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.
You are in the Physics lab.
Energy in during spin-up, energy out during draw-down — and how fast standby loss bleeds the rest away.
What the schematic, the energy-split bar and the two plots are telling you about a rotor that never stops leaking energy.
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.
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.
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.
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.
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.
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.
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.