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== Hours-ahead operation ==
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In the hours prior to the delivery, a system operator might need to deploy additional [[supplemental reserve]]s or even commit more generation units, primarily to ensure the reliability of the supply while still trying to minimize the costs. At the same time, operator must ensure that enough [[reactive power reserve]]s are available to prevent the [[voltage collapse]].{{sfn|Conejo|Baringo|2017|p=10}}
The decisions ("[[economic dispatch]]") are based on the '''dispatch curve''', where the X-axis constitutes the system power, intervals for the generation units are placed on this axis in the ''[[merit order]]'' with the interval length corresponding to the maximum power of the unit, Y-axis values represent the marginal cost of a [[MWh]] of electricity. For cost-based decisions, the units are ordered per increasing incremental (per-MWh) cost. The graph below describes an extremely simplified system, with three generator units: (A) can deliver up to 120 MW at the cost of $30 per MWh, (B) can deliver 80 MW at $60/MWh, (C) is capable of 50 MW at $120/MWh:
{{Graph:Lines
| table=System lambda curve 1.tab
| type=linear| xField=demand
| series="cost"
| title=Dispatch curve (a.k.a. system lambda curve)
| yZero= | xAxis=Demand (MW) | yAxis=Cost ($/MWh) | yMax= | yGrid=y | width=500 | legend=-
| vAnnotationsValues={"text": "Current demand", "x": 150}
| hAnnotationsValues={"text": "Current system lambda", "y": 60}
}}
== Minutes-ahead operation ==
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