This study explores the feasibility of using the vortex impulse approach, based on experimentally generated velocity fields to estimate the energy harvesting performance of a sinusoidally flapping foil. Phase-resolved, two-component particle image velocimetry measurements are conducted in a low-speed wind tunnel to capture the flow field surrounding the flapping foil at reduced frequencies of k = fc/U-infinity = 0.06-0.16, pitching amplitude of theta(0) = 75 degrees and heaving amplitude of h(0)/c = 0.6. The model results show that for the conditions tested, a maximum energy harvesting efficiency of 25% is attained near k = 0.14, agreeing very well with published numerical and experimental results in both accuracy and general trend. The vortex impulse method identifies key contributions to the transient power production from both linear and angular momentum effects. The efficiency reduction at larger values of reduced frequencies is shown to be a result of the reduced power output from the angular momentum. Further, the impulse formulation is decomposed into contributions from positive and negative vorticity in the flow and is used to better understand the fluid dynamic mechanisms responsible for producing a peak in energy harvesting performance at k = 0.14. At the larger k values, there is a reduction of the advective time scales of the leading edge vortex (LEV) formation. Consequently, the LEV that is shed during the previous half cycle interacts with the foil at the current half cycle resulting in a large negative pitching power due to the reversed direction of the kinematic motion. This vortex capture process significantly decreases the total cycle averaged power output and energy harvesting efficiency. These results show the link between the kinematic motion and LEV time scales that affect the overall power production. (C) 2020 Elsevier Ltd. All rights reserved.