What is the merit order—and why can one generator set the price?

Power systems have an unforgiving physical requirement: supply and demand must balance continuously. Market operators therefore need a repeatable way to decide which available resources should generate. A simplified starting point is to rank offers from low to high and dispatch them until demand is met.

That ordered supply stack is the merit order. Low-marginal-cost resources usually enter first. Progressively more expensive units follow. Demand determines how far the operator must move through the stack.

The final block needed is the marginal unit. In a simplified uniform-price energy market, its offer becomes the clearing price paid to dispatched supply. This is why the market price is not the average cost of every generator running at that moment.

The mechanism also explains why modest changes can matter. Demand may move without changing price while it remains inside one block. Cross into the next offer block, however, and the marginal unit changes. An outage or a decline in renewable output can produce the same effect by removing low-cost capacity from the left side of the stack.

For a trader, the useful question is what could move the system into the next price-setting block. For a fundamentals analyst, it is which load, weather, outage, renewable, and fuel assumptions reshape the stack before that happens.