What is the “duck curve”?

ELECTRICITY MARKETS must match supply and demand at every moment. The electricity that consumers use must always, second-by-second, be the same as the amount produced. Demand for electricity, known as “load”, is lowest at night and highest in the early evening, as people return from work and turn on lights and appliances. Plot load, in gigawatts, against the hours of the day starting from midnight and it will look like a slanted “s”, dipping during the early hours of the morning, then rising until the early evening peak, before falling again. But solar panels are transforming energy markets—and regulators have given an avian name to one of these transformations. Why are solar panels creating a “duck curve”, and why is that a problem for electricity grids?

Solar energy is inflexible. Panels are most productive in the middle of the day, when the sun is highest; they generate very little power once it sets. That changes the calculation for the system operators who manage the electricity grid. Instead of serving the total amount of load, ensuring there is sufficient generation to meet demand, they need to concern themselves with the “net load”, or the electricity which solar power cannot provide. Plot this net load against the hours of the day and instead of a slanted “s” it looks like a duck, its belly lying low in the water during the sunniest part of the day while in the evenings its neck rises rapidly. The more solar on a grid, the faster other sources of supply must be added in the evenings to meet this peak load.