The nation's largest retailer is aiming to be the greenest retailer too, announcing Monday that it will install lower-cost thin film solar-generating systems on 20 to 30 sites in California and Arizona.

The announcement is another step toward Walmart's goal of 100% renewable energy. That's an ambitious program, but at $405 billion in annual revenue it has the financial horsepower to be a leader in the industry.

As announced, Walmart newest program calls for thin solar panels, which are less expensive but also less efficient than traditional panels, according to this USA Today story, but should still supply 20% to 30% of the total energy needs for each location, Walmart said in a statement.

That is enough to power about 1,750 houses for a year and will cut more than 11,650 metric tons of carbon dioxide equivalent annually, which is the same as removing 3,000 vehicles from roads.

We've written about Walmart's sustainability efforts here, here and here. The company is using a variety of green technology, including wind turbines (in some cases turbines sit atop parking lot lights) and fuel cells.

In Mexico, it buys energy from a local wind far and has installed solar panels on two facilities. In Canada, it is testing geothermal, fuel cells, solar, wind and is the largest corporate purchaser of low-emission power through a local clean-energy provider.

(Photo of wind turbines upon a Sam's Club from ecoseed.org)