With the continuous increase in distributed energy resources that are being integrated into the utility grid, it becomes necessary for inverters to estimate the grid impedance online to be used for several applications. Despite the fact that online grid impedance estimation techniques using grid-connected inverters gained more attention recently, no comprehensive study has been evaluating the performance of different estimation techniques under several grid conditions. This study proposes an extensive comparison to assess the accuracy of two online frequency-based grid impedance estimation techniques at the fundamental frequency. These two techniques are based on the 75 Hz frequency injection technique and pseudo-random binary sequence injection, respectively. Results from MATLAB/Simulink and PLECS RT Box present the advantages and disadvantages of these estimation techniques and the required trade-offs. This comparison study analyses the effects of disturbance magnitude and its injection time, the variation of grid impedance and non-ideal grid voltage. Furthermore, solutions are proposed to enhance the estimation accuracy of both techniques for non-ideal grid voltage containing inter-harmonics and sub-harmonics. The study concludes with recommendations regarding the best use of these two frequency-based grid impedance estimation techniques to suit a particular low voltage grid scenario.