Duck Curve

The duck curve describes the shape of net demand (demand minus solar and wind output) across a day on a grid with a lot of solar power: it dips around midday as solar output peaks, then rises sharply in the evening as the sun sets and people get home. Drawn out, it resembles a duck's back and neck; the steep evening rise is the operational challenge, because other generation has to increase output quickly to cover both the loss of solar and the evening peak in demand.