The efficiency of an entity (a device, component, or system) in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a fractional expression).

<math>\mathrm{Efficiency}=\frac{\mathrm{Useful\ power\ out}}{\mathrm{Total\ power\ consumed}}<math>

Efficiency should not be confused with effectiveness: a system that wastes most of its input power but produces exactly what it is meant to is effective but not efficient.

The term "efficiency" only makes sense in reference to an intended effect. So a light bulb might be 2% efficient at emitting light yet still be 98% efficient at heating a room. (In practice it is nearly 100% efficient at heating a room as far as the light does not shine through the windows, because the light energy will also be converted to heat eventually).

As an example: an electronic amplifier that delivers 10 Watts RMS of power to its load (for example a loudspeaker), while drawing 20 Watts of power from a power source is 50% efficient. (10/20 x 100% = 50%)

Efficiency of typical electrical devices

• Incandescent light bulb: about 10% (15% for halogen lamps).
• Fluorescent light bulb: 30% to 40% (depending on the exact type).
• Electric kettle: over 90% (comparatively little heat energy is lost during the 2 to 3 minutes a kettle takes to boil water).

Efficiency is an obvious consideration when we wish to design systems that can operate from batteries.

Inefficiency has a cost (either paid to the power company or the cost of the required power supply) to be weighed against the cost of attaining greater efficiency (choosing different components or redesigning the system).

Also, 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.

See also

• Battery
• List of electronics topics
• Electronic amplifier

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