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Tesla Motors, seeking to expand its business beyond electric cars, will announce late Thursday a push into stationary battery units for homes and businesses.

With a $5 billion battery "Gigafactory" under construction outside Reno, creating a business around stationary batteries moves Tesla past focusing entirely on batteries for its electric cars.

Storage batteries could become a huge business because they solve a big problem related to renewable energy. Sure, rooftop solar cells generate juice during the sunny afternoon, but what about the evening?

The extra energy can be stored in batteries to power homes and businesses after the sun goes down. Demand will be helped along by a California Public Utilities Commission mandate that generators of electric power have stationary storage units.

"What Tesla is doing is an all-in strategy to bring the clean-air revolution to the state. And as California goes, so goes the nation," says Roland Hwang, energy and transportation program director for the Natural Resources Defense Council.

Tesla CEO Elon Musk is also chairman of Solar City, a solar energy provider, which now also offers stationary battery units. Solar City says it has sold 300 units already this year.

Analyst Ben Kallo of Baird Equity Research says in a note that developing a stationary storage business can only be good news. "We believe entering storage will allow (Tesla) to more rapidly grow the company while diversifying its revenue streams," he wrote.