Renewable energy in Minnesota is about to take a potentially big step forward.

Connexus Energy is building two electricity "storage" systems - solar-panel arrays connected to big batteries. Each battery can store up to two hours of power, allowing Connexus to inject renewable energy into the grid on command.

"It's a big endeavor, especially since it is new and first of its kind," said Greg Ridderbusch, CEO of Ramsey-based Connexus, the state's largest cooperatively owned electricity provider. "It's not research and development. This is a commercial project."

The $31 million undertaking marks the first commercial-battery deployment in Minnesota, and the largest by an electric co-op in the country. Falling battery prices have made "solar-plus-storage" a viable alternative for Connexus.

The improving economics of storage, combined with regulatory mandates in some states, have prompted a surge in battery projects nationwide. Minneapolis-based Xcel Energy has plans for one of the country's largest energy-storage initiatives in Colorado.

Connexus, like most retail electric co-ops, buys its power from a wholesale co-op, in this case Maple Grove-based Great River Energy. But Great River's contracts allow its customers to generate up to 5 percent of their own power, making room for Connexus' renewable-storage project.

The project will be watched closely by the power industry in Minnesota and beyond, as batteries are viewed as increasingly important to the electric grid of the future.

They could help reduce the use of fossil-fuel generation, particularly so-called "peaker" plants, which operate only at times of maximum demand. Power stored in batteries can be dispatched onto the grid quickly. And batteries can facilitate more solar and wind power.

"We expect that battery storage will allow higher volumes of renewable energy to be integrated on the grid," Ridderbusch said.

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