A flywheel stores energy as the spin of a heavy wheel, and the energy rises with the square of the rotation speed, so doubling the speed stores four times as much. A thin steel ring of 500 kg and 0.6 m radius turning at 1,500 rpm holds about 2.2 MJ (617 Wh), roughly what a 12 V, 50 Ah battery holds, and the same speed-squared law that stores the energy also loads the rim, which is why the wheel’s top speed is set by its material and not by the builder.
Part of Flywheel Energy Storage
Kinetic storage keeps energy in the motion of a rotating mass; the wheel’s mass, its radius and its rotation speed fix both what it holds and how it can break.
A Spinning Mass Is A Battery With No Chemistry
Spin a heavy wheel and every part of it is moving. That motion is energy, and it stays in the wheel until a load takes it back out through the shaft. Nothing wears out chemically, and the wheel can give up its energy in seconds, which a lead-acid battery cannot do without damage. The price is that the energy is small for the size of the machine and leaks away continuously through the bearings and the air.
The energy depends on two things: how much mass is spinning and how fast. Speed counts far more, because it enters squared. A wheel at 2,000 rpm holds (2,000 ÷ 1,000)² = 4 times what the same wheel holds at 1,000 rpm. The full derivation is in 2 × I × omega².
What The Numbers Look Like
Inputs: a thin ring (all the mass at the rim) of 500 kg and 0.6 m radius at 1,500 rpm.
- Angular speed ω = 1,500 × 2π ÷ 60 = 157.1 rad/s.
- Moment of inertia I = m × r² = 500 × 0.6² = 180 kg·m².
- Energy E = ½ × 180 × 157.1² = 2,221,000 J, which is 2.2 MJ.
- In watt-hours: 2,221,000 ÷ 3,600 = 617 Wh.
A 12 V, 50 Ah battery holds 12 × 50 = 600 Wh. So a half-tonne steel wheel doing 94 m/s at its rim banks about what one car battery does. That is the honest scale: a flywheel is a power buffer, not a seasonal store.
Only part of that energy can be drawn out. A generator or load stops being useful as the wheel slows, and a wheel slowed to half speed has given up 1 − (½)² = 75% of its energy and kept 25%. The usable fraction between full and half speed is therefore 75%, or about 463 Wh from the example wheel.
When A Flywheel Is The Right Tool
- Smoothing. Wind gusts and uneven water flow come and go in seconds; a wheel absorbs the surge and fills the gap.
- Bursts. A wheel can hand over its energy in a fraction of a second, for starting a large motor or driving a press.
- Cycle count. The energy store itself does not age chemically, but the rim stress rises and falls with every charge. Metal wheels run near their stress limit can fatigue, so cycles are not literally unlimited.
- Not for hours. Bearing and air losses (see Bearings & Friction) take a meaningful share every hour. For overnight or multi-day storage use a battery or heat storage.
Common Mistakes
- Reading rpm into the formula. The energy formula needs radians per second. Putting 1,500 where 157.1 belongs overstates the energy by (1,500 ÷ 157.1)² = 91 times.
- Confusing weight and mass. The formula takes kilograms. Using newtons (kilograms × 9.81) inflates the result by 9.81 times.
- Assuming a bigger wheel is a safer wheel. Rim stress depends on rim speed, not on diameter; see Burst Energy Calculation.
Safety
The stored energy is the injury energy
The example wheel’s 2.2 MJ is the energy equivalent of about 0.53 kg of TNT (using 4.184 MJ per kilogram), and it is released as flying pieces if the rim fails, not as a bang. The wheel cannot be switched off from where you stand: it takes minutes to hours to coast down. Never stand in the plane of the wheel while it is spinning, never run it faster than the speed calculated from the tensile strength of its material, and read Safety & Containment before you build.
What’s Next
- Learn next: 2 × I × omega² — the derivation and units
- Combine with: Flywheel Construction — what the wheel is made of decides how fast it may turn