The Specific Fan Power (SFP) of a fan driven system is a measure of the efficiency at the duty point by which it can move a unit volume of air. It is not a constant, but varies with air flow and system pressure differential. Specifications for air handling equipment often stipulate an allowable limit for SFP, which is usually expressed in watts per litre per second (W/l/s). 

For combined supply and extract ventilation units with heat recovery, the SFP is calculated by the (total amount of electrical power supplied to fans, actuators and controls) divided by (the larger of supply or extract air flow rate, at design load).