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Sweltering heat and earlier sunsets are a big problem for solar power.
The biggest problem in renewable energy goes by a few names.
Classically, it’s called the “duck curve,” which shows the relationship between solar generation and how much power the rest of the grid uses during the day. At the bottom of the curve, typically around midday, solar can sometimes generate 100% of the power demanded from the grid. But as the sun traces its arc towards the horizon, solar power generation falls and then quickly goes to zero as the sun sets. Often, temperatures and electricity use remains high, especially as people come home from work and start using home appliances, meaning non-solar sources of power must quickly come on line to fill in the gap.
It’s not a coincidence that utilities and grid operators tend to ask consumers to conserve in the later afternoon or evening. The phenomenon is classically associated with solar-heavy California, but it has come to Texas as well, where it goes by the name of the “Armadillo Curve” or the “Dead Armadillo Curve.”
But the relation between the sun and the Earth doesn’t just create darkness and light on daily scales but on annual ones as well. Yes, I know this isn’t breaking news, but it’s important, especially as the power system and climate are changing.
Right now we might be in the neck of the annual duck curve.
In case you haven’t noticed, the sun is setting earlier and it’s still really hot out. Kids are going back to school while much of the country is still facing summer temperatures.
In New York City this week, high temperatures are forecast to get into the 90s, while the sun will set before 7:30; in Washington, D.C., forecast highs are over 100 later this week with a sunset just past 7:30; in Houston, daytime highs to get over 100 later this week, with the sun setting before 7:40; and in Los Angeles , sunsets are at around 7:15 with expected daytime highs in the 90s this weekend.
And Septembers are only getting hotter. Septembers 2021 and 2022 were tied for the fifth hottest on record for the last 143 years, according to the National Oceanic and Atmospheric Administration; 10 of the hottest Septembers have occurred since 2012. The warmest was in 2020.
These higher temperatures mean prolonged periods of high electricity usage, even as one resource — solar — becomes less potent. This matters because, at least in the United States, we tend to organize our lives — and our electricity usage — around the clock, not the sun.
As the sun is setting earlier, our high electricity usage stretches longer compared to the length of the solar day, exacerbating the duck curve dynamics inherent to solar power. A dishwasher that runs when the sun’s still up in July is pulling the same power from the grid as one that runs during fall’s early twilight. The saving grace of shorter days in a grid that uses solar power is supposed to be that air conditioning usage goes down, but that doesn’t happen when summer temperatures persist past Labor Day.
If hot Septembers and even Octobers become the norm, grid conditions could tighten up both during and across the days, with higher cost, less reliable power or increased usage of fossil fuels to fill in the gap.
These longer, hotter summers can make operating electric grids more difficult. ERCOT, the electricity market that covers the vast majority of Texas, restricts power plants from having planned outages between May 15 and September 15 for maintenance. While still in the summer restriction window, ERCOT on Tuesday issued an alert for later this week, warning of “forecasted higher temperatures, higher electrical demand, and the potential for lower reserves.” If ERCOT extends its restrictions on outages for maintenance, there should be more unplanned outages, making power scarcer, meaning higher prices and a greater possibility of blackouts.
Not every country sees peak electricity usage in late summer. In New England, peak electricity demand tends to hit in July. In the sprawling PJM Interconnection last year, the electricity market that spans from the Chicago area to Virginia, demand peaks tended to be in June or August. In New York, peak demand is often in July.
But summer peaks are later in the year in two the country's largest electricity markets: California and Texas.
The Texas energy market had hit its peak day in July in 2022, but it moved out to August this year. And Texas is already bursting through its September demand records. It reached over 78,000 megawatts in just the first week of this month, well over its previous record of 72,370 megawatts, which it set in 2021.
And California hit its power demand record last September amidst a heat wave that covered much of the western United States.
It’s not just there being literally fewer hours of sunlight that drags down solar production later in the year, but also the lower angle of the sun. “As the sun gets lower in the sky we see solar production numbers will drop,” Joshua Rhodes, a senior research scientist at the University of Texas, told me.
“As the sun is lower in the sky it’s up fewer hours ... the photons are coming in at a steeper angle, the panels are not going to get as much light. Even when the sun is at the highest point of the day, the panels are not getting the same level of irradiance as when the sun is at the highest point of the day [at other times of year].”
The best angle for solar panels can change around 15 degrees a year, depending on the year and solar panels are more efficient when they can track the sun during the day. Most homeowners who install solar panels won’t have tracking technology, while utility-scale solar developers are more likely to. This means that a state like Texas, whose renewable mix is more focused on large solar arrays, could see less dramatic drop-offs in solar power throughout the day or throughout the year than a state like California, which has more residential solar.
A roof-mounted four kilowatt-hour solar PV system with standard specs where I grew up in Northern California would get 7.45 kilowatts-hours per meter squared per day in July, generating 689 kilowatt-hours of power, according to National Renewable Energy Laboratory PVWatts tool; in September, solar radiation would drop down to 6.6 kilowatt-hours per meter squared per day and 587 kilowatt-hours per month.
This admittedly basic math suggests it's possible California could struggle this month — and in future Septembers — with meeting electricity demand.
In the past 10 years, California’s annual load peak has occurred in September five times, with the peak loads in 2022 and 2021 occurring on September 6 and 8 respectively.
This year has been, so far, not particularly stressful for the Golden State’s grid thanks to some good luck — no region-wide, prolonged heat waves that max out California’s grid and make imports scarce, mild temperatures on the coasts where the state’s population is concentrated, no major wildfires, and plentiful hydro power thanks to massive snowfall this past winter — as well as massive deployment of batteries across the grid. The batteries especially can help alleviate these duck curve dynamics, as they essentially redistribute power from the sunniest part of the day to the evenings.
While Texas set several new records this year in electricity usage, California has stayed well short of its 52,000 megawatt record last September. California set records for solar power in June and July, with almost 16,000 megawatts, while total demand over 40,000 megawatts.
“While we haven’t seen substantial stress on the grid this summer, we haven’t been fully tested. If we got the kind of west-wide heat we experienced in September 2022, we could need to tap into the state’s emergency or strategic reserves again,” Anne Gonzales, a spokesperson for the California Independent System Operator, told me in an email.
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Current conditions: In the Atlantic, the tropical storm that could, as it develops, take the name Jerry is making its way westward toward the U.S. • In the Pacific, Hurricane Priscilla strengthened into a Category 2 storm en route to Arizona and the Southwest • China broke an October temperature record with thermometers surging near 104 degrees Fahrenheit in the southeastern province of Fujian.
The Department of Energy appears poised to revoke awards to two major Direct Air Capture Hubs funded by the Infrastructure Investment and Jobs Act in Louisiana and Texas, Heatmap’s Emily Pontecorvo reported Tuesday. She got her hands on an internal agency project list that designated nearly $24 billion worth of grants as “terminated,” including Occidental Petroleum’s South Texas DAC Hub and Louisiana's Project Cypress, a joint venture between the DAC startups Heirloom and Climeworks. An Energy Department spokesperson told Emily that he was “unable to verify” the list of canceled grants and said that “no further determinations have been made at this time other than those previously announced,”referring to the canceled grants the department announced last week. Christoph Gebald, the CEO of Climeworks, acknowledged “market rumors” in an email, but said that the company is “prepared for all scenarios.” Heirloom’s head of policy, Vikrum Aiyer, said the company wasn’t aware of any decision the Energy Department had yet made.
While the list floated last week showed the Trump administration’s plans to cancel the two regional hydrogen hubs on the West Coast, the new list indicated that the Energy Department planned to rescind grants for all seven hubs, Emily reported. “If the program is dismantled, it could undermine the development of the domestic hydrogen industry,” Rachel Starr, the senior U.S. policy manager for hydrogen and transportation at Clean Air Task Force told her. “The U.S. will risk its leadership position on the global stage, both in terms of exporting a variety of transportation fuels that rely on hydrogen as a feedstock and in terms of technological development as other countries continue to fund and make progress on a variety of hydrogen production pathways and end uses.”
Remember the Tesla announcement I teased in yesterday’s newsletter? The predictions proved half right: The electric automaker did, indeed, release a cheaper version of its midsize SUV, the Model Y, with a starting price just $10 shy of $40,000. Rather than a new Roadster or potential vacuum cleaner, as the cryptic videos the company posted on CEO Elon Musk’s social media site hinted, the second announcement was a cheaper version of the Model 3, already the lower-end sedan offering. Starting at $36,990, InsideEVs called it “one of the most affordable cars Tesla has ever sold, and the cheapest in 2025.” But it’s still a far cry from Musk’s erstwhile promise to roll out a Tesla for less than $30,000.
That may be part of why the company is losing market share. As Heatmap’s Matthew Zeitlin reported, Tesla’s slice of the U.S. electric vehicle sales sank to its lowest-ever level in August despite Americans’ record scramble to use the federal tax credits before the September 30 deadline President Donald Trump’s new tax law set. General Motors, which sold more electric vehicles in the third quarter of this year than in all of 2024, offers the cheapest battery-powered passenger vehicle on the market today, the Chevrolet Equinox, which starts at $35,100.
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Trump’s pledge to revive the United States’ declining coal industry was always a gamble — even though, as Matthew reported in July, global coal demand is rising. Three separate stories published Tuesday show just how stacked the odds are against a major resurgence:
As you may recall from two consecutive newsletters last month, Secretary of Energy Chris Wright said “permitting reform” was “the biggest remaining thing” in the administration’s agenda. Yet Republican leaders in Congress expressed skepticism about tacking energy policy into the next reconciliation bill. This week, however, Utah Senator Mike Lee, the chairman of the Senate Committee on Energy and Natural Resources, called for a legislative overhaul of the National Environmental Policy Act. On Monday, the pro-development social media account Yimbyland — short for Yes In My Back Yard — posted on X: “Reminder that we built the Golden Gate Bridge in 4.5 years. Today, we wouldn’t even be able to finish the environmental review in 4.5 years.” In response, Lee said: “It’s time for NEPA reform. And permitting reform more broadly.”
Last month, a bipartisan permitting reform bill got a hearing in the House of Representatives. But that was before the government shutdown. And sources familiar with Democrats’ thinking have in recent months suggested to me that the administration’s gutting of so many clean energy policies has left Republicans with little to bargain with ahead of next year’s midterm elections.
Soon-to-be Japanese prime minister Sanae Takaichi.Yuichi Yamazaki - Pool/Getty Images
On Saturday, Japan’s long-ruling Liberal Democratic Party elected its former economic minister, Sanae Takaichi, as its new leader, putting her one step away from becoming the country’s first woman prime minister. Under previous administrations, Japan was already on track to restart the reactors idled after the 2011 Fukushima disaster. But Takaichi, a hardline conservative and nationalist who also vowed to re-militarize the nation, has pushed to speed up deployment of new reactors and technologies such as fusion in hopes of making the country 100% self-sufficient on energy.
“She wants energy security over climate ambition, nuclear over renewables, and national industry over global corporations,” Mika Ohbayashi, director at the pro-clean-energy Renewable Energy Institute, told Bloomberg. Shares of nuclear reactor operators surged by nearly 7% on Monday on the Tokyo Stock Exchange, while renewable energy developers’ stock prices dropped by as much as 15%
Researchers at the United Arab Emirates’ University of Sharjah just outlined a new method to transform spent coffee grounds and a commonly used type of plastic used in packaging into a form of activated carbon that can be used for chemical engineering, food processing, and water and air treatments. By repurposing the waste, it avoids carbon emitting from landfills into the atmosphere and reduces the need for new sources of carbon for industrial processes. “What begins with a Starbucks coffee cup and a discarded plastic water bottle can become a powerful tool in the fight against climate change through the production of activated carbon,” Dr. Haif Aljomard, lead inventor of the newly patented technology, said in a press release.
Last week’s Energy Department grant cancellations included funding for a backup energy system at Valley Children’s Hospital in Madera, California
When the Department of Energy canceled more than 321 grants in an act of apparent retribution against Democrats over the government shutdown, Russ Vought, President Trump’s budget czar, declared that the money represented “Green New Scam funding to fuel the Left's climate agenda.”
At least one of the grants zeroed out last week, however, was supposed to help keep the lights on at a children’s hospital.
The $29 million grant was intended to build a 3.3-megawatt long-duration energy storage system at Valley Children’s Hospital, a large pediatric hospital in Madera, California. The system would “power critical hospital operations during outage events,” such as when the California grid shuts down to avoid starting wildfires, according to project documents.
“The U.S. Department of Energy’s cancellation of funding for [the] long-duration energy storage demonstration grant is disappointing,” Zara Arboleda, a spokesperson for the hospital, told me.
Valley Children’s Hospital is a 358-bed hospital that says it serves more than 1.3 million children across California’s Central Valley. It has 28 neonatal intensive care unit beds and nationally ranked specialties in pediatric neurology, orthopedics, and lung surgery, among others.
Energy Secretary Chris Wright has characterized the more than $7.5 billion in grants canceled last week as part of an ongoing review of financial awards made by the Biden administration. But the timing of the cancellations — and Vought’s gleeful tweets about them — suggests a more vindictive purpose. Republican lawmakers and President Trump himself threatened to unleash Vought as a kind of rogue budget cutter before the federal government shut down last week.
“We don’t control what he’s going to do,” Senator John Thune told Politico last week. “I have a meeting today with Russ Vought, he of PROJECT 2025 Fame, to determine which of the many Democrat Agencies, most of which are a political SCAM, he recommends to be cut,” Trump posted on the same day.
Up until this year, canceling funding that is already under contract with a private party would have been thought to be straightforwardly illegal under federal law. But the Supreme Court’s conservative majority has allowed the Trump administration to act with previously unimaginable freedom while it considers ruling on similar cases.
Faraday Microgrids, the contractor that was due to receive the funding, is already building a microgrid for the hospital. The proposed backup power system — which the grant stipulated should be “non-lithium-ion” — was supposed to be funded by the Energy Department’s Office of Clean Energy Demonstrations, with the goal of finding new ways of storing electricity without using lithium-ion batteries, and was meant to work in concert with that new microgrid and snap on in times of high stress.
That microgrid project is still moving forward, Arboleda, the hospital’s spokesperson, told me. “Valley Children’s Hospital continues to build and soon will operate its microgrid announced in 2023 to ensure our facilities have access to reliable and sustainable energy every minute of every day for our patients and our care providers,” she added. That grid will contain some storage, but not the long-term storage system discussed in the official plan.
Faraday Microgrids, formerly known as Charge Bliss, didn’t respond to a request for comment, but its website touts its ability to secure grants and other government funding for energy projects.
In a statement, a spokesman for the Energy Department said that the grant was canceled because the project wasn’t feasible. “Following an in-depth review of the financial award, it was determined, among other reasons, that the viability of the project was not adequate to warrant further disbursements,” Ben Dietderich, a spokesman for the Energy Department, told me.
The children’s hospital, at least, is in good company. On Tuesday, a Trump administration document obtained by Heatmap News suggested the Energy Department is moving to kill bipartisan-backed funding for two direct air capture hubs in Texas and Louisiana. And although California has lost the most grants of any state, the Energy Department has also sought to terminate funding for new factories and industrial facilities across Republican-governed states.
Rob and Jesse break down China’s electricity generation with UC San Diego’s Michael Davidson.
China announced a new climate commitment under the Paris Agreement at last month’s United Nations General Assembly meeting, pledging to cut its emissions by 7% to 10% by 2035. Many observers were disappointed by the promise, which may not go far enough to forestall 2 degrees Celsius of warming. But the pledge’s conservatism reveals the delicate and shifting politics of China’s grid — and how the country’s central government and its provinces fight over keeping the lights on.
On this week’s episode of Shift Key, Rob and Jesse talk to Michael Davidson, an expert on Chinese electricity and climate policy. He is a professor at the University of California, San Diego, where he holds a joint faculty appointment at the School of Global Policy and Strategy and the Jacobs School of Engineering. He is also a senior associate at the Center for Strategic and International Studies, and he was previously the U.S.-China policy coordinator for the Natural Resources Defense Council.
Shift Key is hosted by Robinson Meyer, the founding executive editor of Heatmap, and Jesse Jenkins, a professor of energy systems engineering at Princeton University.
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Here is an excerpt from our conversation:
Robinson Meyer: Your research and other people’s research has revealed that basically, when China started making capacity payments to coal plants, in some cases, it didn’t have the effect on the bottom line of these plants that was hoped for, and also we didn’t really see coal generation go down or change in the year that it happened. It wasn’t like they were paying these plants to stick around and not run. They were basically paying these plants, it seems like, to do the exact same thing they did the year before, but now they also got paid. And maybe that was needed for their economics, we can talk about it.
Why did coal get those payments and not, say, batteries or other sources of spare capacity, like pumped hydro storage, like nuclear? Why did coal, specifically, get payments for capacity? And does it have to do with spinning reserve? Or does it have to do with the political economy of coal in China?
Michael Davidson: When it came out, we said exactly the same thing. We said, okay, this should be a technology neutral payment scheme, and it should be a market, not a payment, right? But China’s building these things up little by little. Over time we’ve seen, historically, actually, a number of systems internationally started with payments before they move to markets because they realize that you could get a lot more competitive pressure with markets.
The capacity payment scheme for coal is extremely simple, right? It says, okay, for each province, we’re going to say what percentage of our benchmark coal investment costs are we going to subsidize. It’s extremely simple. It does not account for how much you’re using it at a plant by plant level. It does not account for other factors, renewables, etc. It’s a very coarse metric. But I wouldn’t say that it had had some, you know, perverse negative effect on the outcome of what coal generation is. Probably more likely is that these payments were seen, for some, as extra support. But then for some that are really hurting, they’re saying, okay, well then we will maybe put up less obstacles to market reforms.
But then on top of that, you have to put in the hourly energy demand growth story and say, okay, well you have all these renewables, but you don’t have enough storage to shift to evening peaks. You are going to rely on coal to meet that given the current rigid dispatch system. And so you’re dispatching them kind of regardless of whether or not you have the payment schemes.
I will say that I was a skeptic, right? Because when people told me that China should put in place a capacity market, I said, China has overcapacity. So if you have an overcapacity situation, you put in place a market, the prices should be zero. So what’s the point? But actually, when you’re looking out ahead with all of this surplus coal capacity that you’re trying to push down, you’re trying to push those capacity factors of those coal plans from 50%, 60%, down to 20% or even lower, they need to have other revenue schemes if you’re not going to dramatically open up your spot markets, which China is very hesitant to do — very risk averse when it comes to the openness of spot markets, in terms of price gaps. So that’s a necessary part of this transition. But it can be done more efficiently, and it should done technology neutral.
And by the way that is happening in certain places. That’s a national scheme, but we actually see that the implementation — for example, Shaanxi province, we have a technology neutral scheme that would include other resources, not just coal.
Mentioned:
China’s new pledge to cut its emissions by 2035
What an ‘ambitious’ 2035 electricity target looks like for China
China’s Clean Energy Pledge is Clouded by Coal, The Wire China
Jesse’s upshift; Rob’s upshift.
This episode of Shift Key is sponsored by …
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A warmer world is here. Now what? Listen to Shocked, from the University of Chicago’s Institute for Climate and Sustainable Growth, and hear journalist Amy Harder and economist Michael Greenstone share new ways of thinking about climate change and cutting-edge solutions. Find it here.
Music for Shift Key is by Adam Kromelow.