Above: The decommissioned Rancho Seco nuclear plant near Sacramento, California, is now home to a solar farm. Photo by Griffin5 at English Wikipedia.

The aging nuclear power plant fleet in the U.S. poses a challenge to states trying to minimize their greenhouse gas emissions: if these plants retire, how can the electricity they generate be replaced without turning to natural gas? Nuclear power plants are big — often providing one to two gigawatts (GW) of power. For a possible solution, look to California: solar power has grown so quickly in recent years that it has more than made up for the retirement of a 2.3 GW nuclear power plant in 2012.

The nuclear fleet in the United States averages 36 years in age, and many plants are facing potential retirement due to both aging systems and declining competitiveness in electricity markets. Some states, such as New York and Illinois, have proposed giving nuclear power plants “zero emission credits,” effectively serving as subsidies for low-carbon generation to stay online even if it is not economically viable. These subsidies are scaled to reflect the “social cost of carbon” and meant to serve as a bridge to increased renewable energy generation in the future, with the implicit assumption that renewables could not be built fast enough to replace an entire nuclear power plant coming offline.

California has shown that such a bridge may not be necessary. When the San Onofre Nuclear Generating Station (SONGS) retired in early 2012, the state lost a plant that had supplied as much as 18,000 GWh of electricity — nearly 9 percent of total in-state generation. That same year, California generated roughly 3,700 GWh of electricity from a combination of utility-scale and distributed solar. By 2015, solar generation had grown to more than five times that amount, providing 20,800 GWh. And in 2016, California generated more than 27,000 GWh from solar — 13 percent of its in-state generation. Wind generation also grew over these years in California, albeit not as quickly.

Figure 1 shows the annual generation from nuclear and solar resources in California and illustrates this dramatic growth in solar power. Following just a few years of growth, solar generation now exceeds the low-carbon electricity supply lost from SONGS.

It is important to note that the electricity generated from these two resources does not meet the same set of demands on the grid. The nuclear power plant provided electricity at a single location on the grid, while California’s solar generation is spread across the state. Nuclear power plants tend to generate at full capacity all day and night, whereas solar only generates electricity during daylight hours. Both resources are somewhat inflexible. Easily dispatchable resources like hydropower and natural gas are more likely to be called upon to meet daytime demand in a system with lots of nuclear generation and to meet nighttime demand in a system with lots of solar. As a result, the rest of California’s power generation has likely shifted in terms of both the location of power plants being utilized and the hours they operate — with a concurrent shift in the location and time of pollutant emissions from gas plants. From a statewide greenhouse gas standpoint, however, the rapid growth of solar has already mitigated the brief increase in natural gas emissions that followed the retirement of SONGS.

What does this mean for other states? California is sunny, has open land, and is generally well-suited to solar. However, rapid solar growth has also been encouraged through supportive measures such as the state’s 33 percent renewable energy target for the year 2020 and favorable net-metering policies for distributed solar. In New York, the Indian Point power plant is set to retire by 2021. This nuclear power plant supplied about 11 percent of the state’s in-state electricity generation in 2016, meaning that New York has to account for a similar loss in generation as California did for SONGS. New York’s renewable resource potential is different from California’s, so it may have to pursue offshore wind or accelerate efficiency efforts instead of expanding solar. Indian Point also supplies electricity to a transmission-constrained area downstate, introducing additional grid challenges. But from a carbon mitigation standpoint, if New York expanded its renewable generation at the rate that California expanded solar power alone, the state could make up for the lost low-carbon generation from Indian Point in just a few years. Similar considerations apply to potential retirements in Illinois, Pennsylvania, and elsewhere.

From a long-term planning perspective, California’s example illustrates that nuclear power plant retirements don’t have to mean a sustained increase in natural gas generation. Rapid deployment of renewables, encouraged by supportive policies, can help replace the loss of low-carbon electricity generation from nuclear power plant retirements over a relatively short timeframe.

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