The increased number of renewable power plants pose threat to system balance. Their intermittent nature makes it very difficult to predict power output thus either additional reserve power plants are required or new storage and control technologies incorporated. Traditional spinning reserve cannot fully compensate sudden changes in renewable energy power plants. A feasible approach to balance the variation in power and voltage (within seconds) is using new storage technologies such as flow batteries. This paper covers the use of flow batteries and hydro power plant to help wind power plants reach near perfect balance i.e. make the total power output as predicted. It investigates the possibility of such technology to take part in the balance of the Lithuanian power system. A dynamic model of flow battery is demonstrated that evaluates the main parameters of it - power, energy, reaction time and efficiency. A range of hydro power plant reaction times are tested in order to find the required battery size. The research and study cases show that power and energy of a reasonable size flow battery is sufficient to correct the load and wind power imbalance.