The required generating capacity and its breakdown are very different in each of Rocky Mountain Institute’s four scenarios for the future U.S. electricity system (detailed here).
Maintain represents “business-as-usual,” with little smart grid deployment, bulk energy storage, or demand response capability. Most U.S. electricity demand is met with increased generation from natural gas and coal-fired power plants. The 2050 system requires 1,470 GW of nameplate capacity—45% fueled by natural gas and 22% by coal. The gas-fired plants are used more for peaking than the coal, reversing their contributions to annual electricity generation (27% gas and 43% coal).
The Migrate case transitions to a grid relying heavily on nuclear power (18% of capacity, 36% of generation) and gasified coal (IGCC) coal equipped with carbon capture and sequestration (17% of capacity, 30% of generation). Carbon dioxide emissions are significantly reduced, but the system also faces many cost, financial, and technology risks.
In Renew, increased energy efficiency flattens demand growth, and 81% of U.S. electricity comes from utility-scale renewables. Even though the annual electricity demand is lower than in the Maintain and Migrate cases, the required nameplate capacity is higher because of some renewables’ generally lower capacity factors.
Finally, the Transform case radically shifts grid design and operation. Mostly distributed renewables supply 90% of grid electricity, with all remaining generation coming from natural gas combined-cycle plants. As with the Renew case, increased reliance on low- capacity-factor renewables means more gross capacity is needed even though efficiency has reduced 2050 demand by 25% from the “business-as-usual" case.