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Day-ahead operation schedules the generation units that can be called upon to provide the electricity on the next day ('''unit commitment'''). The [[dispatchable generation]] units can produce electricity on demand and thus can be scheduled with accuracy. The production of the weather-dependent [[variable renewable energy]] for the next day is not certain, its sources are thus non-dispatchable. This variability, coupled with uncertain future power demand and the need to accommodate possible generation and [[electricity transmission|transmission]] failures requires scheduling of [[operating reserve]]s that are not expected to produce electricity, but can be dispatched on a very short notice.{{sfn|Conejo|Baringo|2017|p=9}}
Some units have unique features that require their commitment much earlier: for example, the [[nuclear power station]]s take a very long time to start, while hydroelectric plants require planning of water resources usage way in advance, therefore commitment decisions for these are made weeks or even months before prior to the delivery.<ref name=psu>{{cite web |author1-link= |title=Economic Dispatch and Operations of Electric Utilities |url=https://www.e-education.psu.edu/eme801/node/532 |website=psu.edu |publisher=[[Penn State University]] |___location=EME 801 Energy Markets, Policy, and Regulation}}</ref>
For a "traditional" [[vertical integration#Electric utilities|vertically integrated electric utility]] the main goal of the unit commitment is to minimize both the [[marginal cost]] of producing the unit electricity and the (quite significant for fossil fuel generation) start-up costs. In a "restructured" [[electricity market]] a [[market clearing]] algorithm is utilized, frequently in a form of an [[auction]]; the [[merit order]] is sometimes defined not just by the monetary costs, but also by the environmental concerns.{{sfn|Conejo|Baringo|2017|p=9}}
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