As you can see above, SFP can be expressed in units of pressure, since pressure is a measure of energy per m³ air. The relationship between SFP, fan pressure rise, and fan system efficiency is simply:
where:
*  is the overall efficiency of the driven fan system  [-]
*  is the rise in total pressure though the fan  [kPa]
In the case of an ideal lossless fan system (i.e. ) the SFP is exactly equal to the fan pressure rise (i.e. total pressure loss in the ventilation system). In reality the fan system efficiency is often in the range 0 to 60% (i.e. ); it is lowest for small fans or inefficient operating points (e.g. throttled flow or free-flow). The efficiency is a function of the total losses in the fan system, including aerodynamic losses in the fan, friction losses in the drive (e.g. belt), losses in the electric motor, and variable speed drive power electronics. For more insight into how to maximise energy efficiency and minimize noise in fan systems, see ref.1