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\ output}}{\mathrm{Total\ power\ input}} </math>
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Efficiency of typical electrical devices
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 the wanted effect. So a light bulb might have 2% efficiency at emitting light yet still be 98% efficient at heating a room. (In practice it is nearly 100% efficient at heating a room because the light energy will also be converted to heat eventually, apart from the small fraction that leaves through the windows). An electronic amplifier that delivers 10 watts of power to its load (for example a loudspeaker), while drawing 20 watts of power from a power source is 50% efficient. (10/20 × 100% = 50%)
- Electric kettle: over 90% (comparatively little heat energy is lost during the 2 to 3 minutes a kettle takes to boil water).
Efficiency of devices at point of Maximum Power Transfer
As a result of the Maximum Power Theorem, devices transfer maximum power to a load when running at 50% electrical efficiency. This occurs when the load resistance (of the device in question) is equal to the internal Thevenin equivalent resistance of the power source.
Efficiency of light bulbs
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For more details on this topic, see Luminous efficacy.
- Incandescent light bulb: about 2%.
- Compact fluorescent lamp: about 7% to 9%.
- White Light-emitting diode (LED) about 4% to 10%.
Discussion
High efficiency is obviously desirable 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
- Maximum Power Theorem
- Negawatt power
- Battery
- List of electronics topics
- Electronic amplifier
- Thermal efficiency
- Mechanical efficiency
