Transformer explosion at California battery storage site triggers mass outage

On 9 January 2026, around 4:15pm, an explosion occurred near a transformer at ENGIE's Sierra Battery Storage facility near Chinese Camp in Tuolumne County, California. The Tuolumne County Fire Department initially reported the explosion as involving lithium-ion batteries at a neighbouring facility, Pacific Ultrapower, but the sheriff's office later confirmed the explosion was linked to a transformer at ENGIE's site. ENGIE stated the facility experienced an electrical issue, causing some circuits and equipment to trip, but confirmed there was no fire. Nearly 30,000 PG&E customers lost power across the county, with about half restored within an hour and most restored by 7pm. Cal Fire crews responded and assisted with a localised evacuation of the plant as a precaution; all employees were accounted for and evacuation orders were later lifted. No injuries were reported. The facility was disconnected from the grid pending investigation.

ENGIE reported an electrical issue that caused circuits and equipment at the Sierra Battery Storage facility to trip, associated with a transformer explosion; the exact technical cause was still under investigation at the time of reporting.

1. Initial emergency reports from first responders may misidentify the affected facility or equipment type, underscoring the need for rapid, coordinated communication between site operators and emergency services.
2. Precautionary evacuation of battery storage sites during unexplained explosions or smoke is a prudent response even when no fire is confirmed.
3. Adjacent energy facilities can be affected by a neighbouring site's electrical fault, highlighting the importance of clear grid interconnection protocols and rapid disconnection capability to limit cascading impacts.

kcra.com/article/tuolumne-county-power-outage-jan-9/69962119