Using data from the National Renewable Energy Laboratory, Wilson and his colleagues calculated that there are roughly 7.2 billion square feet of roof space on superstores in the continental United States. NREL estimates that about two-thirds of the roofs on large buildings overall are suitable for solar.

Fully building out that potential could produce 84.4 terawatt-hours of electricity each year and avoid more than 52 million metric tons of planet-warming carbon dioxide emissions annually, Wilson’s group reported. That’s roughly half of the electricity that the authors estimated big-box stores use in a year and, they say, the equivalent of powering 8 million American homes, or taking about 11.3 million passenger cars off the road for a year.

Rooftop solar capacity varies widely by both retailer and state. Walmart has by far the greatest potential, with nearly 784 million square feet of available space, more than three times the next closest retailer, Target. Across all companies, California, Texas, Florida and Ohio are the states with the most superstore solar opportunity.

Environment America last published a report on superstore rooftop solar in 2016, and these updated figures represent a roughly 9 percent increase in generation potential since then. “The efficiency of solar and the drop in cost of modules and panels have made the capacity increase,” Wilson explained. “Even though there’s not significantly more big-box stores, the total amount of energy that can be generated has gone up.”

The report acknowledged that the figures overestimate some of the rooftop capacity. “Because total store area was assumed to be equivalent to rooftop area, this is an overestimate for those brands with many stores of more than one story,” according to the report’s methodology.

Nonetheless, current levels of solar installation in the United States remain well below what’s possible. U.S. Energy Information Agency data shows that in 2020 the commercial sector produced roughly 12.9 terawatt-hours of solar power, or about 15 percent of what the latest report calculated was possible on superstore rooftops. But rooftop solar is catching on among big-box stores.

“Ikea is leading the way,” Wilson said. “They’re really ramping up their solar on-site. And Walmart and Target have done that to a lesser extent.”

Ikea reports having solar atop 90 percent of its U.S. locations. And Target ranks first in the amount of on-site solar generation, according to the Solar Energy Industries Association list of retailers with the most on-site solar power in the United States. The company says about a quarter of its more than 1,900 stores nationwide have solar on-site, with the systems providing from 15 percent to 40 percent of a property’s energy needs.

With roughly 2,000 stores in the United States, Home Depot ranks third among superstores in rooftop solar potential. The home-improvement retailer said more than 70 of its stores have solar installed on the roof.

Walmart spokesperson Mariel Messier declined to provide the number of on-site solar installations at the retail chain’s 4,742 stores. “On-site solar, including rooftop solar, is one of the many tools we use to meet our renewable energy goals,” Messier said in a statement. Walmart — along with Target and others — also purchases off-site solar power. Overall, Messier says, Walmart has 550 on-site and off-site solar projects installed or in development.

Jim Fenton is the director of the Florida Solar Energy Center, which encourages companies to take the on-site approach. He said the building must first take steps to make sure it’s operating as efficiently as possible but that on-site solar has “the best internal rates of return.”

“The answer is efficiency, efficiency, efficiency,” Fenton said, “then solar.”

Solar will also be most cost-effective if it’s paired with battery storage, Fenton added. That’s because commercial electricity rates can be much higher during periods of high demand, such as in the evening. So if a company can store power during the day and use it when costs are elevated, it can lead to outsize benefits.

“You need to know your energy loads,” Fenton said. “If they have batteries in combination with the solar, they can really lower their demand charges.”

Fenton said that parking lots can offer even more solar potential, with panels not only providing electricity to the store but shade to cars and perhaps even power for electric vehicle chargers. Wilson’s group didn’t include parking lots in its calculations, but the report touches on the possibility

“Most parking lots could be covered by a solar canopy,” it states, highlighting an Ikea store in Baltimore that installed what looks like solar awnings over the rows of cars in its parking lot. In a news release, Ikea said the move helped cut the store’s energy spending by 57 percent in the fall of 2020.