Redox flow batteries (RFBs) connected to the power grid can absorb load variations which appear in several time scales. An RFB consists of a cell stack as the chemical reaction element and tanks as the storage element. Electrolytes are pumped and circulated between them. The cell stack can supply power as long as reactants are supplied to the stack. The slow velocity of the reactant flow to the cells restricts the output current, and the RFB can not supply the requested power under the current restriction. The reactant flow should be considered for confirming whether the RFB can supply the requested power at load variations. This paper proposes the method to predict the current restriction by the reactant flow through simulations. The method is based on a chemical reaction model. The model enables us to simulate the time evolution of the reactant concentration, which governs the electrical behavior and the reactant flow of the RFB. The current available from the reactant flow is introduced to estimate the current restriction by the slow velocity of the reactant flow. The proposed method predicts well the current restriction of the prototype RFB, which is integrated into a 300-A class discharging circuit.