Forecasting the Future compares the prospects for future decarbonized electric grids principally dependent on nuclear and on renewables, respectively. Grids of near 100% nuclear are achievable. Although, net 100% wind and solar generation can be reached, intermittency dictates that at any time either too much or too little power will be generated, with energy storage insufficient to redistribute the electricity to when it is needed. The chapter concludes that dark doldrums—extended periods of overcast skies and calm winds—are the Achilles heel of grids heavily dependent on wind and solar energy, making a major role for nuclear energy inevitable.