A method for sizing battery energy storage (BES) systems for use in mitigating voltage flicker caused by solar intermittency in photovoltaic generation was developed. The method creates a "design day" from existing solar data and designs the power and energy requirements for a BES system that can help a photovoltaic facility mitigate flicker caused by solar activity associated with the design day. An economic analysis of lead-acid and lithium-ion options for the BES was also developed. The method was then applied to a proposed photovoltaic project in the Midwestern United States.