Flywheel Energy Storage Calculator

A flywheel stores energy as rotational kinetic energy. The ideal stored energy equals one half of the moment of inertia multiplied by angular speed squared. This calculator accepts moment of inertia in kilogram square meters and speed in revolutions per minute. It converts rotational speed to radians per second, evaluates the kinetic-energy equation, and presents the result in joules, kilojoules, and watt-hours. The conversions make it easier to compare a mechanical rotor with electrical or thermal storage quantities. Moment of inertia describes how mass is distributed around the axis, not merely how much the flywheel weighs. Moving the same mass farther from the axis increases inertia and therefore stored energy at a given speed. A thin ring has a larger inertia than a solid disk of equal mass and outer radius. Include every component rotating at the entered speed, such as the rotor, hub, shaft, and coupled hardware, or use a measured system inertia. Components connected through gearing must be reflected to a common shaft speed before their inertias are combined. Energy grows with the square of speed. Doubling speed stores four times as much energy for unchanged inertia, but it also raises material stress sharply. Real flywheel design is therefore limited by tensile strength, fatigue, balance, bearing loads, rotor dynamics, containment, vacuum losses, and maximum safe speed. The ideal calculation does not subtract bearing drag, windage, motor-generator loss, power-electronics loss, or energy that must remain in the rotor above its minimum operating speed. For an operating system, usable energy between two speeds is the difference between the kinetic energy at maximum and minimum speed. Power is separate from energy: a flywheel may hold a given number of watt-hours but its motor-generator, inverter, bearings, and thermal limits determine how quickly that energy can be charged or discharged. Use this calculator for conceptual design, lab exercises, machine coast-down estimates, and consistency checks. High-speed rotors can fail violently, so practical designs require qualified stress analysis, overspeed protection, precision balancing, guarded testing, and containment rated for the rotor's maximum credible energy.