For longtime proponents of renewable energy, figuring out how to keep the lights on when the wind doesn’t blow and the sun doesn’t shine has been a key challenge. Harnessing the excess energy from a turbine or solar panel to use later was the “holy grail” or “golden key,” as one Colorado utility executive calls it.

Times have changed. Xcel Energy-Colorado, the state’s largest electric utility, will add a total of 275 megawatts of large-scale battery storage to solar arrays in its newly approved Colorado Energy Plan. The three separate clusters of batteries — two near Pueblo, one near Commerce City — are part of Xcel Energy’s overall push to increase its use of renewable energy sources to 55 percent by 2026 and cut carbon dioxide emissions by nearly 60 percent.

The storage projects, among some of the largest in the country, are scheduled to go live in December 2022.

“Building a 200- to 300-megawatt project, that’s real stuff. That’s a real deal,” said Paul Denholm, a principal analyst at the National Renewable Energy Laboratory in Golden.

And the pace and extent of the changes that have made it possible for utilities like Xcel Energy and United Power, a Brighton-based electric cooperative, to start using batteries has surprised even industry experts.

“It’s big. We used to talk about individual battery installation,” Denholm said. “Ten years ago, we weren’t talking about (large-scale projects) at all.”

As with solar and wind power, faster-than-expected declines in prices and advances in technology have produced results sooner than anticipated.

“We saw some extremely competitive pricing for these projects, frankly,” Jonathan Adelman, Xcel Energy’s vice president of strategic resources and business planning, said of the battery facilities.

Outside contractors will install and operate the solar and battery projects.

The cost of large-scale battery storage installations has dropped more than 70 percent since 2010, according to the Energy Storage Association, an advocacy and trade organization.

“Costs of grid battery storage are half of what they were four years ago, and in another five to six years, costs will be half of what they are now,” Marissa Gillett of the association said in an email.

At end the of 2017, storage projects with a total capacity of 800 megawatts were in place nationwide.

The technology of stationary storage batteries has benefited from advances in automotive batteries, Denholm said.

“Cell phones and other consumer electronics have driven the scale of the lithium-ion battery market. Electric vehicles are riding the wave, and grid batteries are the next in line. So, we have our device addiction to thank for how cheap the batteries are today,” said Mark Dyson, a principal at the Rocky Mountain Institute, a research organization that focuses on making the transition from fossil fuels to renewable energy.

When Xcel Energy installs the full 275 megawatts of storage, the batteries will help it meet about 5 percent of its peak hourly load on a summer day, when demand is typically the highest, according to the utility.

Until Xcel Energy’s new plan is fully implemented, United Power might have bragging rights for the largest lithium battery in Colorado. The electric cooperative, whose territory forms a kind of horseshoe around Denver to the north, is working with Chicago-based SoCore Energy to install a module of batteries manufactured by Tesla at a substation east of Longmont. The facility will have a capacity of 4 megawatts.

Construction at the 7,000-square-foot site is expected to be completed next week and the facility should be up and running in early November.

“Storage is almost considered like the golden key” to expanding the use of renewable energy, said Jerry Marizza, the cooperative’s new business director.

However, United Power is not pairing the battery with a solar or wind facility. It will store energy generated by all sources overnight, when demand is low, and discharge the energy when demand is high, typically in the afternoon to early evening.

And United Power doesn’t plan to use the battery every day. In fact, Marizza expects to rev it up fewer than a hundred days a year.

“The primary business case for this is peak shaving — lower our peak demand on hot summer days, really any time,” Marizza said.

The battery, which would run for up to four hours, will help United Power offset roughly the equivalent of power for 600 to 700 homes. United Power expects the facility to save about $1 million a year by storing energy that otherwise couldn’t be used, allowing the utility to cover the project’s cost in seven to eight years.

United Power is fielding questions from other rural electric associations in the state and beyond as it gets ready to start using the battery, Marizza said. The utility was in a similar situation when it started a community solar program, which allows customers to pay for a solar panel in a central array in exchange for credit on their bills.

“As a utility, we have to keep our hands in the mix,” Marizza said.