A scheduling decision process that, over a specified planning horizon, selects which generation units will be started, stopped and held online and assigns their production levels so forecasted load and reliability constraints are met, subject to unit technical constraints (minimum up/down times, ramp rates, startup/shutdown characteristics), fuel or operating costs, reserve requirements and network limits.

Principle

Unit Commitment operationalizes a constrained optimization: binary on/off and continuous dispatch decisions are chosen to satisfy forecasted demand and reliability constraints while minimizing objective(s) (e.g., cost) given physical and temporal unit constraints; feasibility and inter-temporal trade‑offs (start‑up costs, minimum run times) drive the scheduling outcome.

Demonstration

Illustrative scenario — Situation: A system operator faces a day‑ahead demand forecast with a morning peak. Recognition: Thermal baseload units cannot follow rapid ramps without violating minimum up/down or incurring high startup costs. Action: The operator runs a mixed‑integer optimization and commits an intermediate‑cycle unit to start before the morning rise, scheduling baseload units to remain on and a peaking unit to be available. Consequence: Sufficient committed capacity exists at the peak with minimized expected operating cost subject to the units' constraints; if the forecast is accurate, real‑time redispatch is small.

Misapplication

Treating Unit Commitment as identical to real‑time dispatch or assuming a UC schedule guarantees cost‑optimality under all future realizations. The error is conflating a planning‑horizon, constraint‑aware binary schedule with subsequent continuous real‑time dispatch and with the uncertainty that can make a planned schedule suboptimal when forecasts deviate.

Consequence

A correct UC produces a legally and physically implementable on/off schedule and availability profile that reduces the risk of resource shortfalls and limits costly emergency actions, but it also imposes startup and cycling costs and may reduce flexibility. An incorrect or infeasible UC can create reserve shortfalls, force emergency unit starts or load curtailment, and increase total system cost through inefficient cycling or insufficient capacity at critical times.

Reversal

When very fast, low‑marginal‑cost resources (e.g., grid‑scale batteries, fast‑ramping gas turbines, flexible demand aggregations) supply most flexibility, the UC horizon, binary decision emphasis and minimum run‑time constraints may be relaxed or reformulated; in microgrids or strictly real‑time markets, UC may be replaced by rolling dispatch with minimal precommitment.

Boundary

Clearly within: day‑ahead or week‑ahead integer optimization that outputs start/stop decisions and scheduled outputs for generating units. Boundary case: co‑optimization of reserves and energy where reserve requirements alter commitment decisions; the label 'UC' still applies but the objective and constraints broaden. Clearly outside: real‑time economic dispatch that adjusts continuous setpoints without committing/off‑lining units, preventive maintenance scheduling that is long term and not driven by near‑term load forecasts.

Semantic Tension

Cost minimization (operational economics) ↔ Reliability/resilience (safety margin and reserve provisioning): tightening one usually increases the other’s cost or risk, so UC must trade off economic efficiency and security of supply under uncertainty.

Synthesis

Unit Commitment is the pre‑real‑time binary planning layer that converts forecasts and technical constraints into explicit availability decisions; its primary role is resolving inter‑temporal feasibility (start‑up, minimum run times, reserves) so that real‑time dispatch can operate within a feasible resource envelope.