California ISO
Leadership and Innovation 2014

Renewables Integration

Progressive growth of preferred resources

The western energy industry is a national leader in advancing a sustainable energy future. In recent years, California, Washington, Nevada, Oregon, Arizona, Utah, Colorado, Montana and New Mexico have enacted policies aimed at reducing greenhouse gases and increasing renewable resource development. California now ranks first in the United States for total megawatts of installed solar and second behind Texas for total megawatts of installed wind.

The growth in these preferred resources is nothing short of phenomenal.

Graphic showing growth of preferred resources

In supporting the renewables portfolio standard, the ISO has steadily integrated more than 10,000 megawatts of zero-emission resources onto the grid and is active in developing the techniques and market rules to manage the variable resources. More importantly, actual production levels of wind and solar are soaring.

Graphic showing wind and solar records

Combined, wind and solar resources to-date account for over 11,400 MW interconnected to the ISO grid. All renewables (solar, wind, geothermal, small hydro, biogas and biomass) make up over 15,220 MW of the ISO generation mix.

With these impressive wind and solar production levels, the ISO is advancing a sustainable energy future to further California’s goal of having renewable resources cover 33 percent of the state’s retail electricity sales by 2020. The state’s three investor-owned utilities have reported to the California Public Utilities Commission that they are meeting 22 percent-24 percent of their retail sales with clean, green resources and are on track to meet the 2020 target.

Grid reliability

Duck chart forecast of rising overgeneration occurrences have begun.

A risk with reliably operating the grid is developing; it’s called overgeneration. ISO analysis reflected in the duck chart illustrates as more renewables interconnect to the grid, the greater the risk we will at times produce more power than what can be used. The chart simply shows the amount of energy we would need from traditional power plants, such as natural gas, in the absence of renewable plants.

Nwt Load

Overgeneration conditions occur when output from planned renewable solar and wind plants combine with generation from new natural gas and other traditional plants creating more electricity that can be consumed and is more pronounced during the spring and fall months.

So, the duck chart is more like the canary in the mine; it is showing us that as each year goes by, the risk rises that mid-day overgeneration will occur, which undermines grid reliability as well as driving up costs. To manage persistent overgeneration, the ISO must curtail production, which only occurs after we have done all we can to cure, such as moving plants to minimum operating levels — but this is more complicated than it sounds. First, a large portion of our fleet is inflexible, so they cannot respond quickly to start and stop dispatches, or adjust output levels. Many of these plants are needed to meet ramps (such as meeting morning demand while solar output is increasing and wind is decreasing) and so must operate on standby at their minimum operating levels prior to the time when they are needed. Also, many power plants have contractual obligations limiting how often they can be curtailed, ramped, started or stopped. Other plants such as nuclear and hydro (especially during spring melt) must continue generating for safety reasons. And solar and wind resources, for instance, operate at maximum capacity and can’t ramp up, if needed.

Grid operators are now seeing overgeneration beginning to shift from overnight hours to the mid-day hours necessitating curtailments just as the duck curve forecasted. The ISO had to curtail wind and solar for reliability reasons four times this past spring representing over 1,700 MW with 1,100 MW of that amount occurring during the morning hours of April 27. Our analysis also shows that by 2024, the occurrences of overgeneration events increase dramatically (see chart below).

Instances of Curtailment

In addition to reliability concerns, our studies show that if we curtail renewables, then we risk not producing enough renewable energy to meet the targets set by the Renewables Portfolio Standard (which says 33 percent of the utility’s retail sales must be served by renewable resources by 2020).

Without some serious consideration of ways to modify current practices, overgeneration will persist and undermine grid reliability. Options the ISO and market participants should consider exist and the likely solution is a combination of actions. This is why deeper regional coordination effort is key in solving curtailment risks as California continues to add renewable resources to the electric grid.

Other options include the following:

  • modify provisions in power purchase agreements (bilateral contracts between utilities and power generators) that deal with curtailment limits to reconcile with renewable portfolio standards priorities
  • electrification of transportation and related managed charging
  • increasing fleet flexibility
  • includes capabilities to start and stop, raise and lower multiple times per day and start with short notice over a wide range of operating levels.


Predicting renewable resources’ variability and optimizing their use to meet consumers’ energy needs is an important element in managing the modern green grid. The ISO, as the operator of the transmission system and the only wholesale real-time electricity market in the west, strongly supports this diversification of generation fuels. With state-of-the-art technologies, including cutting edge geospatial maps, ISO operators stay one step ahead of wind and solar output, which enables them to better match supply with demand. This rich, visual intelligence also helps identify and track wildfires and other grid threats quickly and accurately so operators can act before they become reliability issues.

Photo of solar and wind facility

One recent wholesale market design change involving how often power is scheduled facilitates priority use of renewables in the resource loading order. Now, generators can schedule power in 15-minute intervals, allowing variable resources to better forecast and schedule their green energy closer to the time it is needed.

Forecasting tools

Predicting when winds might fade or when cloud cover might block the sun from solar panels is challenging. As a balancing authority, predicting how renewable resources respond to generation dispatches is a key factor in making sure it carries its weight in contributing to grid reliability. ISO engineers and partners are building new tools that give grid operators better data to more precisely predict wind and solar production output.

Needed information includes determining how fast a flexible generation unit can start and stop, as well as ramp up and down. Another tool predicts when a variable unit can assist with maintaining transmission line voltage, which like water pressure moves water down a hose, keeps electricity flowing. Plus, ongoing technical studies help the ISO identify how to leverage our tools to take advantage of the flexibility of preferred clean resources for daily operations without risking over generation situations.

Accurately predicting when and how variable resources can be used aids greatly in making sure green power can fulfill its role in maintaining grid reliability.