The US electrical grid has insufficient storage capabilities. Thus, power must always be available from generating plants when needed. In total, the US has enough power to meet most any demand. However, getting power from one grid to another can be problematic.
The Texas ERCOT grid was designed to be self-contained and not need outside sources of power, a laudable goal indeed, but it may have backfired a bit on them . ERCOT is projected to be below target capacity this summer. But, due to its design, it is difficult to import electricity into ERCOT quickly. This is a good example of the problems faced by the grid. (And before someone gets snarky about Texas, it has vastly more installed wind power than any other state.)