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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 (per-[[MWh]] of electricity, ignoring the startup costs). For cost-based decisions, the units in the merit order are sorted by the increasing marginal cost. The graph on the right describes an extremely simplified system, with three committed generator units (fully dispatchable, with constant per-MWh cost):<ref name=psu/>
* unit A can deliver up to 120 MW at the cost of $30 per MWh (from 0 to 120 MW of system power);
* unit B can deliver up to 80 MW at $60/MWh (from 120 to 200 MW of system power);
* unit C is capable of 50 MW at $120/MWh (from 200 to 250 MW of system power).
At the expected demand is 150 MW (a vertical line on the graph), unit A will be engaged at full 120 MW power, unit B will run at the '''dispatch level''' of 30 MW, unit C will be kept in reserve. The area under the dispatch curve to the left of this line represents the cost per hour of operation (ignoring the startup costs, $30 * 120 + $60 * 30 = $5,400 per hour), the incremental cost of the next MWh of electricity ($60 in the example, represented by a horizontal line on the graph) is called '''system lambda''' (thus another name for the curve, ''system lambda curve'').
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