Using simulations performed on the ERCOT grid in Texas (United States), the paper examines changes to the electric power system that are required to incorporate high penetration of variable wind and solar electricity generation. ERCOT is a transmission-constrained grid in which different mixes of wind, solar photovoltaics and concentrating solar power meet up to 80% of electric demand. The study shows that a highly flexible system—with must-run baseload generators virtually eliminated—allows for penetrations of up to about 50% variable generation with curtailment rates of less than 10%. It also shows that using a combination of load shifting and storage equal to about one day of average demand, the penetration levels can reach up to 80% with curtailment of less than 10%. The paper highlights the critical role of deploying flexible generation on multiple time scales and the importance of energy storage in achieving penetrations of variable generation beyond 50%.