A flywheel is a black box you spin up and draw down, and it never gives back exactly what you put in: its round-trip efficiency — energy out over energy in — falls short of 100% because the motor that spins the rotor up and the generator that draws it back down each waste a share of the energy passing through them. Stored energy is kinetic — ½Iω², half the rotor's moment of inertia times its spin rate squared — so it climbs with the square of speed, not linearly. Unlike a chemical store, a flywheel's self-discharge is fast: bearing friction and aerodynamic windage drag on the spinning rotor bleed energy away in minutes to hours, not months, reported here as a half-life. Only the top half of the speed range is usable — dropping from full speed to half speed alone gives up 75% of the stored energy, leaving the rest on the table. Spin it faster and the rotor's rim speed climbs toward a material's structural limit that caps how much energy a given rotor can safely hold. Set the rotor mass, radius, top speed, rim-speed limit, standby loss, and hold time, then watch stored energy, power and the standby decay play out live. It all runs on your device.
Energy in during spin-up, energy out during draw-down — and how fast standby loss bleeds the rest away.
Stored energy—
Power—
Round-trip—
Capacity—
Rim speed—
Half-life—
Phase—
Power
Energy split — delivered vs lost
Spin-up / draw-down schematic
Stored energy over the cycle
Standby decay toward half-life
A flywheel loses stored energy far faster than it should if left standing. Bearing friction and aerodynamic windage drag on the spinning rotor constantly bleed away kinetic energy, so a flywheel's self-discharge half-life is measured in minutes to hours — while a chemical energy store held at rest can sit for months. Spin the rotor with the motor and part of the input is lost to conversion inefficiency before it ever becomes stored ½Iω² kinetic energy; draw it back down through the generator and another share is lost the same way, so round-trip efficiency always falls short of 100%. Only the top half of the speed range is usable, since dropping from full speed to half speed alone releases 75% of the stored energy — the rest sits unusable below it. None of this is a currency cost — it's physical accounting in kWh, kW, kg, m and RPM — and the same rim speed that lets a lighter, faster rotor store more energy is capped by the rotor material's structural limit.
Reading the simulation
What the schematic, the energy-split bar and the two plots are telling you about a rotor that never stops leaking energy.
1
The schematic and the power gauge
Energy from the grid flows in as the motor spins the rotor up, some of it lost to conversion inefficiency, the rest stored as kinetic energy; drawing it back down runs the same path in reverse through the generator. The gauge beside the schematic tracks live power against the flywheel's rated power, positive while spinning up and negative while drawing down.
2
The energy-split bar and round-trip efficiency
The energy-split bar breaks the round trip into motor loss, generator loss, and the standby loss bled away by bearing friction and windage while the rotor holds its spin — plus what's left as energy actually delivered. Round-trip efficiency is delivered energy divided by energy in, and it drops as the rim-speed limit is pushed harder or the hold time stretches longer.
3
Stored energy and standby decay
The stored-energy plot tracks percent of capacity through the spin-up, hold and draw-down phases of one cycle, including the fast bleed of standby loss during the hold. The decay plot shows that same standby bleed as an exponential curve toward the flywheel's half-life — how long it takes to lose half its stored energy just sitting at speed — reported live alongside rim speed.
How It Works
A motor loss on the way in, a generator loss on the way out, and a standby leak that never stops — flywheel losses play out in minutes, not months.
Energy from the grid passes through a motor loss on its way into the spinning rotor's kinetic energy, and back out through a generator loss on its way back to the grid. While the rotor just holds its spin, bearing friction and windage bleed energy away as standby loss — fast enough to matter in minutes to hours, not months. The flywheel itself is treated as a black box: only its mass, radius, spin rate and rim-speed limit matter here, not what's inside the motor or generator.
1
Round-trip efficiency: motor in, generator out
Spinning the rotor up costs more energy than the ½Iω² that ends up stored, because the motor converting electrical energy into rotation wastes some of it; drawing the rotor back down through the generator wastes another share converting rotation back to electrical energy. Round-trip efficiency is energy delivered divided by energy in, and a higher rim-speed limit setting — running the rotor closer to its structural limit — changes how much of each pass is lost.
2
Standby loss: friction and windage never sleep
Even with the rotor just holding its spin, no motor and no generator drawing power, bearing friction and aerodynamic windage on the rotor constantly drag energy out of it — this is standby loss. It's fast enough that the flywheel reports a half-life in hours, not the months a chemical energy store can hold a charge, so a long hold time between spin-up and draw-down measurably lowers how much energy is left to deliver.
3
Usable range and the rim-speed material limit
Kinetic energy scales with the square of spin rate, so dropping from full speed to half speed alone releases 75% of the stored energy — the usable range most designs draw from, leaving the bottom half of the speed range essentially spent. Spinning faster packs in more energy for the same rotor mass and radius, but the rotor's rim speed — radius times spin rate — climbs too, and every rotor material has a structural limit on how fast its rim can spin before it fails, set here as a fraction of that limit.
Classroom activity
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Explain rotational kinetic energy storage.
Relate stored energy to moment of inertia and spin rate.
Identify advantages and losses.
Variables to change
Rotation speed
Flywheel mass/inertia
Friction
Procedure
Spin the flywheel up and note the energy stored.
Increase the rotation speed and observe how stored energy grows.
Let it coast and observe energy lost to friction over time.
Observations
Record stored energy versus rotation speed and how quickly it decays.
Questions
What form of energy does a flywheel store?
Write the stored energy.
Which matters more for stored energy, speed or inertia?
What causes a flywheel to lose stored energy?
What are flywheels good for?
Explanation
A flywheel stores rotational kinetic energy E = ½Iω². Because energy scales with the square of spin rate, speeding it up stores far more than adding mass. Friction and drag cause losses, so it suits short-duration, high-power applications.
Answer key (for instructors)
1. Rotational kinetic energy.
2. E = ½·I·ω², where I is the moment of inertia and ω the angular speed.
3. Speed — energy grows with the square of ω, but only linearly with I.
4. Friction in bearings and air drag; advanced units use vacuum and magnetic bearings.
5. Fast charge/discharge and high power for short durations (e.g. grid frequency regulation).
Educators: link or embed this simulation freely in your LMS or course guide.
What is round-trip efficiency in a flywheel?
Round-trip efficiency is the energy you get back out of a flywheel divided by the energy you put in to spin it up. It's never 100% because the motor that spins the rotor up and the generator that draws it back down each convert some of that energy imperfectly, losing a share on both legs of the round trip. A flywheel that keeps those conversion losses lower on both legs reports a higher round-trip efficiency.
Why does a flywheel self-discharge so much faster than a chemical cell?
A flywheel stores energy as motion, and anything that drags on the spinning rotor bleeds that motion away: bearing friction and aerodynamic windage act continuously, even when no motor or generator is drawing power. That standby loss is fast enough that a flywheel's half-life — the time to lose half its stored energy just sitting at speed — is measured in minutes to hours, while a chemical energy store held at rest can sit for months before losing the same fraction. High-performance flywheels cut that drag by spinning the rotor in a vacuum enclosure on magnetic or low-friction bearings, which stretches the standby half-life toward the high end of that range, but even the best designs still fall far short of a chemical cell's months-long shelf life because some drag always remains.
Why is only part of a flywheel's speed range usable?
Stored kinetic energy is proportional to the square of spin rate, so as a flywheel slows from full speed, the energy it has given up grows much faster than the speed drop suggests. Dropping from full speed to just half speed already releases 75% of the stored energy, leaving only 25% in the bottom half of the range — too little to usefully draw from — so most designs treat half speed to full speed as the usable range. Extracting that last quarter would also demand a generator and power electronics built to handle a much wider voltage and frequency swing for a small energy return, so most systems simply stop discharging at half speed and treat the low-speed energy as unusable reserve.
What limits how much energy a flywheel rotor can store?
A rotor's rim speed — its radius times its angular spin rate — sets the centrifugal stress the rotor material has to withstand, and every material has a structural limit past which the rotor would fail. Because stored energy grows with both mass and the square of spin rate, designs push spin rate as close to that rim-speed limit as the material safely allows, then add mass or radius for more capacity within that same speed limit. That's why steel rotors, limited to modest rim speeds, store only tens of watt-hours per kilogram, while carbon-fiber composite rotors — lighter and far stronger for their weight — tolerate much higher rim speeds and can store several times more energy per kilogram of rotor mass.
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