Efficiency and the Efficiency Factor

The figure illustrates that increased efficiency is more than just increasing the efficiency factor. The efficiency factor of the compressor (right hand side) is increased. However - far more important is the increased efficiency by the better insulation of the cabinet. That significantly changes the quotient

(cooled space)/"cooling energy".

"Cooling energy" is not a physical quantity in a strict sense: it is thermal energy to be removed.

More about the 11th Conference on Passive Houses 2007 April 13th/15th 2007: www.passivhaustagung.de.

Another example: Efficiency in transportation. There are vehicles, consuming different amounts of fuel for the same service (carrying 1 person for a distance of 100 km):

12 Ltrs/100 pers.km (an old car)
     (in the US maybe even a new one, sorry fellows:
      you should really go do something to change that)

  7 Ltrs/100 pers.km (an average new car in Germany; not very good as well)
  3 Ltrs/100 pers.km (a quite efficient new car; really, one shouldn't build less      efficient ones for everyday use)

  1 Liter/100 pers.km (prototype-VW-car or "hypercar" or "loremo")
  0 Liter/100 pers.km (a vehicle moving frictionsless using a path on a      brachistochrone)

Path on a brachistochron? See the figure. That was already known by Galilei: It is the fastest path from A to B without energy requirement in a homogeneous gravitational field. Want to know more about this? Download the article

Wissensch_Kultur_Passivhaus.pdf (164 kB, German!),

Science, culture, passive house. Not only brachistochron paths are explained there, but the meaning of "exergy" and "anergy" and why in daily life Galilei is still not established againts Aristotle completely.

Conclusion: To guaranty the most services, one does not need any energy; or, at least, it will be sufficient to use an extremely small amount - if only losses are reduced consequently.

Increased energy efficiency therefore can substitute energy, which is required up to now, almost completely. "Energy efficiency" is almost like a new energy source - but only almost, because using energy efficiency is far better. Energy efficiency is clean, inexhaustible and free of costs during use. Energy efficiency can be made available everywhere. And: Energy efficiency can be produced by ourselves in Europe (and at any other place, too), it is integrated in smart products from the beginning - for the advantage of the users (lower running costs and increased comfort), for the advantage of the manufacturers (higher quality and therefore higher added value), for the advantage of the national economy (employment) and for the advantage of the of the environment (mitigation of global warming).

There are only winners - even the suppliers of conventional energy are winners, looking at an adequate time horizon: E.g. the supply of oil will last for more decades and with lower risks, if the efficiency of using energy is increased.


It is often mistakenly thought that "efficiency" is the synonomous with the efficiency factor. That is only true, if both quantities on the right hand side of the definition

efficiency = benefit / effort

have the dimension of "energy" or "power". But very often the benefit has nothing to do with energy. For example, the benefit can be a mileage (unit "miles"). The effort e.g. is the fuel needed (unit "gallons") and the efficiency of this service will be given by

mileage / (fuel consumption)

with the unit "miles/gallon". In Europe the inverse value

effort coefficient = 1/efficiency

is established: the specific fuel requirement in (liter fuel)/(100 km). Such values characterising the efficiency are not "efficiency factors" - they are values with a dimension. It is not possible to introduce an efficiency factor instead of these quantities. Looking at most services produced by the use of energy you will find, that they do not have the dimension of energy. And, there is no "minimum energy demanded" for those services on physical reasons. The belief in the contrary is widely spread, too - but not valid. The "minimum energy" in most cases is zero (or, in same cases, an extremely small value near to zero).

That sound like an academic question? No, not at all. This is a key insight.

Let us look at an example:
The efficiency factors of
heating boilers can not be increased to more than 100% (law of energy conservation) and contemporary values already are in the range of 90%. But: the efficiency of the total service "heating" (to be measured by the area heated with a given amount of fuel) can be increased almost without any limit - by better insulation and heat recovery. If you only look at the efficiency factor, there will not be a noteworthy potential for energy saving for heating. But if we have a broader look at the service and realise, that the so called "heating demand" can be reduced to values near to zero (by insulation and heat recovery), we will understand that there is a huge potential for better efficiency. This has been demonstrated by the multitude of Passive Houses which have already been built and are occupied.

But it is not only true for heating, it is quite similar with many other applications of energy: At the end of the supply chain, the final use is made out of the energy supplied. At this end there are the major potentials for an increased efficiency: And it is not a few percents which can be saved; it is the major part - in most cases.


  • Heat storage can have an increased insulation; this will reduce the energy needed to hold the temperature level (the principle of the thermos-flask).
  • The same holds for heat distribution pipes (especially domestic hot water and circulation pipes).
  • Heat can be recovered from hot waste water.
  • Insulation, efficient against heat losses, is efficient against "cold losses" as well (see the example on left hand side).
  • Even looking at industrial processes energy can be used far more efficiently, e.g. by heat regeneration. An example is a "counter direction production line": finished baked goods (hot!) are transported on a production line, while the cold, unbaked parts come along in counterflow at another line.
  • Increased efficiency in the use of materials and recycling of energy intensive materials will reduce the energy requirement, too.

A thorough analysis of all services produced with the input of energy reveals: From a physical point of view energy is used predominantly to maintain an unstable situation. But these can be regularly transformed by smart measures into near equilibrium conditions. To realise this, only a very small amount of energy is required. Examples:

  • Comfortable "heated" living space:
    unstable situation: "higher indoor temperature" compared to "cold outdoor environment".
    Smart measure: reduction of heat losses.
    Practise: Passive House.
  • Comfortable "air conditioned" living space:
    unstable situation: "cool indoor temperature / lower humidity" compared to "hot environment / high humidity".
    Smart measure: reduction of heat gains, heat recovery and humidity recovery.
    Practise: Passive House with a compact ventilation system.
  • Cooling chain:
    unstable situation: "cool temperature in the cooling chamber" compared to the "hot environment".
    Smart measure: insulation.
    Practise: vacuum insulated cooling chambers.
  • Transportation:
    unstable situation: "not frictionless movement".
    Smart measure: reduction of friction; recovery of braking energy.
    Practise: Hypercar.

Back to the page on efficiency.

Download article on energy efficiency (492 kB) (pdf, German).

Author: Dr. Wolfgang Feist

updated: 2006-09-23 WF - thanks to Dylan Lamar for proof reading
© Passive House Institute; unchanged copy is permitted, please give reference to this page