Managing the evolving grid

California is a leader in a worldwide shift to reduce carbon in the electricity sector and is well underway in designing the clean, resilient, and affordable power grid of the future. The ISO is supporting that effort by integrating higher amounts of renewable and storage resources onto the grid while protecting system reliability. 

As California transitions to a clean energy fleet, the ISO resource mix continues to evolve and is increasingly comprised of renewable resources, such as solar and wind power. The ISO has also integrated historic amounts of battery storage in the last few years. In light of the changing resource mix, the ISO is identifying operational trends and exploring new policies to reliably run the grid while optimizing the use of renewable and clean resource investments.

In addition to changes to electric supply, electricity demand in California is projected to increase significantly over the next two decades. This growth is driven by a heightened call for clean energy policies and continued electrification of buildings, transportation and other sectors of the economy. At the same, climate change is causing extreme heat, drought, and wildfires at ever-increasing frequency and intensity, introducing new risks to grid resiliency.  

Renewable curtailment

During the middle of the day, California’s robust renewable resources, especially solar generation facilities, sometimes generate more electricity than is needed to serve demand. The ISO’s market automatically reduces, or curtails, renewable generation to match supply with demand. Solar curtailment occurs most frequently in spring and fall when demand is low because of moderate weather, and sunny, breezy days produce an abundant supply of renewable generation. 

While curtailment is an acceptable operational tool, as increasing amounts of renewable generation comes online without commensurate amounts of demand to consume midday generation, oversupply conditions will continue to occur. 

Wind and solar curtailment totals by month



Increased ramping needs

Strong renewable output during the middle of the day steeply declines as the sun sets and solar production rolls off the system. This creates an almost immediate need for energy to replace renewable generation in the late afternoon and early evening. Currently, fast-moving increases in energy demand, called “ramps,” are largely met by natural gas generation and energy imported from outside the ISO footprint. 

The ISO needs a diverse mix of flexible resources to quickly align electricity production to meet sharp changes in demand.

The ISO is pursuing steps to address system ramping needs and manage renewable curtailment:

  • Support emerging inverter-based technologies to enhance operational capabilities of renewable power plants 
  • Increase ISO visibility into “behind-the-meter” commercial and residential distributed energy resources to support operational awareness and short-term load forecasting
  • Continue to integrate energy storage resources, which absorb energy during the middle of the day when solar power is abundant and loads are relatively low, and inject energy back onto the grid as solar production drops and loads remain high.
  • Explore a diverse mix of renewable resources to match output with system needs, including out-of-state and offshore wind development.
  • Continue to advance regional collaboration to find innovative ways to improve flexibility and leverage diversity of geography and resources. 
  • Support development of more dynamic retail rates and expansion of load shifting incentives and programs

Risks of climate change

As the grid faces the increasing threats of climate change, including prolonged extreme heat, severe drought, and wildfires, we are seeing the impact of consecutive days of cloudy, smoky, or low-wind days, which can limit renewable generation. In addition to short-duration storage assets, new clean technologies capable of producing energy for longer durations are needed to address system needs during multiple-day weather events or prolonged reliability events.

To maximize renewable energy use and plan for the risk of multi-day reliability risks, the ISO is exploring the following strategies:

  1. Consider multiple-day reliability needs and events in reliability planning studies
  2. Enhancements to interconnection processes to support the rapid pace of development needed to interconnect a significant amount of new, diverse,  renewable, storage, and new generation technologies in coming years
  3. Diversify resource mix through regional collaboration

The ISO also supports state efforts focused on ensuring reliability during extreme weather events:

  • Development of new demand response and load shifting programs
  • Development of new clean resource investment programs to meet reliability needs when the grid is most stressed

Planning for the grid of the future

Expected growth in electricity demand driven by the electrification of transportation, building infrastructure, and other sectors of the economy, is driving the need for significant new electric generation and transmission to support the delivery of new clean generation across the grid. Accelerated demand for new, clean resources has caused a dramatic rise in proposed projects in the ISO’s interconnection queue, and is driving significant new transmission needs. Through its public stakeholder process, the ISO continues to pursue sweeping reforms to its interconnection queue process to streamline and better prioritize the planning, procurement, and resource development to help keep pace with the renewable transition. The ISO also continues to work with state partners on comprehensive strategies to accelerate clean energy and infrastructure development.


Renewables will continue to grow

  • California continues to transition its resource fleet to lower carbon resources. With Senate Bill 100 guiding the state toward a carbon-free power grid, energy agencies predict that thousands of megawatts of new solar, wind, and battery resources need to be built each year through 2040 to meet both increasing electric demands and state greenhouse gas reduction targets.

Installed renewable resources  

The installed renewable resource values reflect the reporting Net Dependable Capacity only (numbers are rounded). Only fully commercial units are counted, not partials or test energy, as reported via the Master Generating File and captured in the Master Control Area Generating Capability List found on OASIS under “Atlas Reference”.

Battery storage becoming significant

  • Storage is quickly becoming a significant part of the ISO generation mix. Battery storage technologies will be essential to build a clean energy future. Battery storage charge from renewable resources, like solar and wind, so energy can be discharged to serve demand after solar production drops. As the amount of battery storage on our grid has grown exponentially in the past several years, the storage fleet has played a more important role in balancing the power grid during extreme weather events. For more information on storage resources, view the following:

A clean energy future

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