### General Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;[[2]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-ScienceNews-2) full-cycle lifetimes quoted for flywheels range from in excess of 105, up to 107, cycles of use),[[5]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Investire-5) high specific energy (100–130 W·h/kg, or 360–500 kJ/kg),[[5]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Investire-5)[[6]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-pddnet-6) and large maximum power output. The energy efficiency (_ratio of energy out per energy in_) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 kWh to 133 kWh.[[2]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-ScienceNews-2) Rapid charging of a system occurs in less than 15 minutes.[[7]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Distributed_Energy-7) The high specific energies often cited with flywheels can be a little misleading as commercial systems built have much lower specific energy, for example 11 W·h/kg, or 40 kJ/kg.[[8]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Rosseta-8) ### Energy storage efficiency Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two hours.[[17]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Rosseta2-17) Much of the friction responsible for this energy loss results from the flywheel changing orientation due to the rotation of the earth (an effect similar to that shown by a Foucault pendulum). This change in orientation is resisted by the gyroscopic forces exerted by the flywheel's angular momentum, thus exerting a force against the mechanical bearings. This force increases friction. This can be avoided by aligning the flywheel's axis of rotation parallel to that of the earth's axis of rotation.\ _[citation needed_\] Conversely, flywheels with magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.[[18]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-Beacon2-18) ## Comparison to electric batteries Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical rechargeable batteries.[[56]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-56) They are also less potentially damaging to the environment, being largely made of inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored. Unlike most batteries which operate only for a finite period (for example roughly 10[[57]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-57) years in the case of lithium iron phosphate batteries), a flywheel potentially has an indefinite working lifespan. Flywheels built as part of James Watt steam engines have been continuously working for more than two hundred years.[[58]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-58) Working examples of ancient flywheels used mainly in milling and pottery can be found in many locations in Africa, Asia, and Europe.[[59]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-59)[[60]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-60) Most modern flywheels are typically sealed devices that need minimal maintenance throughout their service lives. Magnetic bearing flywheels in vacuum enclosures, such as the NASA model depicted above, do not need any bearing maintenance and are therefore superior to batteries both in terms of total lifetime and energy storage capacity, since their effective service lifespan is still unknown. Flywheel systems with mechanical bearings will have limited lifespans due to wear. High performance flywheels can explode, injuring bystanders with high-speed fragments.[[61]](https://en.wikipedia.org/wiki/Flywheel_energy_storage?utm_source=chatgpt.com#cite_note-61) Flywheels can be installed below-ground to reduce this risk. While batteries can catch fire and release toxins, there is generally time for bystanders to flee and escape injury. The physical arrangement of batteries can be designed to match a wide variety of configurations, whereas a flywheel at a minimum must occupy a certain area and volume, because the energy it stores is proportional to its rotational inertia and to the square of its rotational speed. As a flywheel gets smaller, its mass also decreases, so the speed must increase, and so the stress on the materials increases. Where dimensions are a constraint, (e.g. under the chassis of a train), a flywheel may not be a viable solution.\ _[citation needed_\]