The duck curve

node · earth · first observed in California; now general to high-solar grids

The shape that appears in a grid's net load — total demand minus wind and solar output — once solar penetration passes a threshold, named for its resemblance to a duck in profile. The California Independent System Operator published the original chart around 2013 as a projection of what its own net load would look like by 2020; the projection was not merely met but exceeded, and the shape has since appeared in South Australia, Chile, Hawaii and elsewhere. The belly is midday, when solar suppresses net load toward and sometimes below zero; the neck is the evening ramp, when the sun sets while demand is still high, requiring the system to bring on many gigawatts within about three hours. Two consequences follow, and they are different problems. Midday abundance produces curtailment and negative prices, which is an economic problem for generators and a signal to move demand or charge storage; the evening ramp is an engineering problem for whoever must supply it, and it is the reason four-hour batteries were built in California at scale before they were built almost anywhere else. The duck curve is the cleanest available demonstration that a variable resource changes the shape of the residual problem rather than simply shrinking it.