What is the duck curve?

Take a grid's total electricity demand, subtract what solar and wind are producing at each moment, and you get net load: the demand everything else on the grid has to meet. On a sunny grid, a chart of net load across one day traces a recognizable shape. Moderate through the morning, it sinks through midday as solar floods in, then climbs steeply in the early evening as the sun sets while people arrive home and turn everything on. A sagging belly, a steep neck, a head at the evening peak: a duck, in profile.

The name comes from the California Independent System Operator, whose planners published the original "duck chart" in 2013 while projecting what growing solar would do to the state's net load. The projection came true ahead of schedule, and the chart above is that same measurement, live: CAISO's reported net demand today, drawn against the same calendar date five years ago. The deeper today's belly sits below the dashed line, the more solar is carrying the middle of the day.

Why it matters

The duck's belly means near-free midday power, sometimes so much that solar is curtailed for lack of anywhere to put it. The duck's neck is the hard part: a ramp of tens of gigawatts in about three hours, which must come from batteries discharging, gas plants starting, imports, or demand shifting. Nearly everything this site covers is, one way or another, about that shape: grid-scale storage exists to move the belly into the neck, home batteries and smart panels flatten it from behind the meter, EV charging can fill the belly or worsen the head depending on when it happens, and a growing share of energy policy is an argument about who pays for the ramp.

Reading the two lines

The two curves usually differ in two separate ways, and they have two separate causes. The belly sinks over time because of solar: panels only suppress net load while the sun is up, so five years of buildout show up as a deeper midday trough. The tail and the head rise over time because of load growth: in the hours solar cannot touch, net load is simply demand, and demand keeps growing with electrification, EV charging, data centers, and warm evenings running air conditioning long after sunset. The duck gets deeper and taller at the same time.

The evening ramp is the distance between those two extremes, so both trends steepen it. That widening gap is why battery storage became the fastest-growing resource on this grid, and why so much rate design and charging policy is an argument about the hours between four and nine. One caution when comparing any single pair of days: weather and day of week are not controlled, so part of any one day's gap is weather. The structural trend is what persists across seasons.

About this chart

Data is CAISO's published five-minute net demand, refreshed every half hour, with a one-to-two hour reporting lag. Five years is a long time in this transition; watch the gap between the two lines on a clear spring afternoon.