© ebm-papst

The formula for the energy efficiency

If you want to compare the energy efficiency η of various fans, you should consider the entire fan, i.e. the fan system, and not just its individual parts.


Anyone wanting to purchase a fan system asks themselves two questions: 1. Which fan will be up to the task? 2. Which fan will be up to the task with the greatest efficiency? The second question is about finding a fan that will provide the best efficiency (η).

Uwe Sigloch, Director of Product Management at ebm-papst in Mulfingen (Photo | ebm-papst)

In essence, a fan system consists of three main components: the motor, control electronics, and impeller. The efficiency of each component can be described by the respective individual efficiency level. It is usually specified in terms of the optimum operating range (i.e. ηmax) by the manufacturers of these individual components. If the fan is assembled from the individual components mentioned, the individual efficiency levels are normally multiplied and documented. However, this can only be a theoretical overall efficiency that cannot be achieved later in operation. Why is that?

The efficiency describes the ratio of effort to benefit. In the case of the fan, the benefit is that it moves a volume of air against a resistance (pressure), i.e. the air performance in watts. This is calculated by multiplying the air flow (in cubic meters per second) by the pressure (in Pascal). The effort is the electrical power consumption, also specified in watts. The result is always <1, as energy is always lost. Now you have a value for the level of efficiency, for example η = 0.8. This would appear to be an objective value that can be used to compare the efficiency levels of different fan systems. Unfortunately, this is not always the case.

The crucial factor is how η came about. The question must then be: has the efficiency of the fan system been calculated or measured? There is a theoretical optimum efficiency level, and the efficiency level resulting from actual operation. The optimum η is always higher (i.e. better) than the measured value — sometimes by 20 percentage points! That is why many manufacturers prefer to specify the optimum η.

Measure the fan system as a complete unit

But this is not useful for the application, as it cannot be assumed that all three components will run at their optimum efficiency at the desired operating point. The deviations are often considerable. A fan system has a separate overall level of efficiency in each operating state, which is very difficult to calculate if using the individual efficiency levels of the components. It is better to measure the fan system as a complete unit.

When specifying the individual efficiency level of the impeller, there is also the additional particularity that, in the calculation, often total pressure increase is used, i.e. the sum of static and dynamic pressure. However, only the static pressure is relevant for a ventilation system. In this way, ­ηmaximpeller is often inadmissably calculated to appear better than it actually is.

At ebm-papst, we follow the Wire2Air approach. We only state actual, measured efficiency levels. In any case, we recommend attaching less importance to the efficiency level. In practice, it is much more informative to compare the expected power consumption for a specific air-movement task. 

Required fields: Comment, Name & Mail (Mail will not be published). Please also take note of our Privacy protection.