A power unit connected to a power distribution bus operates in one of several modes to either store excess electrical energy from the power distribution or supply electrical energy to the power distribution bus to account for a detected demand/need. The power unit includes turbomachinery having components connected via a shaft, a generator connected to convert rotational energy associated with the turbomachinery to electrical energy for distribution on the bus, and a motor connected to convert electrical energy distributed by the bus to motive energy used to accelerate the turbomachinery components. A power controller monitors the voltage on the distribution bus. In response to excess voltage on the distribution bus, the power controller connects the motor to the bus to cause the excess electrical energy to be converted to motive energy that is used to accelerate the turbomachinery. In response to a voltage shortage on the distribution bus, the power controller connects the generator to the bus to convert rotational energy stored by the turbomachinery to electrical energy that is supplied to the bus.