How it works
Net load is total demand minus solar output, which is what the rest of the system must supply. Plotted across a sunny day, it sags through the middle as solar rises and reaches its lowest point around early afternoon. As the sun sets, solar falls away just as people return home and demand climbs. Net load then rises steeply into the evening peak. The shape resembles a duck, with its belly at midday and its neck in the evening.
The curve deepens each year as more solar is built. The midday dip pushes prices down, sometimes below zero, and can force curtailment if other plants cannot turn down far enough. The evening ramp requires other resources to add many gigawatts within a few hours. Grid operators measure this ramp in MW per hour and plan reserves around it. Storage that charges at midday and discharges in the evening lifts the belly and lowers the neck.
Why it matters
The duck curve sets the daily price shape that solar and storage projects earn against. A deep midday trough lowers what each extra solar plant earns. A steep evening ramp raises the value of anything that can deliver after sunset, such as batteries, hydro or flexible gas. Developers use it to size storage alongside solar. Industrial buyers on time-of-day tariffs can move flexible load into the cheaper middle of the day.
A worked example
On a spring day, demand is 30 GW at noon and 34 GW at 19:00. Solar supplies 15 GW at noon and 1 GW at 19:00. Net load is 30 − 15 = 15 GW at noon and 34 − 1 = 33 GW at 19:00. Other resources must add 33 − 15 = 18 GW over seven hours, about 2.6 GW per hour on average. Batteries discharging 5 GW at 19:00 would cut the peak net load to 28 GW.
Illustrative figures.
Across markets
The term comes from California, where spring midday net load now falls very low and batteries cover much of the evening ramp. India shows the same pattern as solar grows, with a steep evening ramp and evening exchange prices well above midday ones. In Europe it shows in solar-heavy markets such as Spain, Germany and the Netherlands, where spring midday prices often fall to zero or below.