A system incorporating large amounts of intermittent renewable generation will have greater flexibility needs, but may not cost more. A central finding of this report is that in as little as two decades a system built from scratch, based on renewable energy with investment to support the additional flexibility needs could cost less per unit of energy delivered than a new gas system, even if all of the existing flexibility resources were suddenly unavailable. Employing flexibility inherent in existing hydro-electric plants, demand management, and sparing use of existing gas generation could reduce the additional flexibility costs by half or more, further improving the cost competitiveness of the renewables based system.