​CAISO commissioned 1.24 GW of new grid-scale battery capacity in Q4 2025, bringing total installed BESS to 15.7 GW. The fleet crossed the 15 GW threshold during the quarter, making CAISO the largest battery fleet among US grid operators as of year-end 2025.

​CAISO added 5.0 GWh of energy capacity in Q4, pushing total installed energy to 59.6 GWh. Full-year 2025 additions totaled 4.7 GW and 16.5 GWh, both annual records for CAISO.

​Key takeaways

• 15.7 GW of utility-scale batteries are operating in CAISO as of December 31, 2025. Full-year 2025 additions totaled 4.7 GW, surpassing 2024's 4.0 GW record.
• SP15 accounted for 97% of Q4 growth, adding 1.2 GW. Three projects in Arizona, within CAISO's balancing authority area, contributed 563 MW.
• Standalone BESS now accounts for 59% of the installed fleet, up from 52% at end-2024. Standalone made up 93% of Q4 additions.
• The near-term pipeline contains approximately 9.8 GW of battery capacity across 92 resources progressing through the New Resource Implementation process.

Throughout this report, "hybrid" refers to battery assets that share a DC-coupled connection to the grid with another generation source. "Co-located" BESS share a physical site with another generation asset but connect through separate, independently metered interconnections.

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

​SP15 dominated 2025 Q4 while NP15 held flat

CAISO's southern SP15 zone, covering Southern California Edison and San Diego Gas & Electric territory, accounted for 97% of Q4 additions. The zone added 1.2 GW (+11%), growing from 10.9GW to 12.1GW. SP15 now houses 77% of all CAISO BESS capacity.

​Three Arizona projects contributed nearly half of the quarter's additions. These sites sit within CAISO's balancing authority area but outside California. Arizona previously hosted smaller BESS within the CAISO footprint, but the Q4 additions significantly expand battery capacity in the state.

NP15 added no new BESS capacity in Q4, holding at 2.5 GW. This followed a strong Q3 that added 410 MW to the zone, led by NextEra's 400 MW Kola project. NP15's buildout has historically arrived in large, infrequent additions rather than a steady quarterly cadence.

ZP26 added 40 MW (+3.7%), reaching 1.1 GW. Two projects came online: Idemitsu Renewables' co-located Azalea (38MW | 152MWh) and the standalone Blackwell Test Facility (2MW | 8MWh).

AES’ ​Bellefield battery leads 2025 Q4 with 500MW

Eleven new projects commissioned in Q4 2025. Unlike Q3, which included four augmentations to existing sites, every Q4 addition was a new greenfield project.

The quarter's largest was AES Corporation's Bellefield (500MW | 2GWh) in Kern County, commissioned on December 11. Bellefield is CAISO's fourth-largest battery site, behind Edwards-Sanborn (771MW), Moss Landing (750MW), and Menifee Power Bank (680MW).

​TransGrid Energy delivered two phases of its Atlas Complex in La Paz County, Arizona. Atlas Complex 7 (300MW | 1.2GWh) came online October 6, followed by Atlas Complex 9 (150MW | 600MWh) on October 9.

A third phase, Atlas Complex 8 (382MW across two units), is at "Sync OK" status in CAISO’s near-term pipeline with a planned commissioning date in April 2026.

Leeward Renewable Energy's Dateland Energy Complex (112.5MW | 450MWh) in Yuma County commissioned on October 2. Together, the three Arizona projects delivered 563MW in the first two weeks of the quarter.

Fullmark Energy's San Jacinto Grid (65MW | 260MWh) in Riverside County was the fifth-largest Q4 addition. Middle River Power's Lithium (52MW | 208MWh) in San Diego County was the quarter's only hybrid project.

SDG&E's Buckman Springs microgrid (0.5MW | 1.8MWh), also in San Diego County, was the smallest.

Ameresco's Seal Beach (10MW | 50MWh) in Orange County and esVolta's Black Walnut (15MW | 60MWh) in Ventura County round out the remaining California projects.

​Standalone batteries widened their lead over co-located assets

Standalone batteries dominated the Q4 buildout. New standalone capacity of 1.16 GW grew the segment from 8.2 to 9.3 GW. Standalone now represents 59% of CAISO's installed fleet, up from 57% at Q3's end.

Co-located capacity grew by 38 MW via the Azalea project, reaching 5.3 GW. Hybrid BESS added 52MW through the Lithium project, reaching 1.1 GW.

​At the end of 2024, standalone and co-located BESS sat at 5.7 GW and 4.5 GW, a gap of 1.2 GW. By December 2025, that gap had widened to 4.1 GW. Every quarter in 2025 favored standalone additions, and Q4's 93% standalone share was the most concentrated yet.

Subscribers to Modo Energy’s CAISO research can read about

• How the near-term queue is distributed across CAISO’s zones and coupling arrangements,
• How the near-term queue has evolved since our last buildout report,
• And how the 80GW of battery capacity in the long-term queue will be realized over the next decade.

Subscribers can download all the data, including the full BESS pipeline, alongside this report.

Near-term pipeline: 23GW by 2027

The Generation Interconnection Resource ID report (GIRID) tracks generation assets progressing through CAISO's New Resource Implementation (NRI) process. The NRI is the final stage of interconnection before commercial operation. A resource enters the GIRID when CAISO creates its resource ID, typically around 210 days before planned synchronization.

The GIRID data as of January 2026 shows 68 battery resources totaling approximately 9.8GW in the pre-COD pipeline. Of these, 54 resources (7.9 GW) hold "Active" status in the early NRI buckets.