Duck curve
The shape of net demand in a solar-heavy grid: a deep midday dip as solar floods in, then a steep evening ramp as it disappears.
Take a grid's demand curve and subtract the output of solar and wind. What remains — net load, the demand that conventional and storage resources must serve — sags deeply in the middle of a sunny day, then climbs steeply as the sun sets while people turn on lights and air conditioning. Plotted over 24 hours, the line looks like a duck: a belly at midday, a steep neck in the evening.
The name was coined at CAISO, where the shape is most pronounced: California routinely serves a large share of midday demand with solar, then replaces it within about three hours as the sun sets. That evening ramp is the operational challenge — thousands of megawatts of flexible capacity must start up or ramp quickly, every single day.
The duck curve is why batteries and solar are natural partners. Storage charges into the belly, when energy is abundant and often nearly free, and discharges into the neck, when the grid needs fast capacity most. As solar penetration grows in ERCOT and other markets, their net-load curves are growing duck-shaped too — and midday prices increasingly sag toward zero while evening scarcity sharpens.
For the technical reader
Net load = system demand − variable renewable output. The duck curve's belly deepens with installed solar capacity, which steepens the evening ramp (MW/hour) and compresses the hours over which the daily energy price spread concentrates. The operational binding constraints are ramp capability and minimum-generation limits: thermal units that must stay online for evening capacity set floors on midday dispatch, contributing to midday oversupply and curtailment or negative prices.
For storage economics, the duck curve concentrates arbitrage value into a predictable daily cycle — charge in the belly, discharge across the neck — and shifts marginal system needs from energy toward ramping and capacity products. Deep solar penetration also moves the hours of scarcity risk later into the evening, which shows up directly in the shape of real-time price duration curves.
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