Calculate energy stored in a spinning flywheel using E = ½Iω². Supports different geometries, RPM-energy tables, rim speed safety analysis, and energy density comparisons.
A flywheel stores energy in rotation, with stored energy given by E = ½Iω². The key inputs are the mass, the radius, and how that mass is distributed around the shaft. That is why a rim-weighted rotor stores more energy than a solid disk of the same mass.
Flywheels matter anywhere fast charge and fast discharge are useful: backup power, regenerative braking, grid balancing, and short-duration storage. The tradeoff is that higher RPM raises both stored energy and mechanical stress, so geometry and rim speed matter as much as mass.
This calculator compares common flywheel shapes, computes stored energy and rim speed, and shows how energy changes across RPM. It also helps you compare flywheel storage against battery-style energy density.
Flywheel design is easy to misread if you only look at mass or RPM. Energy depends on moment of inertia, so the geometry you choose can change the result as much as the speed does. This calculator keeps the shape math, unit conversions, and rim-speed check in one place.
Rotational Kinetic Energy: E = ½Iω² Angular Velocity: ω = 2πn/60 Moments of Inertia: Solid cylinder/disk: I = ½mr² Hollow cylinder: I = mr² Solid sphere: I = ⅖mr² Hollow sphere: I = ⅔mr² Rim Speed: v = ωr Where: I = moment of inertia (kg·m²) ω = angular velocity (rad/s) m = mass (kg), r = radius (m)
Result: 8,333 J (2.31 Wh)
A 15 kg solid cylinder flywheel with 0.15 m radius at 3000 RPM: I = ½ × 15 × 0.15² = 0.169 kg·m², ω = 2π × 3000/60 = 314.2 rad/s, E = ½ × 0.169 × 314.2² = 8,333 J ≈ 2.31 Wh.
A flywheel energy storage system uses a motor/generator to spin up a massive rotating disk. Energy is stored as rotational kinetic energy. To extract energy, the motor operates as a generator, converting kinetic energy back to electricity as the flywheel slows down.
Modern systems use composite rotors spinning at 20,000-60,000 RPM in vacuum enclosures with magnetic bearings to minimize friction. The motor/generator is typically built into the flywheel assembly for compact design.
Grid-scale flywheel plants provide frequency regulation services, smoothing the output of wind and solar farms. The Beacon Power Stephentown facility in New York uses 200 flywheels storing 5 MWh total. In transportation, Formula 1 cars use flywheel-based KERS (Kinetic Energy Recovery Systems). NASA has explored flywheels for spacecraft energy storage as an alternative to batteries, offering longer life and higher power density.
A spinning flywheel contains enormous energy concentrated in a small volume. If the flywheel fails mechanically, the release of this energy can be explosive. This is why flywheel systems require robust containment vessels, careful material selection, and continuous monitoring of vibration and temperature. High-speed composite flywheels have a safety advantage: when they fail, the rotor tends to disintegrate into small fibers rather than large fragments.
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Because E = ½Iω² and angular velocity is proportional to RPM. Doubling the speed quadruples the stored energy, which is why flywheels reward higher RPM so strongly.
The rim sees the highest stress because it moves fastest. As rim speed rises, centrifugal stress rises too, so the material limit is what eventually caps performance.
Flywheels usually store less energy per kilogram than lithium-ion batteries, but they can deliver and absorb power much faster and tolerate far more cycles.
Shapes that place more mass near the rim have the greatest moment of inertia for a given mass, so they store more energy at the same speed.
High-speed systems often use vacuum enclosures to reduce drag losses. Lower-speed flywheels can run in air, but they lose energy faster.
With low-loss bearings and a vacuum enclosure, a flywheel can hold useful energy for hours. The exact hold time depends on drag, bearing losses, and operating speed.