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Electrical efficiency

The efficiency of an entity (a device, component or system) in electronics and electrical engineering is defined as a fractional expression. One factor is the power it absorbs; the other is the power it delivers in the form whose production is the purpose of the entity. (Efficiency should not be confused with effectiveness; a system that wastes power in producing exactly what it is meant to, is both effective and inefficient.)

Specifically, calculating such an an entity's efficiency requires knowing the (input) power going into it, and the (output) power it delivers. The output power divided by the input power is the efficiency, usually expressed as a percentage. For example, an electronic amplifier that delivers 10 Watts RMS to its load, while drawing 15 Watts of DC power from a power source is 67% efficient. (10/15 x 100% = 67%)

Efficiency is an obvious consideration when we wish to design systems that can operate from batteries.  For more subtle reasons, the same is true when the amounts of power are large: any difference in the input and output power probably produces heat within the system (though noise and other mechanical vibrations involve at least theoretically separate inefficiencies), and that heat must be removed from the system if it is to remain within its operating temperature range. And in general, since power is seldom free, every inefficiency in effect gets its costs weighed against the cost of attaining greater efficiency.