At the end of 2010， the United States had the second highest installed wind power capacity in the world after China. With supportive policies and considerable wind resources， some have predicted that the United States could supply 40% of its electric energy from wind in the foreseeable future. This paper presents challenges of large-scale wind power penetration， focusing on its operating impacts on power systems in the context of the United States. It demonstrates that key challenges come from variability in wind power output， which translates into variability in the net load（load minus wind generation）. The paper then presents various grid-scale storage options that are already in practice or in demonstration to mitigate the adverse impacts of wind power on the electric power grid. The impact of grid-scale storage on the power plants' utilization factors is also discussed.