This paper looks at fuel consumption due to shaft power off-takes from the engine and the related increase in the aircraft’s fuel consumption. It presents a review and comparison of published and unpublished data on this kind of consumption. Insight is given into the effects caused by off-takes, looking at phenomena inside the engine when shaft power is extracted. The paper presents results from the TURBOMATCH engine simulation model, calibrated to real world engine data. Generic equations are derived for the calculation of fuel consumption due to shaft power extraction and values are presented for different flight altitudes and Mach numbers. Main result is the shaft power factor kP found to be in the order of 0.002 N/W for a typical cruise flight. This yields an amazingly high efficiency for power generation by shaft power extraction from a turbo fan engine of more than 70 %.