energy. According to GigaOM:
According to the report there’s 2.3 GW of solar panel systems installed, delivering over 24,000 systems, for companies and governments (non-residential). In comparison 1 GW is the size of a large coal or nuclear plant.