Four Korean ESS materials, parts, and equipment companies are simultaneously establishing production sites in the United States. Seojin System began final assembly of battery enclosures for Samsung SDI America in Indiana in July 2026; Shinsung ST is preparing liquid-cooled thermal management parts in Kentucky, Hanjoong NCS liquid-cooled cooling systems in Indiana, and Sebang Global Battery an ESS business base through its U.S. subsidiary. This article reads this trend not as “component-by-component localization” but through a system variable: “the number of supply interfaces connected in series to the local assembly line.” In an assumed scenario, even if each supplier’s on-time supply probability is 95 %, increasing the number of serial suppliers…