ISO Board of Governors approves 2025-2026 transmission plan

FOLSOM, Calif. – The California Independent System Operator Board of Governors today approved the ISO’s 2025-2026 Transmission Plan, recommending 38 infrastructure projects to accommodate rising electricity demand and support the state’s energy goals.

In addition to continuing to expand access to lower-cost resources, the plan prioritizes transmission needed to serve growing customer demand reliably and economically.

More than half the projects – and more than half the $6.7 billion estimated cost – are driven by forecasted load growth. Most of the remaining projects are needed to access resource development basins identified by the California Public Utilities Commission (CPUC) and to serve load and meet state reliability and greenhouse gas reduction goals.

Initial draft estimates released in April projected $7 billion in upgrades but updated cost assumptions have reduced the total projected costs to $6.7 billion at full buildout over the next decade.

“We are constantly striving to find ways to meet system needs in the most affordable way possible,” Neil Millar, the ISO’s Vice President, Transmission Planning and Infrastructure Development, said. “This year’s plan does that in a number of different ways while also making sure we have the right infrastructure in place to accommodate all of the new resources that are being added to the system.”

This year’s plan includes 12 reconductoring projects that increase transmission capacity without having to build new transmission from scratch. Three of these will employ advanced conductors, which are one of a suite of grid-enhancing-technologies, or GETs, that the ISO routinely considers in its planning efforts.

Responding to updated cost information from transmission owners, the approved plan cancels the Serrano-Del Amo-Mesa 500-kilovolt Transmission Reinforcement project in the LA Basin, originally approved in the 2022-2023 planning cycle. The reliability needs will instead be addressed by a number of smaller, lower-cost upgrades and increased reliance on energy storage development planned in key areas on the grid.

Another hallmark of the ISO’s planning process is ongoing analysis of grid congestion which can result in dispatching higher-priced generation to serve load when transmission limitations restrict access to lower-cost power. Planners use production cost modeling to compare the cost of new infrastructure against the customer savings from reducing congestion. This helps determine when it is more economical to reinforce the system rather than continue operating with recurring bottlenecks.

In the 2025-2026 plan, this analysis identified the need for a new 500-kilovolt line to relieve congestion along the Path 15 corridor, a major north-south transmission corridor. The recommended alternative needs additional engineering and will be refined in next year’s planning cycle prior to a final recommendation. This upgrade will also support renewable development in the Westlands area in Fresno and Kings counties in southern California.

The plan is based on California Energy Commission (CEC) projections showing that California’s load will grow by 15 gigawatts (GW) by 2035 and 20 GW by 2040, while the installed resource capacity, provided by the CPUC, will need to increase by more than 74 GW and 107 GW by those same timelines. As described in the CEC’s load forecast, the increasing demand is being driven by building and transportation electrification, manufacturing, and large loads, including data centers.

Along with projects from previous cycles, this year’s plan enables the grid to accommodate forecasted load growth and critical resource development identified by the CPUC in its Integrated Resource Plan, including:

• 45 GW of solar generation in the Westlands area in the Central Valley, Tehachapi, the Kramer area in San Bernardino County, Riverside County, as well as southern Nevada and western Arizona;
• 8 GW of in-state wind generation in Tehachapi;
• Over 2 GW of geothermal development, primarily in the Imperial Valley and in southern Nevada;
• Access for battery storage projects co-located with renewable generation projects across the state, as well as stand-alone storage located closer to major load centers in the LA Basin, greater Bay Area, and San Diego;
• The import of over 10 GW of wind generation from Idaho, Wyoming and New Mexico, largely over out-of-state transmission that is already in development, by enhancing corridors from the ISO border in southeastern Nevada and from western Arizona into California load centers. (The in-development transmission projects include SWIP North and Subscriber Participating Transmission Owner projects); and
• Over 4.5 GW of offshore wind with 2.9 GW in Central Coast Morro Bay and 1.6 GW in the North Coast Humboldt.

Specific transmission upgrades in the current plan, most notably related to load growth in the Greater Bay area, include:

• Greater Bay Area Tesla – Trimble – Metcalf 230 kV Corridor Expansion to supply the south Greater Bay area;
• Trout Canyon – Lugo 500 kV Line to access resources in the East of Pisgah area;
• Short-circuit duty upgrades at a number of stations to accommodate increases in resources in the CPUC portfolio;
• Gates – Los Banos #3 500 kV Line Series Compensation to address congestion on Path 15; and
• A host of smaller upgrades improving supply of load and access to other smaller resource zones.

Visit the ISO's 2025-2026 Transmission Planning Process webpage for more information.

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About the California ISO 
250 Outcropping Way, Folsom, CA 95630 | www.caiso.com

The California Independent System Operator (ISO) is a nonprofit public-benefit corporation dedicated with its partners to continuous improvement and secure operation of a reliable grid operated for the benefit of electricity consumers.

The ISO provides comprehensive grid planning, open and nondiscriminatory access to one of the largest networks of high-voltage transmission power lines in the world and operates a competitive electricity market that in 2024 included $16.1 billion in transactions. In helping to carry out California’s energy policy objectives, the ISO is at the forefront of integrating renewable power and advanced technologies that will help provide a reliable and sustainable energy future efficiently and cleanly.