Electric power systems must maintain continuous balance between electricity production and electricity consumption. If a generation source on the system fails, the power system must be prepared to react. But it doesn't use one-to-one backup generation to respond.
To create balance between electricity production and consumption, electric power system operators call on a stable of controllable generators to "follow," or respond to, changes in total system demand, or load. They don't follow a single generator or a load source. Therefore, when variable generation such as solar and wind energy is added to a system, the variability in overall load is still the operating target for the system operator.
NREL is addressing system balancing issues on the transmission system through its work on: