Four Peoria firefighters who were seriously injured from an explosion at an Arizona Public Service facility in Surprise are now in stable condition.

They were identified as Capt. Hunter Clare, Engineer Justin Lopez and firefighters Matt Cottini and Jake Ciulla, Peoria fire officials tweeted Saturday afternoon.

Three of the firefighters were flown to Maricopa Medical Center's burn unit in Phoenix while another was taken to a West Valley hospital on Friday night. Mike Selmer, a spokesman for the Peoria Fire Department, said one of the firefighters was in critical condition and in surgery until 1 a.m. before becoming stable.

Lopez was identified in the tweet as the "most severely injured firefighter." Selmer said two others who were seriously injured are now stable.

A fourth firefighter, identified as Ciulla in the tweet, was discharged from the hospital Friday evening.

"The guys are doing good," Selmer said in a written statement Saturday.

The four Peoria firefighters were joined by four firefighters from the Surprise Fire Department after responding to a call about 6 p.m. Friday about smoke rising from the APS McMicken Energy Storage facility near Grand Avenue and Deer Valley Road.

The facility contains a utility-scaled battery to store and distribute solar energy, according to APS' website.

The firefighters were evaluating the lithium battery when there was an explosion that left the Peoria firefighters with chemical and chemical-inhalation burns, according to Selmer.

Battalion Chief Julie Moore, a spokeswoman for the Surprise Fire Department, said the Surprise firefighters were evaluated at a hospital but didn't suffer any injuries.

The four Surprise firefighters remain unidentified as of late Saturday afternoon.

Annie DeGraw, a spokeswoman for APS, said the company didn't have any updates on what caused the fire or an estimate on how long the investigation would take.

What we know about the facility

The batteries are stored wall-to-wall in cabinets that look like school lockers inside a facility that's about the size of a large manufactured home. They require constant cooling to maintain a temperature of 75 degrees and prevent them from overheating.

Workers can enter the facility — often referred to as a battery itself — but they do not require constant staffing. The batteries are monitored remotely.

APS has an identical battery several miles away in the Festival Ranch community about 10 miles west of Loop 303 off of Bell Road and Sun Valley Parkway.

The APS battery in Festival Ranch had a large tank of dry chemicals to extinguish fires when The Arizona Republic visited in 2017.

DeGraw said the McMicken facility was identical to the Festival Ranch one, but refused to say whether the tank could have prevented the fire from happening. She also wouldn't comment as to whether an APS employee was monitoring the facility when the fire broke out.

"All of our assets are constantly monitored," DeGraw said. "I can't speculate as to any of the calls or anything that was being monitored at that exact moment."

Moore said firefighters responded to the facility after a passer-by noticed smoke coming from the site.

Friday's fire marks the second fire APS has had at a battery installation. The utility installed a large battery in Flagstaff in 2010 before it caught fire in 2012. The Flagstaff battery was made by Canada-based Electrovaya Inc. while AES Energy Storage built the one that burned at the Surprise facility.

The batteries are part of a program where APS installs solar panels on customers’ homes and pays them a monthly credit for the use of their roof. The batteries are separated from the homes but used to balance the fluctuating energy on the grid caused by things like passing clouds that affect solar output.

APS announced in February that it would install an additional 850 megawatts of batteries on the grid. The McMicken and Festival Ranch batteries, by comparison, are only 2 megawatts in size each. One megawatt is enough capacity to power about 250 homes at once.

Energy Secretary Rick Perry visited the Festival Ranch site on Feb. 22 after APS announced it would add many more batteries to help integrate more solar power from customers' roofs and from large solar power plants into the grid.

Reach the reporter Perry Vandell at 602-444-2474 or perry.vandell@gannett.com. Follow him on Twitter @PerryVandell.

Reach reporter Ryan Randazzo at ryan.randazzo@arizonarepublic.com or 602-444-4331. Follow him on Twitter @UtilityReporter.

Support local journalism. Subscribe to azcentral.com today.