The duck curve is a graph showing the gap between how much electricity consumers use and how much solar energy is available throughout the day. When you plot this gap over 24 hours, the shape resembles a duck: a sagging belly during midday when solar panels flood the grid with power, and a steep neck in the evening when solar drops off but demand surges. The California Independent System Operator (CAISO) first published the chart in 2013, and it has since become one of the most referenced concepts in energy policy.
How the Curve Gets Its Shape
To understand the duck curve, you need to know the difference between two measurements. Total demand is the actual amount of electricity consumers use at any given time. Net load is total demand minus the electricity generated by renewable sources, primarily solar. Net load tells grid operators how much power they still need to produce from other sources like natural gas, nuclear, or hydroelectric plants.
On a sunny day, solar panels generate enormous amounts of electricity during late morning and early afternoon. That drives net load down sharply, creating the duck’s belly. Then, as the sun sets between roughly 4 p.m. and 8 p.m., solar output drops to zero while millions of people arrive home, turn on lights, cook dinner, and crank up air conditioning. Net load shoots upward. That steep climb is the duck’s neck, and it forces grid operators to bring large amounts of conventional power online very quickly.
Why the Belly Creates Problems
The midday dip might sound like good news. After all, solar panels are producing so much electricity that the grid barely needs anything else. But the grid was not designed to handle wild swings in supply. When solar output exceeds what the grid can absorb, operators face overgeneration, a situation where there is literally more electricity than anyone can use or store.
Overgeneration creates real consequences. In 2024, CAISO curtailed 3.4 million megawatt-hours of wind and solar output, a 29% increase over 2023. Solar accounted for 93% of all that wasted energy. Curtailment means grid operators told solar farms to stop sending power to the grid, essentially throwing away clean electricity because there was nowhere for it to go.
The oversupply also drives wholesale electricity prices down, sometimes below zero. Across U.S. electricity markets, negative prices occurred in about 4% of all hours and market locations. In some regions the problem is far more concentrated. In the Permian Basin in western Texas and parts of Kansas and western Oklahoma, negative prices appeared in more than 25% of all hours. Negative prices mean generators are paying the grid to take their electricity, which sounds absurd but reflects the real cost of having too much power with no way to store or redirect it.
Why the Evening Ramp Is Dangerous
The belly is a headache. The neck is a genuine reliability risk. As solar generation fades in late afternoon, grid operators must ramp up conventional power plants fast enough to cover both the loss of solar and the simultaneous spike in evening demand. This ramp can require adding thousands of megawatts of generation capacity in just a few hours.
Most conventional power plants, particularly natural gas turbines, were not designed to go from idle to full output that quickly. The faster and steeper the ramp, the harder it becomes to keep the grid’s voltage and frequency stable. If operators can’t keep pace, the result can range from voltage fluctuations to rolling blackouts. California has experienced exactly this kind of grid stress during heat waves, when the evening ramp coincides with extreme air conditioning demand.
The Curve Gets Deeper Every Year
The original 2013 chart showed projected duck curves for several years into the future, and the belly got deeper with each passing year as more solar capacity was installed. That prediction has played out. Some energy analysts now describe a “canyon curve” in regions with very high solar penetration, where midday net load doesn’t just dip but plunges so far below the baseline that the duck shape becomes more like a steep ravine. The deeper the canyon, the more extreme the evening ramp, and the more difficult and expensive it becomes to manage the grid.
Battery Storage as a Solution
The most direct fix for the duck curve is storing midday solar energy and releasing it during the evening peak. Battery energy storage systems do exactly this. They charge during the hours when solar output exceeds demand, absorbing the surplus that would otherwise be curtailed. Then they discharge during the steep evening ramp, reducing how quickly conventional plants need to fire up.
This approach attacks both halves of the problem simultaneously. It raises the belly by soaking up excess solar, and it flattens the neck by supplying stored energy during peak hours. Batteries also help stabilize voltage and frequency in real time, acting as a buffer that smooths out the rapid fluctuations renewable sources introduce. Grid operators can schedule batteries to charge during low-price periods and discharge when demand and prices peak, which reduces stress on the grid while improving the economics of solar generation.
California has deployed battery storage aggressively in recent years, with several gigawatts of capacity now online. During summer evenings, batteries regularly supply a meaningful share of the state’s electricity during the critical ramp hours, a role that barely existed five years ago.
Other Ways to Flatten the Curve
Battery storage gets the most attention, but several other strategies help reshape the duck curve:
- Time-of-use pricing: Utilities charge more for electricity during the evening peak and less during midday, encouraging customers to shift energy use (running dishwashers, charging electric vehicles, doing laundry) to the hours when solar is abundant.
- Demand response programs: Large commercial and industrial customers agree to reduce their electricity use during peak hours in exchange for lower rates. This shaves the top off the evening spike.
- Geographic diversity: Connecting grids across wider regions means solar generation in one area can serve demand in another where the sun has already set. Longer transmission lines effectively stretch the solar production window.
- Flexible generation: Newer natural gas plants and hydroelectric facilities can ramp up and down more quickly than older plants, making the evening transition smoother.
No single solution eliminates the duck curve. The combination of battery storage, smarter pricing, and grid flexibility is gradually reshaping the net load profile, turning the duck into something flatter and more manageable. But as solar capacity continues to grow faster than storage capacity, the curve remains one of the central engineering and economic challenges of the clean energy transition.