A kinetic energy storage device, a pumpjack inertia capacitor (PIC), for use with an oil well pumpjack, the PIC includes a primary shaft assembly to transfer power and torque between a prime mover of the oil well pumpjack, a flywheel assembly having one or more flywheels, and an output assembly, the one or more flywheels absorb and then transfer torque to an oil well pumping unit to enable the oil well pumping unit to operate at a near constant speed; and the one or more flywheels enable the prime mover to operate at a near constant speed and a near constant power.