A generator's maximum power output is called its rated power level. This capacity is normally expressed in megawatts -- millions of watts, or thousands of kilowatts. Most generators can be operated any where between about 20 and 100 percent of this maximum rating, although operation at the lower end of this range could be quite inefficient. The dispatcher controls the generator by "chasing" the load by maintaining the system's frequency and voltage and varying the fuel input into the plant's boiler as needed. If loads are higher than what the generator is producing, the frequency will drop, and visa versa. The amount of this loading is generally express as a percentage of the generator's capability. For example: a 100 megawatt generating unit that is 25 percent loaded is producing 25 megawatts.
Generating units also have the mechanical ability to produce more than their rated capacity for short periods of time. It is generally not a good idea to run a generating unit above its rated capacity since this will reduce the unit's operating life.