From InsideSources:

“Since electricity cannot be stored on a large scale, a functioning grid requires that there be enough capacity to meet demand at all times. This means that for most of the year, a significant amount of generating capacity has to be kept in reserve, only to be used during times of peak demand (which in Texas is in the summer). How to ensure that there will be enough capacity has been a matter of much consternation and controversy,” says Josiah Neeley, director of energy policy at the R Street Institute.