Duck curve

The duck curve is the name given to the shape of the net load curve in a market with a significant penetration of solar energy. The net load curve is the demand curve less all renewable generation. This curve is important because it demonstrates the amount of load remaining to be served by non-renewable generation after loads have been served with all available renewable generation. The term first came into wide use in California as system planners studied the potential effects of increasing solar power. 

Source: California ISO

The curve is so named because the net load curve resembles a duck: with a tail during the morning, a belly in the middle of the day as solar output is at a maximum, and a head in the early evening as solar power wanes and overall system demand is at its peak. As shown in the above graphic, the “belly” becomes increasingly pronounced as the amount of solar grows year by year.

The chart below from the California ISO (CAISO) for April 24, 2021, shows the duck curve using actual data. On this day, net loads in the middle of the day dropped to just over 1,000 MW. Note that during the four-hour period between the hours of 1600 and 2000, the CAISO needed to ramp almost 16,000 MW of non-renewable resources to match supply and demand. The duck curve is also observed in other regions besides California. In recent years it has been seen in states such as Texas and even Massachusetts. 

The duck curve highlights the operational changes required to manage an electric system with growing penetrations of solar energy. System operators require increased amounts of flexible resources that ramp up or down quickly to match changes in solar output. Such resources include:

Also important is the ability to import or export power from neighboring systems. In the above example, California was exporting over 2,600 MW at 1 p.m., but it was importing over 6,000 MW during the evening hours. As solar power continues to grow, system operators will need increasing amounts of flexible resources and will need to trade more power across large geographical regions in real time.