The result is that officials are frequently forced to jettison solar power production while the sun is shining, just hours before customer demand peaks in the late afternoon and evening. The same thing happens to a lesser extent with wind energy — and the issue is surfacing in many other states as well.
“It all comes down to this problem of it’s not how much energy we have, it’s the when and the where the energy is being produced,” said James Bushnell, an economics professor at the University of California at Davis. “Particularly the solar resources — it’s just in the wrong places and at the wrong times.”
Some solar power operators accept curtailment as the cost of doing business, because large plants can soak up more sun later in the day even if they’re overproducing during the sunnier hours. This can benefit consumers, who can see lower rates when solar is running, because it’s generally a cheaper energy source than fossil fuels. Defenders of the system say some inefficiency is to be expected as California embarks on the nation’s most ambitious transition to clean energy, and that eventually there should be enough battery storage to ensure excess power doesn’t go to waste.
But solar and wind generation in California is at this point far ahead of storage capacity, and in some cases isn’t located near adequate transmission lines. Batteries and transmission lines can be costly to build and find space for, feeding skepticism from the fossil fuel industry about green energy.
The need to use not lose excess power was underlined during the record heat wave that broiled California for 10 straight days earlier this month, busting heat records as temperatures soared well past 100 degrees. Demand for energy spiked as residents went indoors and jacked up their air conditioners.
State officials pleaded for conservation and issued daily alerts advising Californians to limit their power consumption from 4 pm to 9 pm Officials urged residents to set their thermostats at 78 degrees during those hours and hold off on charging cars or using large appliances.
Despite these efforts, on Sept. 6, the state set a record for power consumption — and came perilously close to imposing targeted blackouts to safeguard the energy grid, something that hasn’t happened in two years. State officials say they avoided blackouts that evening only by blasting an urgently worded emergency text message to residents, who responded by quickly lowering their energy use.
Yet just hours earlier, California had been awash in energy. Solar production was booming by midmorning as the sun beat down on hundreds of solar panel plants all over the state. By 10 am, the California Independent System Operator, the state’s electric grid manager, was rejecting hundreds of megawatts of solar energy power — unable to use it in the moment, make room for it on the state’s congested power grid or save it for later when consumer demand would peak.
By 5 pm, with massive consumer demand straining the grid, officials had turned down more than 3,000 megawatts of solar power energy. Customer demand was soaring, but solar production declined as evening fell, and officials no longer had access to that overabundant solar power from earlier in the day. Nevertheless, officials managed to make it through the day without blackouts by turning to other energy sources and blast-texting residents.
Some experts noted that extreme heat waves caused by climate change will only become more frequent, while one of the chosen solutions to climate change — renewable energy, specifically solar — is not always there when we need it.
“The very technology that the state is relying on to reduce carbon emissions, solar power, drops exactly when electricity demand is at its peak,” said Kyle Meng, co-director of the climate and energy program at the University of California at Santa Barbara’s Environmental Markets Solutions Lab. “One of our main remedies for addressing climate change may also be making us more vulnerable to climate change’s impacts.”
“The irony is that the very technology we’re relying on to fight climate change is making us vulnerable exactly at those moments when climate impacts are at their worst.”
The practice of rejecting renewable energy generation is called “curtailment,” and it’s risen swiftly in California in recent years — before dropping off slightly last year — as the state pushes aggressively to add more renewables to its power mix, according to calculations by the US Energy Information Administration. In 2020, the California Independent System Operator curtailed some 1.5 million megawatt hours, or 5 percent of its total solar output, according to the EIA; last year the percentage stood closer to 4.2 percent.
Anne F. Gonzales, senior public information officer at the Independent System Operator, explained via email that “it helps to stabilize the grid to clear some of the oversupply off the system.” She added that during the heat wave, Californians were encouraged to precool their homes during the middle of the day — allowing them to benefit from cheap and abundant solar power during daylight hours and set thermostats higher later on.
Even so, grid operators had more solar power on their hands than they could use.
“People get concerned when there’s any curtailment of solar but the reality is we have too many renewables some of the time and not enough at other times,” said Bushnell, of UC-Davis. “If we had enough storage capacity we could soak up that excess. … That’s where everybody hopes this goes.”
Indeed, battery storage has been growing quickly in California, including an enormous facility in Northern California on the site of a former Pacific Gas and Electric power plant. Overall capacity doubled last year and is projected to keep growing rapidly, helped by tax credits included in the recent federal Inflation Reduction Act. Batteries kicked in during the heat wave, and state officials praised their performance.
Industry experts expect that the amount of available storage — or potentially other uses for excess energy — will eventually grow to the point where curtailment happens on a much smaller scale, or not at all. They note that California is undergoing a rapid energy transition, moving from heavy reliance on fossil fuels including coal just a decade ago, to a grid that is supposed to consist of 100 percent clean energy by 2045. The grid of the future is being built in real time, with inefficiencies and perverse incentives still being ironed out.
“I think of curtailment as something that is a leading indicator for how you’re going to be adding more storage,” said Alex Morris, executive director of the California Energy Storage Alliance, an industry group. “In California you see increasing amounts of storage every year absorbing and positioning themselves to capture extra energy.”
Another possible solution to the oversupply of renewables would be a mass shifting of consumer demand, so that residents use power in the middle of the day instead of the afternoon or night, said Severin Borenstein, faculty director of the Energy Institute at Haas business school at the University of California at Berkeley. That’s been discussed by policymakers but would require cooperation from utilities to redesign rate structures.
In Texas, where wind energy is curtailed in massive quantities, many of the same questions have arisen around the practice. Some are asking how excess energy could be used on-site at wind or solar plants, obviating the need for transmission somewhere else. Bitcoin mining and mobile data labs have been put forward as possible candidates to soak up excess power.