Years ago, big retailers and tech companies installed solar panels as a way to take an environmental stance. But these days it’s often an economic choice that is fueled by the promise of lower and less volatile energy costs.

On Tuesday, Whole Foods WFM said that it planned a huge project to cover nearly one-fourth of its stores with solar panels. After construction is complete, Whole Foods says it could be among the top 25 biggest commercial U.S. solar suppliers alongside Walmart WMT , Walgreens WBA , and Target TGT .

According to a report last year by the Solar Energy Industry Association: “While solar has long been viewed as an environmentally responsible energy choice, businesses now deploy solar because it is a smart fiscal choice as well.”

Alternative Energy And Jobs
SolarCity workers install solar electrical panels on the roof of a home in Palo Alto, Calif. in 2011.Photograph by Tony Avelar — Christian Science Monitor Christian Science Monitor—Getty Images

Whole Foods’ global sustainability leader, Kathy Loftus, said in a statement that the move was about “lower energy costs,” among other goals. Whole Food’s global energy coordinator, Aaron Daly, told Fortune that the solar project is about “environmental stewardship while saving money and reducing the power price volatility for our stores.”

Another report from SEIA found that in every quarter in 2015, the average cost of solar systems for commercial businesses dropped steadily. Across 2015, the cost of solar systems for commercial businesses slid by an average of 10% to a low of around $2 per watt by the end the year.

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Whole Foods is working with solar panel suppliers NRG NRG and SolarCity SCTY to cover its stores in solar. These companies, which build solar projects for homes and businesses in huge numbers, can provide Whole Foods and others with attractive deals that potentially make solar cheaper than a typical monthly utility bill. These solar deals also fix the rate that companies pay for solar power over time so companies can hedge against a spike in grid prices.

Add in attractive state and federal incentives, and solar looks like a good deal. That is particularly true in California, which is expected to be home to a third of the solar installations for commercial companies and community solar farms next year.

Overall, U.S. solar is growing rapidly. Last year, the U.S. built more solar power than natural gas power for the first time ever.

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Indeed, SEIA’s list of the top 25 commercial solar companies reads like a who’s who of the Fortune 500 including Walmart, Apple AAPL , Intel INTC , Costco COST , and General Motors GM .

Don’t expect the trend to reverse. There are still ample ways to reduce the cost of solar for commercial companies.

In contrast to the really cheap solar deals that utilities are doing, commercial companies are still facing hurdles with so-called soft costs, or the added costs of everything that isn’t hardware like marketing, software, and paper work. The soft costs edge up the total cost of commercial solar. But solar companies expect to be able to reduce these soft costs for commercial solar deployments, too, through new algorithms, use of data and even new startups.