We present numerical investigations on the power-based efficiency of vanadium redox flow battery (VRFB). A three-dimensional numerical model is developed to capture the complexities of electrochemical reactions and fluid dynamics when considering different serpentine channel sizes and electrolyte flow rates. It is shown that the reduced channel size and increased electrolyte flow rate improve the electrochemical performance of the VRFB due to the enhanced distribution of molar centration at the electrodes. Nonetheless, the channel size reduction and increased electrolyte flow rate also increases pressure drop between inlet and outlet of the serpentine channels for negative and positive sides. In this, we calculate the power-based efficiency by considering the generated power of VRFB and power loss due to overpotentials, ohmic loss, and required pump power. The maximum power-based efficiency of 96.6% is calculated with the channel size of 1.9 mm at 60 mL min -1, while it is 95.5% with 9.6 mm in channel size at 100 mL min(-1). The proposed numerical approach can be useful to determine the channel size with optimized electrolyte flow rate and maximum VRFB efficiency. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.