The world’s biggest retailer is taking a big step toward its goal of eventually getting 100 percent of its energy from renewable resources.

Wal-Mart announced earlier this week that it will install 20 to 30 thin film solar arrays on the roofs of stores in California and Arizona.

Each array is expected to generate 20 to 30 percent of power for each of the stores where they’re installed, said Wal-Mart spokesman Kory Lundberg. Together, the arrays are expected to generate 22.5 million kilowatt hours of energy.

Wal-Mart decided to go with thin film panels rather than the more traditional photovoltaic panels for a relatively simple reason—to see how well they work.

“We know it’s going to take a number of different technologies to help Wal-Mart reach its goal of being supplied 100 percent by renewable energy,” Lundberg said. “This is another example of testing different technologies and helping make the business case for their use.”

Wal-Mart announced its clean energy goal in 2005, but gave no timeline for the goal and doesn’t announce its progress. But the company has mixed and blended a number of clean energy technologies, including wind generators in California Sam’s Club parking lots and fuel cell generation at one Lancaster, Calif. store.

Lundberg said Wal-Mart knows it’s a leader and that where it goes, other companies will follow. As a result, the company is trying to make responsible energy choices and do the legwork, experimenting with different technologies. The retail giant has enough different locations (8,400 to be exact) and enough roofs to try a number of different renewable energy options, Lundberg said.

“We certainly have interest in all kinds of technologies, and we know our involvement helps strengthen the business case for those resources and help bring them to market and make them a more attractive option quicker,” Lundberg said.

The California solar firm SolarCity will design, install, own and maintain the thin film panels on the new Wal-Mart stores. The panels will be a mix of both copper indium gallium selenide (CIGS) thin film solar panels and cadmium telluride thin film technology.

These installations will almost double the number of Wal-Mart stores with solar power generation. There are already 31 stores in California, Hawaii, and Puerto Rico with Photovoltaic panels.