The usual reason for this limit is heat, although in certain electromechanical devices, particularly speakers, it is to prevent mechanical damage.
Equipment is generally rated by the power it will deliver, for example, at the shaft of an electric or hydraulic motor.
For electrical motors, a similar kind of information is conveyed by the service factor, which is a multiplier that, when applied to the rated output power, gives the power level a motor can sustain for shorter periods of time.
Exceeding the power rating of most devices for a very short period of time is not harmful, although doing so regularly can sometimes cause cumulative damage.
Power ratings for electrical apparatus and transmission lines are a function of the duration of the proposed load and the ambient temperature; a transmission line or transformer, for example, can carry significantly more load in cold weather than in hot weather.
Momentary overloads, causing high temperatures and deterioration of insulation, may be considered an acceptable trade-off in emergency situations.
The power rating of switching devices varies depending on the circuit voltage as well as the current.
Audio amplifier power ratings are typically established by driving the device under test to the onset of clipping, to a predetermined distortion level, variable per manufacturer or per product line.
Manufacturers can use these methods to market amplifiers whose published maximum power output includes some amount of clipping in order to show higher numbers.
[14] For instance, the Federal Trade Commission (FTC) established an amplifier rating system in which the device is tested with both channels driven throughout its advertised frequency range, at no more than its published distortion level.
The Electronic Industries Association (EIA) rating system, however, determines amplifier power by measuring a single channel at 1,000 Hz, with a 1% distortion level—1% clipping.
[14] The nominal power of a photovoltaic module is determined by measuring current and voltage while varying resistance under defined illumination.
The power is measured while varying the resistive load on the module between open and closed circuit.