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This paper highlights the compromises between good steady-state accuracy and the speed of convergence in Perturb and Observe (P&O) maximum power point tracking (MPPT) algorithm. Three fixed step-sizes and a variable step-size have been defined as the perturbation step-sizes and the effect of applying different step-sizes for P&O algorithm are discussed. The perturbation step-sizes have been simulated using Matlab/Simulink and the maximum power tracking efficiency for each step-size is analysed. These step-sizes have been experimentally tested using a PV illumination test rig to emulate rapid changes in shadow effect on a PV panel. The fixed step-sizes are tested using a direct duty-cycle control boost converter while the variable step-size is examined by applying current-mode controlled boost converter. It is concluded that the application of fixed perturbation step-size has a limitation in performing MPPT while a variable step-size is necessary to balance the competing aims of speed and accuracy.