How is flywheel energy calculated?

E = 0.5*J*omega^2, where J is moment of inertia and omega is angular velocity.

What is useful energy range?

Energy between max and min operating speed: 0.5*J*(omega_max^2 - omega_min^2).

1 kJ = 0.000278 kWh = 1.341 hp*s.

Material tensile strength via hoop stress sigma = rho*v^2.

Ring has J=mR^2 vs disc J=mR^2/2. Ring more mass-efficient but higher stress.

Free Engineering Tool #079

Calculate kinetic energy stored in a flywheel. Enter J (or mass + radius) and rotational speed. Results in kJ, kWh, and hp·s.

Energy StorageMultiple UnitsAuto-Calc

Kinetic Energy

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Energy (kWh)

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Energy (hp·s)

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Inertia J

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Angular Velocity

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Max safe rim speed: 100–200 m/s for steel. Hoop stress: σ = ρv².