Tucson Electric Power doubles battery storage capacity at Roadrunner Reserve facility

For Tucson residents facing another summer of extreme heat, Tucson Electric Power’s expanded battery storage could provide an important buffer during the hours when air conditioners are running hardest. Roadrunner Reserve now can discharge up to 400 megawatts, enough to supply about 85,000 homes for four hours when fully charged. The added capacity is designed to reduce pressure on the electric grid during peak demand, when reliable power can be especially important for older adults, low income households and people with medical needs.

TEP and DEPCOM Power executives joined Tucson Mayor Regina Romero and other local leaders Tuesday for a ribbon cutting marking the expansion. The new installation, Roadrunner 2, doubles the storage at the facility. Roadrunner 1 began operating last summer with 200 megawatts of capacity. Together, the two units store electricity earlier in the day, when solar generation is more available, then send that power back to the grid as temperatures rise and customers use more electricity.

The need is particularly clear in a region where summer heat can drive record demand. TEP spokesperson J.D. Wallace said the utility’s peak demand record occurred on Aug. 6, 2025, when temperatures reached 111 degrees and customers used 2,502 megawatts. Roadrunner 1 was already operating at that time and helped TEP meet the demand. TEP says the expanded system has also been tested during the current summer. When a transmission line went offline and more than 400 megawatts of power were suddenly needed, the facility supplied most of that amount.

The two battery units do not have identical equipment. Roadrunner 1 contains 360 batteries, while Roadrunner 2 has 244. Wallace said the newer batteries can hold more energy individually, allowing the second unit to provide the additional capacity with fewer batteries. Beyond emergency support, TEP says storage can help manage fuel expenses by capturing lower priced energy and using it later, when electricity demand and prices are higher.

Romero emphasized the connection between grid reliability and public health during heat waves. Fewer outages can allow residents to remain safely in their homes when air conditioning systems are placing heavy demands on the electrical system. For Tucson’s mayor, cooling is not an optional comfort during the region’s hottest months. She described air conditioning as a necessity, particularly for vulnerable residents facing extreme heat.

The expansion arrives as TEP seeks approval for a major increase in the rates paid by its customers. The investor owned utility serves about 455,000 customers in Pima County and at Fort Huachuca in Cochise County, with residential customers making up roughly 90% of its customer base. TEP has asked the Arizona Corporation Commission to increase retail revenues by about $172 million, or approximately 13%. If approved, the new rates would begin Sept. 1, 2026, and most residents would see a similar increase in their bills.

TEP says it has invested $1.7 billion since 2021 to maintain and improve service, including the Roadrunner Reserve project. The company argues that inflation, the cost of maintaining the grid and the need to earn a competitive return for investors have made it harder to recover those expenses. The utility says the requested increase would support safe and reliable service and strengthen its ability to finance future infrastructure. The Attorney General’s Office has proposed a considerably smaller increase of 4%, arguing that TEP could maintain the same level of reliability by better aligning customer payments with its actual costs. TEP has responded that the smaller increase could weaken reliability and make it harder to attract investment for long term grid projects.

Affordability advocates and local officials have raised concerns about any increase, especially as many Tucsonans already struggle with utility bills. During the rate case, residents described having to choose between electricity, food and medication, or keeping their thermostats at temperatures that could endanger their health. The Residential Utility Consumer Office said people across Arizona reported difficulty managing current utility costs, while Wildfire AZ testified that some of TEP’s lowest income customers spend 11% of their earnings on electricity. The Pima County Board of Supervisors opposed both the proposed increase and TEP’s requested changes to the rate setting process. Tucson also intervened in the case, saying higher rates could make clean energy goals more difficult and increase the city’s own electric costs by $2.1 million, potentially creating pressure for additional taxes or service fees.

TEP is also asking regulators to establish an Annual Rate Adjustment Mechanism, known as ARAM, which would allow rates to be updated each year instead of requiring the type of full formal proceeding now underway. The company says annual adjustments would make rates reflect costs more quickly and predictably. Opponents, including RUCO, have called the proposal too permissive and warned that it could reduce public oversight and encourage costs to be shifted onto customers. Tucson City Council member Miranda Schubert has described the mechanism as a serious threat to oversight, and the council voted to oppose it while supporting the Attorney General’s 4% proposal. TEP’s case hearing concluded May 12, with closing arguments due in July. The commission must ultimately set rates it considers just and reasonable, rather than simply reject the utility’s request.

Roadrunner Reserve is not expected to be TEP’s final storage investment. Wallace said the utility plans to bring the Winchester solar and battery storage project online next year. That facility is expected to add 80 megawatts of solar generation and another 80 megawatts of battery storage. For Tucson residents, the projects point toward a grid that relies more heavily on storing daytime solar power for evening use, while the rate case will determine how much of the cost of that transition appears on their bills.