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Walmart’s massive scale allows it to dominate a lot of sectors it enters — turns out the retailer is also a behemoth in solar panel power. According to a list of the top 20 companies in the U.S. using solar panel systems, released by the Solar Energy Industries Association (SEIA) on Wednesday, Walmart comes out on top for both how much solar power its stores are generating as well as how many individual systems it’s installed.

Other major U.S. solar customers include Costco, Kohl’s, Walgreens, IKEA, and Macy’s. It’s not surprising that the companies that have a lot of department and grocery stores — with large roof space — would have the most opportunity for installing solar panel systems. 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.

Remember these are already installed systems. Other companies have sizable plans in the works. For example, Apple is building a 20 MW system in North Carolina for its data center.

As I reported yesterday, the trends behind this interest by companies in solar panels is due to rock bottom solar panel prices, policies in some key states, like California, Arizona, and New Jersey, and also new business models for financing solar panel systems. Companies like SolarCity have created businesses around raising money from investors like banks (or even Google) to finance the upfront cost of a solar panel system. SolarCity’s customer can then pay for the solar system over a 20-year, or so, time frame (which is more convenient and affordable), and the bank can get back a return on that investment (like 12 percent).

However, according to the report released on Tuesday about the growth of solar panel systems in the U.S., this rapid adoption is expected to slow down in the second half of this year and next year.