Giant magnetically levitated flywheels store electricity to keep the power grid on frequency.
AC frequency fluctuates from second to second as generators turn on and off and consumer demand varies.
When demand exceeds supply, the extra load slows down power plant turbines, thus depressing frequency. The turbines accelerate when supply exceeds demand.
Frequency regulators add or remove power to restore balance.
Beacon Power Corp operates more than 160 1150-kilogram magnetically levitated flywheel motor-generators which continuously accelerate and decelerate to balance electrical supply and demand to keep New York state's grid tuned to 60 hertz.