Wind and solar now have to feed a grid demand curve nobody designed for

Grid operators call it net load: the demand left over once wind and solar output is subtracted from total electricity use. Plot that number across a single day in a place with heavy solar adoption, and it sags in the middle, then rockets upward in the evening in a shape utilities have nicknamed the duck curve.

The pattern isn’t a glitch. It’s what happens when a grid built around plants that could be told to run whenever demand rose gets fed by panels and turbines that generate on their own schedule instead of the utility’s. Nowhere has that mismatch shown up more clearly, or for longer, than California.

What net load and the duck curve actually measure

The concept has a specific, technical definition. The U.S. Energy Information Administration describes net load as demand minus the electricity generated by wind and solar, and it says the pattern created by the midday dip in that curve, followed by a steep rise in the evening as solar drops off, “looks like the outline of a duck.” The California Independent System Operator first published a chart illustrating the shape back in 2013, and the U.S. Department of Energy’s Solar Energy Technologies Office has spent years funding research aimed at softening it.

“High solar adoption creates a challenge for utilities to balance supply and demand on the grid,” Becca Jones-Albertus, the office’s director, wrote in a Department of Energy explainer of the phenomenon, noting that conventional plants must ramp up quickly once the sun sets and solar output falls. The same EIA analysis of the curve warned of a longer-term economic effect: if reduced run-time cuts too deeply into the revenue of conventional plants, some may retire without a dispatchable replacement ready to take their place.

California’s swing from a fifth of days to four-fifths

That warning was written when the trend was still an emerging one. It no longer is. EIA’s Hourly Electric Grid Monitor data show utility-scale solar generated more electricity than natural gas on 82% of days in CAISO during the first five months of 2026, up from just 21% over the same stretch in 2024 and 2025. Solar generation in the system rose 21% between those two periods while gas generation fell 60%, even as electricity demand climbed 7%.

The capacity mix underneath those numbers shifted just as fast. Between April 2024 and April 2026, utility-scale solar capacity in CAISO grew 19% to 25 gigawatts and net battery storage capacity grew 79% to 16 gigawatts, while natural gas capacity stayed essentially flat at 29 gigawatts. Battery discharge over the first five months of 2026 tripled compared with the same period two years earlier, according to the same EIA report, as batteries increasingly soaked up midday solar and released it after sunset.

The evening ramp CAISO built its planning around

The reason batteries matter so much traces back to a specific engineering problem CAISO modeled a decade ago. In planning materials examining net load curves from 2012 through 2020, the grid operator calculated that a spring day’s duck chart could require an additional 13,000 megawatts of generation, brought online within roughly three hours, purely to replace the electricity lost as solar output fell at sunset. That figure describes CAISO’s own planning scenario rather than a fixed daily event, but the underlying shape it illustrates, a fast, steep evening climb, is the same one now showing up in the state’s real-time data every year solar capacity grows.

Batteries have become the tool grid planners lean on to blunt that climb, and the buildout has been fast by utility-industry standards. Nationally, utility-scale battery storage capacity grew at an average annual rate of 70% over the three years through 2025, reaching 43.6 gigawatts by the end of that year and nearly 52 gigawatts by mid-2026, with another 54 gigawatts of additional capacity in developers’ plans through 2028.

California’s own fleet illustrates the scale of that shift most directly. The California Energy Commission reported the state’s battery storage capacity passed 21,000 megawatts in August 2026, up from less than 700 megawatts in 2019, with nearly 16,000 megawatts of that total sited within the state as part of the CAISO grid. “Seeing solar surpass gas on the grid is proof that our vision of a 100% clean energy future is absolutely possible,” CEC Chair David Hochschild said in the commission’s announcement of the milestone.

Batteries now increasingly absorb the portion of the evening ramp that gas plants used to cover, according to the commission, which credited the shift with helping natural gas generation drop even as overall electricity demand kept rising. The tradeoff has an economic dimension EIA flagged as far back as 2023: as batteries and solar together erode the number of hours a gas plant can profitably run, some of that capacity risks becoming uneconomical to keep online, the same dispatchable backup a grid still needs on the days wind and solar underperform. Whether enough of it survives that squeeze, or whether storage fully replaces it, remains the open question CAISO’s own planners are still working through.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview