Curtailment
Deliberately reducing a power plant's output below what it could produce, usually because the grid can't absorb it or prices make it uneconomic.
Curtailment is turning down power that could have been made. A wind farm feels 30 mph of wind but is instructed — or chooses — to produce less than the wind allows; a solar plant clips its output on a cloudless afternoon. The fuel was free, so the forgone energy looks like pure waste, and in accounting terms it is: energy the resource could have delivered but didn't.
It happens for two broad reasons. Grid-driven curtailment occurs when the wires physically can't carry the power to where it's needed — the local line is full, so someone must back down. Price-driven curtailment occurs when the local price falls so low, often negative, that producing loses money; plants without subsidies or contracts that pay them to run will simply stop.
Curtailment is best read as a signal rather than a scandal. Persistent curtailment in a region says the grid there is saturated: transmission is behind, or generation has clustered faster than demand can follow. It marks exactly the places where new transmission earns its keep and where batteries have an opening — charging on power that would otherwise have been thrown away.
For the technical reader
In nodal markets the two flavors blur, because congestion is expressed through price: a constrained-off wind plant in ERCOT typically isn't manually ordered down but sees its nodal LMP fall below its offer floor and is dispatched down economically. Basis collapse and curtailment are then the same phenomenon viewed as price and as quantity. Subsidised resources (with PTCs or contract structures paying per MWh generated) have negative effective marginal costs and keep producing at moderately negative prices, deepening negative-price episodes.
Measuring curtailment requires a counterfactual — potential generation modeled from weather and availability minus actual output — which makes published figures methodology-sensitive. For siting and storage analysis, curtailment frequency and depth by node are a direct map of trapped energy: hours of near-zero or negative local prices are the raw feedstock of battery charging economics.