Grace Hood/CPR News The Tesla-made lithium ion battery unveiled by United Power Cooperative Tuesday, Dec. 18, 2018. The battery storage system is the largest in Colorado.

Brighton-based United Power Cooperative unveiled the largest battery storage system in Colorado Tuesday. The lithium ion battery cost millions, and could take decades to pay off — yet officials expect it to to save customers $1 million per year.

“We think this battery and others like it will be one of the things that will have to happen to make a carbon free future happen,” United Power President John Parker said.

The lithium ion battery was supplied by Tesla and built in the company’s new Gigafactory. The battery is based just east of Longmont.

It’s a big deal because energy storage will become increasingly important as companies, utilities and the state of Colorado seek to add more wind and solar to the grid. Gov.-elect Jared Polis is expected to pursue policy that promotes his 100 percent renewable energy bill.

The big challenge in pursuing a higher level of renewable power stems from the fact that the sun doesn’t always shine and the wind doesn’t always blow. The grid needs more backup energy in storage. And that’s why batteries like the one near Longmont are important.

“There will be other technologies that we will need on the grid,” said Xcel Energy Chairman and CEO Ben Fowke at a December announcement where the utility declared a 100 percent carbon free goal.

Technological innovation is just one piece of the puzzle. Another is the future restructuring of utilities. Clean Cooperative reports that a new policy adopted by Tri-State Generation and Transmission could discourage other rural electric coops from pursuing battery storage.

In the coming months, United Power will focus its attention on testing how much the battery stores and how unloading the power onto the grid works. A key focus will be how that works during peak hours of the day when energy demand is the highest.

United Power will use that data to decide whether to unroll a battery storage subscription program, one that could work similar to how community solar programs work.

Editor's Note: A previous version of this story misstated the storage capacity of the ion battery.