Integrating large amounts of variable generation (VG) resources such as wind and solar into a region's power grid without causing significant VG curtailment will likely require increased system flexibility via changing grid operation and deploying enabling technologies such as energy storage. This article analyzes the storage duration required to reduce VG curtailment under high-VG scenarios. The three analysis scenarios assume VG provides 55% of the electricity demand in the largely isolated Electricity Reliability Council of Texas grid system in 2050, with three different proportions of wind and solar generation. Across the three scenarios, 11%-16% of VG energy is curtailed without storage due to system-generation constraints. When 8.5 GW of storage capacity with 411 of duration are added, curtailment is reduced to 8%-10% of VG. Additional storage duration further reduces curtailment, but with rapidly diminishing returns. At least half the potential avoided-curtailment benefits are realized with 8 h of storage, and the first 4 h provide the largest benefit. At VG penetrations up to 55%, there appears to be little incremental benefit in deploying very-long-duration or seasonal storage. (C) 2018 Elsevier Ltd. All rights reserved.