​Nearly 1.6GW of new grid-scale batteries was commissioned in CAISO during Q3 2025, bringing total installed BESS capacity to 14.7GW. CAISO continues to host the largest battery fleet among US grid operators, but ERCOT is rapidly closing the gap.

​In terms of energy capacity, CAISO remains the clear leader: the addition of 5.9GWh last quarter increased total installed storage to 55.2GWh — the largest total worldwide outside of China.

Key Takeaways

• CAISO operates 14.7GW of utility-scale batteries as of September 30, 2025.
• SP15 hosts 11GW of that buildout. BESS in that region can leverage the extreme price canyons generated by the extensive solar buildout.
• Northern and Central California zones grew battery capacity by 26-30%, signaling geographic diversification beyond SP15's historical dominance.

Subscribers to Modo Energy’s CAISO research will also be able to read about the buildout by co-location status and Modo Energy’s long-term BESS buildout view. Subscribers will also be able to download all the data — including the BESS pipeline — in the report.

If you have any questions about our CAISO research, please reach out to our CAISO Market lead, Logan, at logan@modoenergy.com

​Most BESS are in SP15, but ZP26 and NP15 capacity surged by 25-30% last quarter

CAISO’s southern SP15 congestion zone remains home to most of the utility-scale batteries in California. The extensive solar farms in the region generate price profiles with deep canyons, making them ideal for Energy arbitrage. However, SP15’s advantage continues to erode.

​SP15’s rated power surpassed 11GW last quarter after adding 755MW (+7.3%) of new BESS capacity. Additions or augmentations to existing sites accounted for more than half of the increase (429MW).

The other two zones historically hosted fewer BESS but now grow much faster. ZP26 increased capacity 26%, from 850MW at Q2 2025's end to 1073MW now. NP15's total BESS rated power grew 30% to 2.6GW. The zone's 610MW of newly commissioned BESS almost matched SP15's growth in absolute terms.

New battery projects launch in Northern and Central California; completions concentrate in the South

That 610MW increase in NP15 was almost entirely due to the commissioning of NextEra Energy Resources’ 600MW/2.4GWh Kola project in San Joaquin County. ENGIE’s Ultrapower Chinese Station BESS (10MW | 40MWh), sited east of Kola at the existing Ultrapower Chinese Station biomass facility in Tuolomne County, contributed the remaining 10MW.