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Wednesday, 5 February 2014

Efficiency Vs Efficacy in techno speak

Often, the words efficiency and efficacy are used confusingly and interchangeably. This regularly leads to an ambiguous understanding of what is being discussed. In this post I hope to discuss the words effective/efficient their terminology and how they are used in a technical environment.

Efficiency is an objective word most often misused in a subjective way. The word efficiency can also be used to disguise poor information. Its use has the ability to trigger an emotional response, especially with the spectre of impending doom looming large.


What do you think of the following sentences?:

  1. This shower-head is more efficient, as it restricts water flow.

  2. This radiator is more efficient due to additional radiating fins.

  3. These pipes are more efficient at delivering water.

  4. This tap aerator adds air, making your tap flow more efficient.

  5. The toilet flush is not efficient if it uses 12 litres of water.

  6. This heater is more efficient due to its special heating element.

Subjectively:  you get notion that random product, "x" is better than random product "y" and it has something to do with using less but, actually, you are not quite sure what the amount of, "less" you are using.

Objectively: you understand nothing at all!  All 6 statements convey no information just a vague notion.  You are guessing what the person is trying to indicate but the actual statement means nothing.  If it cannot be expressed as a number then the word efficient is probably the wrong word.  In the example sentences the word efficient should be replaced with vague terms such as economical or effective.


Efficiency has become one of those watch words, which gives an illusion of authority but it is often cloaked by a veil of confusion and misdirection.  Culprits rely in the fact that you don't know what the word efficiency actually means and the reliance that you have assumed, they must be in possession of the facts. Regular culprits know its use is associated with academic and technical competence, which should only be used by the most qualified individuals. Consequently, this misguided assumption deters any challenge to them.  If ever you see or hear the word, "efficient," you should ask, "how efficient?" If you are met with no answer or a blank look, just walk away.

You may hear an argument that it is used colloquially. If that is true, then why do only sales people seem to use it, "colloquially," to rubbish other products?

So what is the definition of efficiency?

Essentially, it is a non-scalar relative term meaning the amount of "x" you get out compared to the amount of "x" you put in.

  • -  Non-scalar - meaning it has no scale.
  • -  Relative - meaning it is related, usually something in to something out.
  • -  "x" - meaning whatever you're trying to measure.


It's a ratio usually expressed as a percentage as follows:

Efficiency = {( amount of "x" out ) / ( amount of "x" in )} then multiplied by 100% if expressed as a percentage.

  • -  Where "x" is the quantifiable something you are expressing.
  • -  Quantifiable meaning it can be expresses as a number.

So to be clear, if you are measuring something in to something "different" out, then it is "not" a statement of efficiency!



Efficacy, is an under used word and usually incorrectly supplanted by the word efficiency. Essentially, efficacy is how effective, "x" is at causing, "y," and is generally defined, "ad hoc" (i.e. on the spot).

An example of the efficacy is the effectiveness a tungsten filament bulb creates light.  According to some sources the efficacy is 10 lumens per watt often misunderstood as 10% efficient. In actual fact a tungsten filament bulb is only 5% efficient according to Energy Saving Trust.  i.e. only 5% of the power is converted to usable light and the remaining 95% in wasted as unwanted heat.  This says nothing about the quality or colour of the light just the conversion ratio.


Efficiency has become one of those watch words which gives the user the illusion of authority but, sinisterly, it's often cloaked by the veil of confusion. In the world of the impassioned eco warrior or the general population who make a real effort to take care in the way they consume the worlds resources, the incorrect use of the word efficiency is forgiveable.  I can bite my lip with the casual and the mathematically naive but when I regularly hear and read it misused, by the mainstream media and marketers, I feel disheartened.

  • A classic example is the following: 

    This electric radiator is more efficient than this other inferior radiator.  The translation being:  If you buy my unnecessary and expensive radiator we will save the planet together. Otherwise you are a planet hater! Essentially emotional blackmail.

As energy can neither be created nor destroyed, how would that be possible? If you put in 100watts of electrical energy you will get the equivalent of 100watts of heat out.

To be pedantic 100watts is 100 joules per second. So, in 10 seconds that will be 1000 joules of energy. So if the heater was on for only 10 seconds, you will get the 1000 joules out. It may take longer than 10 seconds though.

If there is a difference between the rate of energy "in" and energy "out," there will be a rise in temperature.  The element can only hold so much energy, which is reflected in its temperature.  If it goes over temperature it will switch off, until it has radiated sufficient heat energy to allow it to switch back on again.  Either that or break! By definition it can only be 100% efficient. It might be ineffective at heating a room, especially if the rate of cooling in the room is greater than the speed heat can be radiated.  None the less the heater is 100% efficient. (Even fan blown air or light, from an indicator, eventually becomes heat)

Energy or work is measured in Joules. The rate at which work happens is (joules/second) and called power. Power is measured in Watts.

                        Watts = Rate of work = joules/second

Unless heat energy is being gained from a second source such as an air or ground source heat pump, it cannot be more "efficient," just more "effective."

So try and be more observant of the confusion in these two words and who is confusing them because there is probably a why!





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