Electrical 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).

Contents 1 Efficiency of typical electrical devices 2 Efficiency of devices at point of maximum power transfer 3 Efficiency of light bulbs 4 Discussion 5 Implications in discussion of power generating equipment 6 See also 7 References 8 External links

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%)

• Electric kettle: more than 90% (comparatively little heat energy is lost during the 2 to 3 minutes a kettle takes to boil water).
• A premium efficiency electric motor: more than 90% (see Main Articles: Premium efficiency and Copper in energy efficient motors).
• An electric fire is 100% efficient in terms of converting electrical energy into heat.

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.

Diagram of efficiency for various types of lamps

• Incandescent light bulb: about 2%.
• Compact fluorescent lamp: about 7%-9%.
• White light-emitting diode (LED) about 4%-18%.

Discussion

High efficiency is useful in the design of systems that can operate from batteries. Inefficiency requires weighing the cost of the required power supply) against the cost of attaining greater efficiency (through 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, like a home or office, heat generated may reduce heating costs or increase ventilation and air conditioning costs and are quick to run out.

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.