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 It presents a review and comparison of published and unpublished data on this kind of consumption. The paper presents results from the TURBOMATCH engine simulation model, calibrated to real world engine data. A generic equation is derived for the calculation of fuel consumption due to shaft power extraction. Main result is the shaft power factor k_P 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 %.