Brighton-based United Power, the state’s largest electric cooperative, and Ameresco, a leading cleantech firm, are teaming up to install Colorado’s largest battery storage project to date.

Under a 20-year agreement, Ameresco will install 78.3 megawatts (MW) of battery capacity at eight substations on United Power’s distribution system to store power so the cooperative can better balance its system during periods of high demand as it takes on more generation from variable wind and solar sources.

“The use of batteries on our distribution network is essential to a resilient and responsive power system, and we are excited to be moving ahead with one of the most aggressive plans for such a system,” Mark Gabriel, president and CEO of United Power, said in a release. “These battery arrays will allow us to balance our power needs throughout the day and incorporate local renewables more efficiently.”

Under the agreement, Ameresco will construct four 11.75 MW and four 7.84 MW battery arrays at eight different substations located in Adams, Bromfield and Weld counties. The batteries will draw down during periods of heavy consumption — days like Tuesday, when air conditioning units were cranked up.

“To our knowledge, no other Colorado utilities are deploying batteries on this scale at this time. We believe this is a pioneering effort — one that continues our commitment to being an innovator in the industry,” said Trista Fugate, United Power’s chief marketing officer, in an email.

In 2018, United Power was the first utility in Colorado to install a large-scale battery, at 4 megawatts, for its Carbon Valley Service, Fugate said. It was also an early supporter of community solar projects.

United Power said the battery arrays, which are set to operate next year, are necessary as it prepares to end its longtime relationship on May 1 with wholesale power provider, Westminster-based Tri-State Generation and Transmission. Although United Power is Tri-State’s largest customer and technically one of its owners, the two parties could not agree on how quickly coal-sourced power would be phased out and replaced by renewable sources, including from within United Power’s own territory and customer base.

Last week, United Power said it had struck an agreement to buy a third of its power from Guzman Energy, a firm that rural electric cooperatives that broke away from Tri-State have turned to. Guzman obtains more of its energy from owners of solar and wind projects.

United Power has more than 6,500 miles of distribution lines over a 900-square-mile service territory located mostly northeastern side of the metro area and into Weld County and also serves the Golden Gate and Coal Creek canyons. The member-owned cooperative counts about 110,000 customers, primarily residential. It is also an important power supplier to Colorado oil and gas operators in the Denver-Julesburg Basin and to farmers in the region.

Updated 1:25 p.m. July 13, 2023 The headline of this story has been updated to correct the name of United Power.