The Hidden Cost of Going Renewable: Losing Our Rotational Inertia

As renewable generation has increased, one underappreciated thing we have lost is momentum, in the literal sense.

Traditional fossil fuel plants relied on massive rotating machines such as turbines and generator shafts that stored large amounts of kinetic energy. This spinning mass acted as an instantaneous stabilizer for the grid. When a sudden change in load or generation occurred, those rotors resisted the change and absorbed the shock, giving grid controls precious milliseconds to respond. It was an elegant, physics-based form of stability that came for free with synchronous generation.

As we replace these synchronous machines with inverter-based resources like solar, wind, and battery storage, that stabilizing momentum disappears. The physical inertia that once maintained grid frequency and power quality now has to be recreated artificially. Modern “grid-forming” inverters can emulate inertia through power electronics, but doing so requires precise coordination and active control.

This shortfall has been recognized by grid operators and is now reflected in evolving market structures. Frequency regulation and fast frequency response (FFR) programs in markets like CAISO, ERCOT, and PJM are beginning to compensate resources that can inject power within fractions of a second. Similarly, at the behind-the-meter level, the decline in local system stiffness has created opportunities for storage systems and smart inverters to provide voltage and power-quality support that traditional rotating machinery once supplied inherently.

As the grid becomes cleaner, it also becomes less inherently stable. Rebuilding this stability through storage, control systems, and properly designed ancillary service markets will be one of the defining technical challenges of the energy transition.