Wind and solar energy are currently the most promising carbon-free sources of electrical energy. However, because of their unpredictable and intermittent nature, high penetration of solar and wind into the electrical grid will likely require energy storage in order to provide grid power balance. In this work we consider the storage requirements for 100% and nearly 100% wind and solar power, examining the effects of source diversity, geographical distribution of sources, overcapacity, and balancing power. As a case study we use load data from the Midcontinent Independent System Operator (MISO) energy market in the central region of the United States for the years 2007, 2008, 2009, and 2010. Solar and wind production is modeled using data from the National Renewable Energy Laboratory Solar and Wind Integration Data Sets. Our results show nearly an order of magnitude decrease in required energy storage capacity is achieved as the overcapacity is increased from 0 to 30% for optimized solar to wind ratios. Significant reductions also occur by allowing a balancing power of only a few percent. However, the corresponding storage discharge and balancing power capacities are a significant fraction of the solar and wind capacities with correspondingly low capacity factors.