In this Insight, we describe the many challenges to replacing ERCOT's remaining coal plants with renewables alone. Coal plants reside at important locations on the grid, while most existing and planned renewables are far from load centers. Additionally, renewables have temporal patterns not properly accounted for by ERCOT’s current reliability calculations. We will demonstrate that at high levels of intermittent renewables, a new definition of peak demand and renewable reliability is required. Because of recent excitement around 100% renewable energy goals, we also briefly look at the challenges of ERCOT meeting 100% of annual demand with only wind and solar resources. We conclude that large amounts of energy storage (i.e., batteries) would be required to enable such high renewable deployment.