Rating of alternator is expressed in two units.
Capacity in MVA
Generation in MW
Capacity is the maximum electric output a generator can produce under specific conditions. Nameplate capacity is determined by the generator’s manufacturer and indicates the maximum output a generator can produce without exceeding design thermal limits.
Generation is the amount of electricity a generator produces over a specific period of time.
For example, a generator with 1 megawatt (mW) capacity that operates at that capacity consistently for one hour will produce 1 megawatthour (mWh) of electricity. If it operates at only half that capacity for one hour, it will produce 0.5 mWh of electricity. Many generators do not operate at their full capacity all the time; they may vary their output according to conditions at the power plant, fuel costs, and/or as instructed from the electric power grid operator.