Curtailment

California sometimes pays neighboring states to take its electricity and still has to shut off solar farms anyway.

Curtailment is the deliberate reduction of a generator's output below what it could physically produce, ordered when supply would exceed demand or when transmission lines cannot carry the power to where it is needed. Three causes trigger it: oversupply (solar exceeds regional load on a mild spring afternoon), transmission congestion (a West Texas wind farm is at capacity but the lines to Houston are full), and insufficient flexibility (no dispatchable resources available to ramp down).

Some curtailment is economically rational. Building enough storage and transmission to capture every last megawatt-hour would cost more than the energy is worth. Grid planners accept rates of 1-5% as a cost-effective design choice. California curtailed about 2.4 million MWh in 2022, a few percent of its wind and solar output. At 15-20%, though, economics deteriorate: each new plant earns revenue only a fraction of the time it could run.

A curtailment decision in real time. A mild spring Sunday in California. Air conditioning load is low. Solar output is climbing toward 15,000 MW. Gas peakers and imports can reduce output by about 4,000 MW before hitting technical minimums.

If solar keeps climbing and load stays flat, what must the operator do?

Issue a curtailment order. Doing nothing would push frequency above 60.1 Hz, triggering automatic shutdowns across the system. Curtailing 500 MW on purpose is cheaper and safer than an uncontrolled cascade. The energy is not stored; it is foregone.

Curtailment is where storage and transmission investment becomes concrete: each dollar spent on flexibility directly reduces wasted clean generation.

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