Duck Curve in Power Systems

The duck curve shows how the electricity that must be supplied by sources other than solar power changes over a day when solar generation is substantial. It plots net load, meaning total electricity demand minus variable renewable generation, especially solar; its midday dip and steep evening rise can resemble a duck.

How the curve forms

The curve reflects the mismatch between the timing of solar generation and electricity consumption.

• During daylight hours, increasing solar output reduces net load, producing a deep midday dip even though consumers continue using electricity.
• Towards sunset, solar output falls while electricity demand may remain high, creating a steep evening ramp that other resources must meet.
• Greater solar penetration can deepen the midday dip and increase the required evening ramp; the curve represents residual demand, not total electricity consumption.

Why it challenges grid operation

The central challenge is operational flexibility: the ability to adjust electricity supply or demand quickly enough to maintain balance.

• Conventional generators may need to reduce output during sunny hours and increase it rapidly as solar generation declines.
• If available generation exceeds demand and the system cannot absorb or export the surplus, operators may require curtailment, deliberately reducing renewable electricity output.
• Adequate generating capacity alone is insufficient if resources cannot respond quickly enough to the changing net load.

How electricity systems respond

• Energy storage can absorb electricity during solar-rich hours and discharge it during the evening.
• Demand response can shift suitable electricity consumption towards periods of abundant solar generation.
• Flexible generation, stronger transmission links and improved forecasting help balance changing supply and demand.