Why does solar power make the grid's evening so hard?

Solar panels have a schedule: they work hardest at noon and stop at sunset. People have a different one, and in some energy markets the daily peak in demand comes after sunset, when solar power is no longer available. Put the two together and you get the duck curve, the reason a sunny grid can have a very hard evening.

The duck curve is a graph of the electricity demand left over after subtracting the supply from variable renewable sources, mainly solar, over a day. Where a lot of solar is installed, that leftover drops at midday, which is the duck's belly, then rises rapidly around sunset and peaks in the mid-evening hours: the neck and head. The term was coined in 2012 by the California Independent System Operator. In 2020, the neck represented a ramp of 10 to 17 gigawatts in three hours, which flexible generation had to supply. In Hawaii the effect is more pronounced, and it is known as the Nessie curve.

The challenge is timing. Without storage, power companies must rapidly increase other generation at sunset, and the oversupply at midday against a shortage in the evening explains the large gap between midday and evening prices. When there is too much power at noon, operators can use curtailment, an intentional reduction of generation to keep the grid stable. In Hawaii, curtailment reached 20 percent on Maui in the second and third quarters of 2020.

The duck is shrinking in places that act. By 2024, massive battery systems in California had greatly decreased its severity, discharging in the evening to meet the sunset peak.