Wikipedia Deep Dive

Duck curve

8 min read

Every evening around sunset, grid operators across California face a challenge that would have seemed absurd just two decades ago: they have too much electricity during the day and not enough in the evening. The visual representation of this problem, when graphed over a typical day, looks remarkably like a duck sitting on water—a plump belly in the middle of the day and a long neck stretching up toward evening peak hours. This is the duck curve, and it has become one of the defining puzzles of the renewable energy transition.

The Shape of the Problem

The duck curve emerged as a recognized phenomenon in 2012, when the California Independent System Operator—the organization responsible for managing most of the state's electrical grid—first coined the term. What they were seeing was a fundamental mismatch between when solar panels produce electricity and when people actually use it.

Think about a typical day. Solar panels begin generating power at sunrise, ramp up through the morning, and reach peak production around midday when the sun is highest. Meanwhile, electricity demand follows a different pattern entirely. People wake up, turn on lights and appliances, head to work or school, and demand stays relatively steady through the day. Then evening arrives.

This is when everything changes.

As the sun sets, solar production plummets toward zero over the course of just a few hours. But human activity is ramping up. People return home, turn on air conditioning that's been off all day, cook dinner, do laundry, watch television. Demand surges just as solar supply vanishes. Grid operators must somehow fill this gap, and they must do it fast.

Why Ducks Matter to Electrical Engineers

The belly of the duck represents midday, when solar panels are flooding the grid with cheap, abundant electricity. The neck represents the evening hours when operators must rapidly bring other power sources online to compensate for the loss of solar. The steepness of that neck—how quickly generation must increase—is what makes grid operators lose sleep.

In 2020, California's grid faced ramp rates of between ten and seventeen gigawatts in just three hours during the afternoon transition. To put that in perspective, a gigawatt is roughly the output of a large nuclear power plant. So grid operators were essentially being asked to turn on the equivalent of ten to seventeen nuclear plants in the span of three hours, every single day, just to keep the lights on through dinner.

Traditional power plants were not designed for this kind of rapid cycling. A coal plant or a nuclear plant prefers to run at a steady output around the clock. Natural gas plants are more flexible, but even they have limits on how quickly they can ramp up and down. The duck curve forces these plants to operate in ways that increase wear, reduce efficiency, and drive up costs.

Hawaii's Loch Ness Monster

If California has a duck, Hawaii has something more dramatic. The islands' isolated grids, combined with their enthusiastic adoption of rooftop solar, have produced a curve so pronounced that local operators dubbed it the Nessie curve, after the Loch Ness Monster. Where California's duck has a gentle belly, Hawaii's curve plunges so deep during midday that it creates serious stability problems.

Island grids cannot draw power from neighboring states the way California can. When Hawaiian solar panels produce more electricity than the islands need, there's nowhere for that power to go except into storage or, increasingly, into deliberate waste through a process called curtailment—simply telling solar panels to stop producing even though the sun is shining.

The Price of Sunshine

The duck curve has created a strange new economics of electricity. During midday hours, when solar panels are pumping out power, wholesale electricity prices have collapsed. In California, midday prices have dropped to around fifteen dollars per megawatt-hour—a remarkable bargain compared to historical norms.

But the neck of the duck tells a different story.

During the evening hours from five to eight in the evening, when solar has faded but demand remains high, wholesale prices have climbed to around sixty dollars per megawatt-hour—four times the midday rate. This price gap keeps widening. As more solar capacity comes online, midday prices fall further. As the evening ramp becomes steeper, peak prices rise higher.

For traditional power plant operators, this creates an uncomfortable business model. Their plants sit idle during the sunny, cheap hours, then must fire up rapidly for just a few profitable hours each evening. The economics of building and maintaining a power plant that only runs a few hours per day are challenging at best.

Taming the Duck

Grid operators and engineers have developed a toolkit of strategies for managing the duck curve, each with its own tradeoffs.

The most direct approach is energy storage—capturing that abundant midday solar electricity and saving it for evening use. Battery storage facilities have become increasingly important in this role. California's Moss Landing Power Plant, once a natural gas facility, now hosts one of the world's largest battery installations. By 2022, California was shifting up to six gigawatt-hours of electricity per day from low-price midday periods to high-price evening hours.

Pumped-storage hydroelectricity works on a similar principle but at larger scales. During periods of excess solar production, water is pumped uphill into a reservoir. When evening demand peaks, that water is released through turbines to generate electricity. The approach is elegant but geographically constrained—you need the right combination of elevation differences and available water.

Some utilities have experimented with orienting new solar installations toward the west rather than due south. West-facing panels produce less total energy over the course of a day, but they shift production later into the afternoon, partially smoothing out the duck's neck. The tradeoff is efficiency for timing.

Demand as a Resource

Another approach flips the problem around: instead of trying to match supply to demand, change when demand occurs.

Time-of-use pricing gives consumers financial incentives to shift their electricity consumption. Charge your electric vehicle during midday rather than overnight. Run your dishwasher in the early afternoon. Pre-cool your house before sunset so your air conditioner can coast through the evening peak. When electricity prices accurately reflect the duck curve, rational consumers should naturally flatten it.

Smart grid technology takes this further by automating demand response. A smart water heater can be programmed to heat water during solar surplus hours, storing thermal energy that will be available whenever you need it. A smart air conditioner can pre-cool your home a few degrees before the evening price spike hits. The house becomes, in effect, a form of energy storage.

Electric vehicles represent an intriguing opportunity. Most cars sit parked for the vast majority of the day. An electric vehicle plugged in at work could charge from midday solar surplus, then potentially feed some of that power back to the grid during the evening peak—a concept called vehicle-to-grid, often abbreviated as V2G. If millions of electric vehicles participated in such a program, their combined battery capacity would represent a massive distributed storage resource.

The Geography of Sunsets

One elegant solution to the duck curve requires no batteries at all: simply connect grids across wider geographic areas. When the sun sets in California, it's still shining in Arizona. When Arizona's solar panels go dark, New Mexico still has light. A sufficiently interconnected grid could, in theory, pass solar electricity eastward as the earth rotates, chasing the sunset around the planet.

The practical challenges are substantial. Long-distance power transmission loses energy along the way. Building new transmission lines faces permitting battles and local opposition. But the physical principle is sound, and expanded regional transmission capacity has become a key part of many decarbonization plans.

Running Faster to Stay in Place

Perhaps the most striking aspect of the duck curve is how quickly it has evolved. When California grid operators first identified the phenomenon in 2012, they projected it would become a significant challenge by the early 2020s. The actual curve has grown faster than those projections. Each year brings more solar capacity, a deeper belly, and a steeper neck.

This creates a kind of arms race. Storage and demand management solutions must deploy at a pace that matches or exceeds the growth of solar generation. Fall behind, and grid reliability suffers. The evening of August 2020, when California experienced rolling blackouts during a heat wave, offered a preview of what happens when the duck curve overwhelms available resources.

Beyond the Duck

The duck curve is ultimately a transitional phenomenon—a visible symptom of an electrical grid caught between its fossil fuel past and its renewable future. A grid with abundant storage, smart demand response, and wide geographic interconnection would not display a duck curve at all. Supply and demand would flow smoothly into each other, electrons generated at noon powering televisions at nine in the evening.

We are not there yet. The duck remains, sitting on its graph, reminding us that the renewable energy transition involves not just building solar panels but fundamentally reimagining how electricity systems work. The sun shines when it shines, not when we flip a switch. Learning to work with that constraint, rather than against it, is one of the defining engineering challenges of our time.

The duck curve is what happens when physics meets human behavior meets economics. It is neither good nor bad—it simply is. And increasingly, it is the shape that determines how we will power our civilization through the coming decades.