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.
