Merit Order

Merit order is the ranking of available generation resources from lowest to highest marginal cost for the purpose of determining dispatch sequence. In its simplest form, the cheapest available units are used first, followed by progressively more expensive units until demand and reserve requirements are met.

The concept is fundamental to electricity-market economics because it links plant operating cost to dispatch and price formation. Changes in fuel cost, renewable output, carbon pricing, or outages can all shift the merit order and change which units set the market price.

Key Aspects of Merit Order:

• Marginal-Cost Ranking: Units are ordered according to the cost of producing the next increment of electricity. In competitive settings, this ranking often guides which generators clear first in the market or are dispatched first operationally.
• Price-Setting Mechanism: The last unit needed to satisfy demand typically sets the clearing price in marginal pricing systems. This means the market price reflects the marginal unit, not the average cost of all generation online.
• Renewable Effect: Wind and solar resources often have very low short-run marginal cost, so they can enter early in the merit order when available. This can displace higher-cost thermal plants and lower market prices during periods of high renewable output.
• Constraint Interaction: The ideal merit-order dispatch is often modified by transmission congestion, reserve requirements, ramp limits, or unit-commitment constraints. Real systems therefore dispatch according to an economically optimized but physically constrained version of the merit order.
• Market Interpretation: Merit-order shifts explain many observable market effects, including lower midday prices in solar-heavy systems, reduced run hours for mid-merit thermal plant, and changing investment signals as the generation mix evolves.