The maximum amount of power, normally expressed in megawatts, that a given system or subsystem can carry or produce at a particular moment, and is typically used to represent the real production capability rating of a generation or transmission system.

Generating capacity denotes the maximum amount of power that can be produced by a given facility. Load capacity or transmission capacity denotes the maximum amount of energy that can be transmitted through a system. Flow capacity is measured in tons, tonnes, cubic feet or cubic metres, and indicates the maximum amount of gas or liquid that can be transported through a pipeline.

Capacity can vary depending on the current state of the system or its capabilities at a given moment, so it is not a fixed or absolute value. For example, a generator's capacity may increase or decrease depending on the type or mixture of fuel used, and a gas pipeline's capacity will vary with changes in the ambient air temperature.

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