Recently, small scale H-Darrieus vertical axis wind turbines (VAWTs) have got great attention of researchers and wind turbine manufacturers due to their application at roof-tops of residential buildings. They are generally preferred over horizontal axis wind turbines (HAWTs) in terms of lower levels of noise production, operation at low wind speeds and lower manufacturing costs. But the main problem with such wind turbines is their low efficiency, which is mainly due to the complex flow around the turbine blades. It is of great importance to first fully understand the nature of complex aerodynamics associated with such wind turbines. Present study analyzes the instantaneous tangential velocity (ITV) and its impact on the power performance of H-Darrieus wind turbine at five different tip speed ratios (TSRs) i.e. 1-5. Computational models based on large eddy simulations (LES) have been developed using commercial software SC/Tetra V12. Throughout the numerical simulation, it is found that ITV of single blade has a negative value (reverse rotation) for TSRs 1-3 when the azimuth position ranges from 80 degrees to 140 degrees whereas it has positive values during the remainder of the rotation for the mentioned TSRs. Furthermore, the magnitude of the negative ITV for a single blade was found to increase from TSR 1 to 3; the maximum value of the negative peak is equal to approximately -17.2 m/s while at the azimuth position of 110 degrees for blade number 1 at TSR 3. On the other hand, no negative value of ITV is found for any of the three blades during a complete rotation at relatively higher values of TSRs i.e. 4 and 5. The possible physical reasons behind this behavior have been explained through aerodynamic forces and 2D color contours of important parameters, such as vorticity and static pressure. Finally, a discussion has been included in the final section of this paper explaining how negative ITV affects the power performance of H-Darrieus wind turbines. The current CFD model has been compared and validated with experimental data.