Providing peaking capacity could be a significant U.S. market for energy storage. Of particular focus are batteries with 4-h duration due to rules in several regions along with these batteries' potential to achieve life-cycle cost parity with combustion turbines compared to longer-duration batteries. However, whether 4-h energy storage can provide peak capacity depends largely on the shape of electricity demand. Under historical grid conditions, beyond about 28 GW nationally the ability of 4-h batteries to provide peak capacity begins to fall. We find that the addition of renewable generation can significantly increase storage's potential by changing the shape of net demand patterns: for example, beyond about 10% penetration of solar photovoltaics, the national practical potential for 4-h storage to provide peak capacity doubles. The impact of wind generation is less clear and likely requires more detailed study considering the exchange of wind power across multiple regions. (C) 2019 Elsevier Ltd. All rights reserved.