Aqueous organic redox-flow batteries are an emerging technological solution in the field of grid-scale energy storage, owing to their long lifetime, safety, chemical flexibility, potential for low cost and environmental friendliness. In this work we present a physics-based dimensionality reduced model for the performance prediction of aqueous organic redox flow batteries. The model allows for fast evaluations of the cell voltage and power density, which are expressed explicitly in terms of the electric current density and state of charge. The model takes into consideration important phenomena, such as the activation and concentration overpotentials in the electrodes as well as the non-negligible electro-osmotic drag of water through the membrane. A sensitivity analysis of the model indicates the influence of various model parameters at different current densities on the predicted cell voltage. In this work we found the formal potentials and ohmic cell resistance to be the most critical parameters for performance prediction. Experimental parameterization and validation on cycling and polarization experiments revealed good agreement with the experiments within a predicted range of validity due to the different simplifying assumptions.