The merit order is a way of ranking available sources of energy, especially electrical generation, based on ascending order of price and sometimes pollution, together with amount of energy that will be generated. In a centralized management scheme, the ranking is such that those with the lowest marginal costs are the first sources to be brought online to meet demand, and the plants with the highest marginal costs are the last to be brought on line. Dispatching power generation in this way, known as economic dispatch, minimizes the cost of production of electricity. Sometimes generating units must be started out of merit order, due to transmission congestion, system reliability or other reasons.