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KAI RYSSDAL: More than a year ago, Wal-Mart CEO Lee Scott made a speech to his employees. He promised the world's largest retailer would someday run on 100 percent renewable energy. On top of that, he said Wal-Mart would eventually produce zero net waste.

Tomorrow marks one of the first major milestones in Wal-Mart's green makeover. Bids are due for a solar power project that could become the biggest in history. From the Marketplace Sustainability Desk, Sam Eaton reports.

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SAM EATON: When you talk about Wal-Mart, it's hard not to address the company's sheer size. This is the world's largest retailer. It ranks second on the Fortune 500 list, with annual revenues some small countries can only dream of. So when Wal-Mart puts its feelers out for what could become the largest solar energy project ever, industry players like Andrew Beebe listen.

ANDREW BEEBE: If they were to install solar on a half or a third of the stores around the country, they could literally quickly take up the entire supply, you know, for the next few years — at least on a projected basis.

Beebe's company, Energy Innovations, is now installing what is currently the biggest solar energy system at Google's corporate headquarters. Wal-Mart's proposal, if fully realized, would be about 60 times the size of that. Clean energy analyst Joel Makower says a project on that scale would do for the solar industry what McDonald's did for the hamburger: introduce it to the masses.

JOEL MAKOWER: This is what Wal-Mart does best. They look at a technology, at a purchase, at a supplier and figure out how to optimize it and how to ring out the excess costs — and how to use their buying power to make it affordable.

Wal-Mart says it will begin installing solar panels on stores in five states, including California and Colorado. But Makower notes it is only a proposal. And even if the project does go forward, he says the challenge will be to do it in a way that doesn't chase the industry overseas in search of cheaper labor.

In Los Angeles, I'm Sam Eaton for Marketplace.