What is curtailment and why is clean power thrown away?

Curtailment is when wind turbines and solar farms that are already physically connected, with the resource available, are ordered by grid dispatch to reduce output — throwing away clean electricity that could have been generated. The cause is never a shortage of generation but an inability to move it or use it: in 2016 wind curtailment rates in Gansu and Xinjiang in north-west China reached 30%—40%.

The first cause is power that cannot be moved. Regions where wind and solar capacity grows fastest are often far from load centres, and inter-provincial ultra-high-voltage (UHV) transmission has lagged behind generation build-out. In 2016, curtailment rates in Gansu, Xinjiang and other north-western Chinese provinces reached 30%—40%, meaning three to four tenths of the available wind energy was wasted rather than turned into useful electricity.

Since 2017 China has pushed UHV transmission construction hard, with cumulative investment in the hundreds of billions of yuan, alongside integrated 'wind-solar-thermal-storage' base planning that bundles wind and solar with peaking thermal units and storage to relieve immediate pressure on transmission corridors. By 2023 the national average wind curtailment rate had fallen to about 4% and solar curtailment to about 1.7%, a marked improvement, though the problem persists in parts of the north-west and north-east. The most extreme piece of that corridor is the ±1,100 kV Changji–Guquan HVDC link, rated at 12 GW over roughly 3,300 km, carrying Xinjiang wind, solar and coal power to eastern China's load centres.

The second cause is power nobody needs at that moment — the duck curve. The chart published by the California Independent System Operator (CAISO) in 2013 plots net load (total load minus solar output) across a day: solar ramps up in the morning and net load collapses into the duck's belly; then the sun sets just as demand peaks, and net load climbs vertically within a few hours to form the duck's neck. When the belly gets deep enough that solar output exceeds instantaneous grid demand, the system has no choice but to curtail — instructing solar plants to stop generating and discarding free clean energy. In 2022 California curtailed 2,400 GWh of solar, equivalent to the annual consumption of 500,000 households.

There are two ways out. One is storage: capture the midday surplus and release it into the evening peak, which also flattens the duck's neck ramp. The other is demand-side management — running EV charging, smart water heaters and pool pumps only at the deepest point of the duck's belly harvests the surplus solar for free, converting 'dumb demand' into a flexible resource. If a grid operator can control 20% of load, it can eliminate the need for billions of dollars of peaking gas plants; this is the logic of the virtual power plant (VPP). Transmission remains the structural answer: interconnection queues and insufficient network capacity are the same bottleneck seen from the other end of the wire.

When the duck's belly runs deep enough that solar output exceeds instantaneous grid demand, the system has no choice but to curtail — instructing solar plants to stop generating and discarding free clean energy.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第十一章 11.4、第十三章 13.3