Flywheel Energy Storage Calculator — Rotational Kinetic Energy
Find how much kinetic energy a flywheel stores and how much power it can deliver. Enter the mass, radius, rotational speed and rotor shape to get stored energy in joules, kilowatt-hours and the maximum discharge power.
kg
m
RPM
Flywheel geometry
s
Kinetic energy in the spinning flywheel: E = ½ · I · ω²
22.207 kJ
stored energy- 1
Moment of inertia
I = 0.5 × 10 kg × 0.3 m² = 0.45k = 0.5 for solid disk, 1.0 for thin-walled hollow cylinder. - 2
Angular velocity
ω = 2π × 3,000 ÷ 60 = 314.1593 - 3
Kinetic energy (J)
½ × 0.45 × 314.1593² = 22,206.61 - 4
Stored energy (kJ)
22,206.61 ÷ 1,000 = 22.207
How does this calculator work?
A flywheel stores E = ½Iω² joules, where I = k·m·r² (k = 0.5 solid disk, 1.0 hollow ring) and ω = 2π·RPM/60. Enter mass, radius and speed to get stored energy in kJ and Wh, plus the maximum discharge power for a given release time.
Formula
How this is calculated
A flywheel stores kinetic energy by spinning. The energy is E = ½Iω², where I is the moment of inertia in kg·m² and ω is angular velocity in radians per second (converted from RPM via ω = 2π × RPM / 60). The moment of inertia depends on how the mass is distributed: for a solid disk or cylinder I = ½mr², while a thin-walled hollow cylinder (all mass at the rim) gives I = mr². The k coefficient equals 0.5 for solid and 1.0 for hollow — hollow rotors store twice the energy for the same mass and radius at the same speed.
Real flywheel energy storage (FES) systems use composite rotors spinning at 20 000–50 000 RPM in near-vacuum to minimise air drag. A 10 kg solid steel disk of radius 0.3 m at 3 000 RPM stores roughly 13 kJ — enough to power a 100 W lamp for 2 minutes. Maximum discharge power is E divided by the discharge time, assuming the rotor slows from full speed to rest; real inverter and mechanical losses reduce usable output.
This calculator assumes a uniform-density rotor. It does not account for material tensile-strength limits (hoop stress scales with ρ·v²), bearing friction, motor-generator efficiency, or vacuum enclosure losses. Treat results as theoretical upper bounds.
Frequently asked questions
A hollow cylinder concentrates mass at the outer rim, maximising the moment of inertia for a given total mass and radius. The k coefficient is 1.0 for a thin-walled hollow ring vs 0.5 for a solid disk, so the hollow design stores exactly twice the energy at the same speed. Modern high-energy flywheels use annular composite rotors for this reason.
Material tensile strength. The centrifugal hoop stress at the rim is proportional to ρ·v² (density times rim speed squared). Structural steel tops out around 200 m/s rim speed; carbon-fibre composite rotors withstand 700–1 000 m/s, allowing far higher stored energy for the same rotor size.
Flywheels excel at power density and cycle life — millions of full charge/discharge cycles with negligible degradation. Lithium-ion batteries have higher energy density (Wh/kg). Flywheels are used for grid frequency regulation, UPS backup and regenerative braking where fast response and long cycle life matter more than total energy capacity.
Also known as
TG we-Calculate Editorial Team. (2026). Flywheel Energy Storage Calculator — Rotational Kinetic Energy [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/flywheel-energy-storage-calculator
TG we-Calculate Editorial Team. "Flywheel Energy Storage Calculator — Rotational Kinetic Energy." TG we-Calculate. 2026. https://we-calculate.com/calculator/flywheel-energy-storage-calculator.
TG we-Calculate Editorial Team, "Flywheel Energy Storage Calculator — Rotational Kinetic Energy," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/flywheel-energy-storage-calculator
@misc{wecalculate_flywheel_energy_storage_calculator, title = {Flywheel Energy Storage Calculator — Rotational Kinetic Energy}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/flywheel-energy-storage-calculator}}, year = {2026}, note = {TG we-Calculate} }
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