Optimal transmission expansion minimally reduces decarbonization costs of U.S. electricity by

The relative savings of expanded transmission are greater in the high-renewable zero-emissions system than in the least-direct-cost system with about one-third the amount of wind and solar. [...] The overall cost of decarbonization, defined as the difference between the average marginal cost of a zero-emissions system and a least-direct-cost system, is 25.2–27.4 $/MWh, or 68–73% of the least- direct-cost system, depending on the transmission scenario. [...] The main effect of transmission is to slightly increase the frequency of hours in the $30 to $80 and $100 and $200 range. [...] The length of time that energy may be stored is typically limited to the time series, which means that the amount of energy storage at the beginning of the time series is constrained to equal the ending amount at the end of the time series. [...] Panels a and b show the least- direct-cost systems under existing and optimized transmission (LE and LO), and panels c and d show the zero-emissions systems under existing and optimized transmission (ZE and ZO).