Lawrence Berkeley National Laboratory

There is growing interest in leveraging interregional transmission as a resource adequacy (RA) asset as the U.S. electric grid faces challenges from extreme weather, load growth, and a changing generation mix. However, empirical evidence of how transmission functions during periods of system stress remains limited. This paper addresses that gap by analyzing nine years of historical peak demand patterns and interregional power flows across 18 continental U.S. transmission planning regions from 2016–2024. We identify RA-critical periods based on peak net load and assess when regions experience these peaks at simultaneous or opposite times. We then analyze actual power flows during these high-stress periods to evaluate how effectively interregional transmission was used to support stressed regions. Our results show substantial geographic variation in the theoretical potential for transmission to provide RA support. Furthermore, realization of this potential is often poor: approximately half of examined region pairs show transmission flows that support the high-need region 50%–60% of the time during non-coincident peaks, barely better than random. Several pairs exhibit counter-supportive flow patterns, and benefits are often asymmetric between neighboring regions. These findings suggest that operational, market, institutional, or other barriers are constraining the reliability value of interregional transmission.