Electrical efficiency

See also: energy efficiency  and Energy conversion efficiency

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), typically denoted by the Greek letter small Eta (η).

\mathrm{Efficiency}=\frac{\mathrm{Useful\ power\ output}}{\mathrm{Total\ power\ input}}

Contents

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" makes sense only in reference to the wanted effect. A light bulb, for example, 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 (e.g., a loudspeaker), while drawing 20 watts of power from a power source is 50% efficient. (10/20 × 100% = 50%)

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. This is valid only for non-reactive source and load impedances.

Efficiency of light bulbs

For more details on this topic, see Luminous efficacy.

Discussion

High efficiency is useful in the design of systems that can operate from batteries. Inefficiency has a cost (either the cost 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 (although noise and other mechanical vibrations involve at least theoretically separate and generally negligible inefficiencies), and that heat must be removed from the system if it is to remain within its operating temperature range. If the system is in a climate-controlled environment (e.g., a home or office), heat generated may reduce heating costs or increase ventilation and air conditioning costs.

Implications in discussion of power generating equipment

There are two different definitions of caloric value, and the difference between the HCV and LCV definitions causes much confusion when quoters fail to state the convention being used[1] as there is typically a 10% difference between the two methods for a power plant on natural gas.

See also

Other measures of efficiency:

Efficiency improvement initiatives:

References

  1. ^ Claverton Energy Research Group. "The difference between LCV and HCV (or Lower and Higher Heating Value, or Net and Gross) is clearly understood by all energy engineers. There is no ‘right’ or ‘wrong’ definition."

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