Passing clouds and wind gusts can create unacceptable rapid voltage/power variations in power networks. Simulation results using a real Australian distribution feeder with real load demand and PV output profile show that with a high penetration of PV the voltage variations can increase beyond the allowable limit given by the standards. Similarly, wind gusts can create wind power output variations. This paper addresses the rapid voltage/power variations caused by solar or wind power outputs and presents a control strategy using the energy buffer in energy storage for their impact mitigation. By controlling the discharging/charging operation of the energy storage based on the available energy buffer in the storage unit, not only the rate of power output variations can be maintained at the desired level, but also the voltage variations can be controlled within acceptable limits. An experimental setup, consisting of a PV emulator, inverter, and grid simulator, demonstrates the potential threat of unacceptable voltage variations. The proposed mitigation strategy using energy buffer in the storage for smoothing the power outputs of solar/wind is tested and validated through simulation. Simulation results show that with the proposed control, the fast variations of the voltage/power variations instigated by solar/wind can be maintained within the acceptable limits.