Rows of solar photovoltaic panels and wind turbines behind a chain link fence.

California’s Growing Solar and Wind Problem

Phillippe Phanivong

Here in California, we are working to be world leaders in the fight against climate change. Part of this fight requires decarbonizing our electricity sector, and California has set ambitious targets for this transition from fossil fuels to carbon-free sources of electrical energy. State law requires that by 2030, 60% of electricity be supplied by renewable energy, and by 2045, 100% of electricity be from carbon-free or renewable energy sources. To reach these goals, we will need to build and use more renewable energy, primarily solar and wind generation. Understanding our current solar and wind utilization can highlight potential issues as we connect more renewables to the electric grid.

How Much Solar and Wind Does California Currently Use?

California is one of the top renewable energy-producing states in the country. Based on September 2023 capacity data from the U.S. Energy Information Administration (EIA), we are the number one state in installed utility-scale solar photovoltaic (PV) systems and sixth in utility-scale wind systems. But how well are we using the renewable energy from these systems? Let’s try to answer this question by looking at data from the California Independent System Operator (CAISO), the market operator for California’s wholesale electricity market. The CAISO market is where most utility companies buy electricity, giving a reasonably comprehensive picture of California’s energy mix.

According to CAISO’s Monthly Renewables Performance Reports, on average, 32.23% of the combined total electrical load in 2022 was served by renewable energy. In 2021, the annual average was 30.56%. Going back five years, in 2017, 24.17% of the combined load was served by renewable energy. It’s great to see this type of growth. However, we need to do a lot better if we are to reach our targets. While we must build more renewable energy capacity, there is an issue with how we use our solar and wind resources. Solar and wind curtailment is siphoning away some of the renewable energy we could be using today.

Bar plot showing an upward trend in the average annual percentage of electrical load served by renewable energy sources from 2017 through 2022.
There is an upward trend in the average percentage of electrical load served by renewable energy sources from 2017 to 2022. The author created this plot using data from CAISO’s Monthly Renewables Performance Reports.

What is Solar and Wind Curtailment? Is It Really a Concern?

Solar and wind curtailment is the name we give to any intentional cutting back of output from solar and wind generators. For example, suppose a solar farm could produce 100 megawatt-hours (MWh) in a given hour of the day, but there is congestion in the transmission network. In that case, we might reduce the solar output to prevent exacerbating this congestion and only ask for the solar farm to generate 75MWh. In this situation, the curtailed solar energy would be the 25MWh we could have generated in that hour because the sunlight was available. Another reason we might need to curtail is that we may produce more energy at a given time than we can use. If we can’t store or use energy, we need to curtail it.

However, solar and wind curtailment isn’t always a bad thing. In a 100% carbon-free electric grid, we will likely have more solar and wind than load at certain times. In this scenario, curtailing a little extra energy now and then would not be a problem. The concern is if we are curtailing a large amount of energy or if curtailment is growing as we move towards our climate goals. To see if curtailment is an issue in California, let’s ask two questions: How much solar and wind energy is California curtailing? And how fast is the rate of curtailment growing?

In 2017, we curtailed 379,418 MWh of solar and wind energy. If we used the 2022 EIA monthly residential average consumption for CA, this would be equivalent to providing over 59,000 residential customers energy for an entire year. In 2022, curtailment skyrocketed to over 2.4 million MWh—the energy consumption of nearly 382,000 residential customers annually—which is more than the total number of households in San Francisco! This is a lot of carbon-free energy sitting and available on the power grid that we are wasting. Unfortunately, solar and wind curtailment is only getting worse in 2023. On October 15, we exceeded the total curtailment of 2022; and by the end of October 2023, we had curtailed more than 2.5 million MWh for the year. This level of curtailment is far too high for the amount of renewable energy we use on the grid.

However, what might be more concerning is the growth rate of curtailment. From 2017 to 2022, we have seen a 546% increase in solar and wind curtailment with only a 41% increase in wholesale solar and wind energy generation. This is an alarming growth rate. If we only supply one-third of our energy needs with renewable energy, how much curtailment will we see as we get closer to 100% renewable energy? Can we even get to 100% renewable energy with this growth rate of curtailment?

Bar plot showing an upward trend in Annual CAISO solar and wind curtailment from 2017 to October 2023.
Annual CAISO solar and wind curtailment from 2017 to October 2023. The author created this plot using data from CAISO Production and Curtailment Data.

In addition to the growth in annual solar and wind curtailment, we are starting to see strange curtailment changes at the hourly level. On June 7, 2020, from 8 am to 11 am, CAISO curtailed more MWh of solar than was generated in those hours. This means that in those hours, more than half of all utility-scale solar generation in California was being curtailed. This was the first time that in a single hour, more solar was curtailed than was generated in California’s wholesale electricity market. However, this was a Sunday morning during the COVID-19 pandemic. Sunday is typically the lowest load day of the week, and the system had minimal load that day. Not much generation was needed.

A bar chart of hourly solar generation and solar curtailment on June 7, 2020. The solar curtailment exceeds the solar generation in several hours during the morning market hours.
Hourly solar generation and solar curtailment on June 7, 2020. The author created this plot using data from CAISO Production and Curtailment Data.

A similar curtailment event happened again on two Sundays in Spring 2022: March 13 and May 29. There was more generation than needed on lightly loaded Sundays, leading to more solar curtailment than solar generation at certain hours. However, in 2023, we experienced the first weekdays where solar curtailment exceeded solar generation: first on Friday, March 24, then on Friday, March 31, and again on Tuesday, April 4. These weekday curtailment events are significant because the load on the system is significantly higher on weekdays, as more people are at work and are using more energy. We should not be curtailing the majority of available solar energy on days like these. To reach our goal of a 100% carbon-free electricity grid, we need to use the solar resources we have already connected to the grid.

A grouped bar chart of hourly solar and wind oversupply curtailment from 2017 to 2023. Each bar represents the amount of oversupply in a CAISO market operating hour. The bars are taller in more recent years and in later hours of the day.
Hourly solar and wind curtailment for oversupply by year. The author created this plot using data from CAISO Production and Curtailment Data.

Why Are We Curtailing So Much?

The reason behind these curtailments is not entirely clear. CAISO categorizes curtailment as one of two broad categories: “Local” and “System.” A “Local” curtailment happens if congestion on the transmission network prevents the full delivery of the energy. (Unfortunately, CAISO does not provide additional details on where this congestion is or what transmission network components are affected). A “System” curtailment happens when there is a system-wide oversupply of energy. Essentially, too many generating resources supply power to the grid at once. Since you need a balance between load and generation for proper power grid operation, if we have too much generation and not enough load, we need to reduce generation. Curtailing solar and wind is a fast and effective way to reduce generation.

While CAISO uses “Local” and “System” to describe curtailment, many in the research community prefer to use “Congestion” and “Oversupply” to avoid confusion about the proximity associated with the word “local.” Congestion-related curtailment is the dominant reason for curtailment. However, that does not mean oversupply is also not a critical issue. The amount of energy curtailed for oversupply is still considerable and growing. From January to October 2023, we’ve had over 566,000 MWh of oversupply curtailment, more than the total curtailment we experienced in 2017 or 2018. This is an almost 21% increase in oversupply curtailment compared to the same period in 2022. Oversupply curtailment is also the easiest curtailment to address. While congestion curtailment may require system upgrades, oversupply can be addressed by shifting loads like electric vehicles or charging batteries at times when we would curtail energy instead. However, some of those solutions have their own challenges as well.

Oversupply curtailment has also been exhibiting an interesting hourly change over the years. In 2022, oversupply curtailment started to skew later into the day. We are now seeing significant amounts of oversupply curtailment occurring at the start of typical peak electricity pricing times, between 4-9 p.m. This change is meaningful because the most efficient way to reduce oversupply curtailment would be to shift load into hours where oversupply occurs. However, if customers pay peak prices for electricity, they will move load away from these times, potentially leading to the oversupply curtailment we are trying to avoid. One way to address this issue is to create more dynamic electricity rates.

A bar chart of hourly solar generation and solar curtailment on March 24, 2023. The solar curtailment exceeds the solar generation in several hours during the afternoon market hours.
Hourly solar generation and solar curtailment on March 24, 2023. The author created this plot using data from CAISO Production and Curtailment Data.

The Future of Curtailment

Solar and wind curtailment is a problem in California. While some curtailment should be expected in the power grid with significant solar and wind generation, we see too much for our current solar and wind generation levels. We are also seeing the growth rate of curtailment increase much faster than our annual increase in solar and wind energy. More information is required to fully understand where congestion is causing curtailment. However, oversupply can be addressed if loads are incentivized to consume at the hours when oversupply is occurring. This requires more dynamic prices and flexible loads, such as electric vehicles (EVs) and batteries. Expanding markets to sell this energy to other states is also valuable. CAISO has been expanding its energy markets throughout the Western U.S., which has helped connect loads to our excess solar and wind generation. However, there is still a lot of work to be done, and we’re running out of time.

Cover image credit: Kindel Media via Pexels

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