Flywheel Energy Storage Calculator
How much energy a flywheel stores, how long it can supply a load, and whether the rim stress is survivable.
A spinning mass used as a battery.
How the flywheel energy storage calculator works
Stored energy is ½Iω², so speed matters far more than mass — doubling the rpm quadruples the energy while doubling the mass only doubles it.
The limit is material, not magnetic. Rim stress in a spinning ring is ρv², where v is the rim speed, so the useful ceiling is set by the specific strength of the material. That is why modern flywheels use carbon composite rather than steel.
Formula: E = ½Iω²; σ = ρ v²
Worked examples
| Inputs | Stored energy | Note |
|---|---|---|
| 100 kg ring, 0.5 m, 10,000 rpm | 13.708 MJ | 13.7 MJ, but 2.1 GPa rim stress |
| A slower, safer rotor | 2.193 MJ | a sixth of the energy, a sixth of the stress |
| A solid disc instead | 6.854 MJ | half the energy for the same mass |
FAQFrequently asked questions
How much energy can a flywheel store?
A 100 kg steel ring at a survivable speed stores a few megajoules — around one kilowatt-hour. Composite rotors reach several times that per kilogram.
Why is rim speed the limit rather than rpm?
Because hoop stress depends on rim speed alone: σ = ρv². A larger rotor must spin slower for the same stress.
Why use carbon composite?
Its specific strength — strength divided by density — is far higher than steel's, and specific strength is exactly what sets the energy per kilogram.
What are flywheels used for?
Grid frequency regulation, uninterruptible power supplies, and regenerative braking. They excel at high power for short durations, where batteries wear out.
What happens if one fails?
Catastrophically — the stored energy releases at once. Containment vessels and vacuum housings are mandatory, and modern designs favour fibre rotors that shred rather than fragment.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Measurement Institute — Australia's measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.