What is the duck curve and why does more solar make the grid harder to run?
The duck curve is the daily shape of grid net load — total demand minus solar output. Abundant midday solar presses net load into a low 'belly'; at sunset solar vanishes just as evening demand peaks, so net load ramps almost vertically into the 'neck'. CAISO named it in 2013; in California the grid must add 16,000 MW within a three-hour ramp.
A traditional grid has a 'camel curve': a trough overnight, a peak in the afternoon when businesses are running, and a taper in the evening. As solar grows from a marginal 1% of the grid to shares of 20% or even 40%, millions of panels inject power during daylight and strip demand away from conventional plants, rewriting the shape of the curve. Solar output surges after sunrise, the net load that conventional plants must serve collapses into the duck's belly, and by noon the grid may face outright surplus energy.
The real difficulty comes at sunset. Solar output disappears while millions of people arrive home, switch on lights and start cooking, so net load must rocket upward to fill the gap. In California the grid needs to add 16,000 MW during the three-hour evening ramp (CAISO) — the equivalent of 16 large nuclear reactors, delivered in three hours. Conventional coal and nuclear units cannot cycle that fast, forcing the system to lean on gas peaker plants. Every year of added solar makes the neck steeper; if it becomes too steep, the grid destabilises and blackout risk rises.
The belly side carries its own cost. When the belly runs too deep and solar output exceeds instantaneous demand, the system must curtail — instructing solar plants to stop generating and throwing free clean energy away. In 2022 California curtailed 2,400 GWh of solar, roughly the annual consumption of 500,000 households. Storage exists precisely to capture that discarded midday surplus and shift it into the evening peak.
Taming the duck takes four strategies. Storage charges batteries on the midday belly and discharges them into the evening neck. Demand response uses smart appliances to move energy-intensive tasks — dishwashers, EV charging — from the evening peak into the sunny afternoon. Transmission interconnection links regions so surplus desert solar can serve a cloudy city hundreds of kilometres away. And inverter intelligence lets modern smart inverters help stabilise grid frequency and voltage, taking on a role once reserved for large spinning generators.
| Strategy | Part of the curve addressed | How it works |
|---|---|---|
| Storage | Belly and neck | Batteries absorb the midday solar surplus and discharge into the evening ramp |
| Demand response | Neck | Shift dishwashers, EV charging and similar loads from the evening peak to the afternoon |
| Transmission interconnection | Belly | Move surplus solar across regions to serve cloudy cities far away |
| Smart inverters | Whole curve | Power electronics help hold frequency and voltage, replacing part of the role of spinning machines |
The duck curve is, in the end, a marker of success. It means so much solar is being produced that the old grid has been bent out of shape. The answer is not to slow solar deployment but to accelerate the transformation of grid infrastructure.
Sources
- CAISO, Managing the Duck Curve: 2023 System Update (net load curve; 16,000 MW three-hour ramp)
- The Full Spectrum, Chapter 13.3 (California curtailed 2,400 GWh of solar in 2022)
- The Full Spectrum: Every Energy Source Explained — A Singapore Perspective, Chapter 7.8
This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第七章 7.8 / 第十三章 13.3