The California solar duck curve: what it is and why it matters

What the duck curve shows

Grid operators care less about total demand than about net load, which the California ISO defines as the difference between forecast load and the expected output of variable resources such as solar and wind. Net load is what the rest of the fleet, from gas plants to hydro, imports and batteries, has to cover.

On a sunny spring day, solar output climbs through the morning and peaks around midday, so net load drops into a deep trough: the duck’s belly. Late in the afternoon solar falls away just as people come home, cook and run air conditioning, so net load rises sharply into the evening peak: the duck’s neck. The California ISO’s 2016 fact sheet described a spring scenario in which it had to bring on an additional 13,000 megawatts within about three hours as the sun set (CAISO).

Why the belly keeps getting deeper

Every new solar plant adds to midday supply and does little for the evening. The EIA put it plainly in 2023: as solar capacity in California continues to grow, “the midday dip in net load is getting lower” (EIA). The California Energy Commission counts in-state solar plants of 1 megawatt and larger at 14,981 megawatts at the end of 2021 and 23,749 megawatts at the end of 2025 (CEC, updated June 16, 2026). Rooftop solar is not in that figure, and it deepens the belly too, because it lowers the demand the grid sees in the middle of the day. How big solar’s share of the state’s power has become is covered in what percentage of California’s power is solar.

Curtailment: when California switches solar off

When the belly gets too deep, there is more renewable power than the grid can use or export. The California ISO says that in those hours its market “automatically reduces, or curtails, renewable generation to match supply with demand,” and that it happens most often in spring and fall, when “moderate weather, and sunny, breezy days produce an abundant supply of renewable generation” (CAISO). It applies to generators in the ISO’s market, which are mostly large solar and wind farms, rather than to a home rooftop system.

These totals are our sums of the California ISO’s monthly wind and solar curtailment chart data, in a file dated September 10, 2026 (CAISO). Curtailment roughly quadrupled from 2019 to 2025, and the first eight months of 2026 already exceeded any full year, with April and May each near 1.45 million MWh. The California ISO stopped publishing its separate production-and-curtailment reports on June 1, 2025, so check the chart file for newer months.

Batteries are flattening the neck

Batteries do the obvious thing: charge on cheap midday solar and discharge into the evening ramp. The EIA reported that California’s battery capacity grew from 0.2 gigawatts in 2018 to 4.9 gigawatts in April 2023. By August 7, 2026, the California Energy Commission counted 21,112 megawatts of battery storage serving the state’s grid, about 3,000 megawatts of it in more than 300,000 smaller systems at homes, schools, farms and businesses (CEC). The year-by-year build-out is on California battery storage capacity. The curtailment numbers above show that batteries have not kept pace with midday solar in spring, but they carry a growing share of the evening peak.

What the duck curve means for your electric bill

The California ISO’s own list of fixes included time-of-use rates that encourage daytime use, more storage and more electric vehicles (CAISO). All three now show up on a California bill. Time-of-use plans charge the most in the late afternoon and evening; the hours for each utility are in electricity peak hours in California.

Rooftop solar is paid the same way. For PG&E, SCE and SDG&E customers on the Net Billing Tariff, the CPUC credits exports using its Avoided Cost Calculator values, which it says are “usually lower than the retail rate” but “can rise above the retail rate on late summer evenings.” The CPUC also reports that nearly 70 percent of net billing customers had paired a battery with their solar by the end of 2024 (CPUC). In other words, the duck curve is why a new system’s midday exports earn little and why a battery that shifts that energy into the evening is now the usual design. The hourly values are in NEM 3.0 export rates by hour, and the tariff change itself is in NEM 2.0 versus NEM 3.0.

What you can do with it at home

You cannot change the grid’s shape, but you can stop paying for it. Run the dishwasher, laundry, pool pump and EV charging in the middle of the day where your plan prices those hours lower, and move as little as you can into the evening peak. If you have solar on net billing, using your own midday output is worth more than exporting it. If you are pricing a battery, size it to your evening use, not to a sales chart; the method is in how many batteries you actually need, and whether it pays back on your rate is in the battery payback analysis.