The user possesses an unlimited energy source that will never run out, allowing them to use attacks and accomplish other feats that require energy indefinitely. The source can supply any form of energy, such as electrical, kinetic energy, etc, which can provide power to a physical body, technology, etc. Different types of energy have different effects on the users, for example, life energy, which effectively grants the user unlimited regenerative power and eternal youth.
Magicule Breeder Reactor (That Time I Got Reincarnated as a Slime)
Silver Sonic v2.0 (Archie's Sonic the Hedgehog) possesses an unlimited power supply, allowing it to fight indefinitely.
Kain (Legacy of Kain) as Scion of Balance has the powers of the Conflict Guardian,that Draw Energy from the Battle,giving him limitless energy.
After being implanted with the Mantra Reactor, Asura (Asura's Wrath) was able to channel an infinite amount of Mantra without fear of it backfiring and destroying his body.
Through the Azure Grimoire, Ragna the Bloodedge (BlazBlue) has direct access to the Boundary, granting him an unlimited supply of seithr.
Due to the Uranium-235 core that animates and powers his body, Adam (Buffy the Vampire Slayer) has a supply of energy that lasts "essentially forever", allowing him to go indefinitely without food, water, or sleep and heal from any wound, even beheading.
Priscilla (Claymore) possesses an infinite amount of youma life energy, which grants her transfinite stamina and the ability to regenerate from all wounds instantly, which combined to her lack of weak points makes her nigh-indestructible.
Pariah Dark (Danny Phantom) possesses infinite ghost energy as long as he wears the Ring of Rage and Crown of Fire.
The Neurotomic Protocore (Dexter's Laboratory) a powerful energy source that can give near-infinite energy to any device, enabling the user to enhance their own intellect and create planetary utopias or dystopias depending on their intentions.
Captain Atom's (DC Comics) connection to the quantum field enables him to draw upon infinite amounts of energy, allowing him to use it for purposes which are solely limited by his willpower and imagination
Wally West/The Flash's (DC Comics) direct link to the Speed Force ensures he'll never run out of energy, allowing him to overload a kinetic energy siphoning device.
Both Androids #17 and #18 (Dragon Ball series) possesses an infinite energy supply,...
...allowing them to fight indefinitely without ever tiring.
Broly's (Dragon Ball Super) Legendary Super Saiyan form supplies him with a seemingly infinite supply of growing energy, to the point where he needs to vent it out.
In his Legendary Super Saiyan form, Broly (Dragon Ball Z: Broly- The Legendary Super Saiyan) possesses a bottomless supply of growing energy...
...to the point where he needs to vent it out to relieve the strain.
Nuova Shenron's (Dragon Ball GT) Nova Death Ray gets its power from the sun itself, ensuring it won't run out of energy.
Prelati's Spellbook (Fate/Zero) is both a spell-book and a perpetual prana generator, constantly producing an everlasting supply of magical energy unless destroyed.
After spending several years in the Lifestream, Sephiroth (Final Fantasy VII) now possesses an unlimited pool of magic power and unparalleled access to any spell he has ever cast, completely eliminating the need to use Materia.
Powered by the sun, the Perpetual Motion Solar Jacket (Grenadier) has an infinite amount of energy, allowing it to destroy entire nations with its blasts.
As a Kaiser, Shinichi Ozaki (Godzilla: Final Wars) possesses an infinite amount of Kaiser Energy...
...which he grants to Godzilla to empower him against Keizer Ghidorah.
Ophis (High School DxD) possesses an infinite supply of power, truly living up to her epithet of "Infinite Dragon God."
The Celestial Robe (InuYasha) acts as a source of infinite power for Kaguya and greatly strengthens her abilities.
The Omega Spawn (Image Comics) has an unlimited supply of necroplasmic energy, whereas most other Hellspawns have a finite pool of power that slowly runs down.
As the Conflict Guardian and a wraith, Malek the Paladin (Legacy of Kain) draws energy from battle and can fight indefinitely without ever tiring.
The Triforce parts (The Legend of Zelda) grant their chosen bearers tremendous power...
...with Ganondorf, the bearer of the Triforce of Power, having an unlimited amount of magic power.
Bruce Banner/Hulk (Marvel Comics) possesses a near-limitless amount of energy connected directly to his anger levels.
The Reapers (Mass Effect) generate a nigh-limitless amount of energy, allowing them to live for millions of years without needing any kind of fuel or sustenance and making a war of attrition against them impossible.
Tohru (Miss Kobayashi's Dragon Maid) generates her own mana...
...while other dragons need to convert it from other sources.
Jūgo's (Naruto) body constantly absorbs natural energy, granting him an infinitely growing supply of life energy to use as he pleases, though it also corrupts his sanity from time to time.
Mukade (Naruto) drawing from the infinite Ryūmyaku chakra to regenerate at an immortal rate, so long as the source remains open for him.
Victini (Pokémon) can produce an infinite amount of energy that it can share with others, ensuring absolute victory.
Haja (Rave Master) possesses the Type-61 Dark Bring that gives him infinite magical power.
Hexadecimal (ReBoot) possesses transfinite power reserves, which, though not absolutely infinite, is larger than all finite numbers, allowing her tremendous feats and destructions.
The Chaos Emeralds (Sonic the Hedgehog) are objects of great and mysterious power: just one of them is powerful enough to power a machine at infinite scales (if not overload some) and all seven could destroy the Earth.
Master Emerald (Sonic the Hedgehog)
Perfect Chaos (Sonic the Hedgehog) commands infinite energy after absorbing the negative Chaos Energy from the seven Emeralds.
Sonic the Hedgehog (Sonic the Hedgehog Movie) has an infinite supply of energy, to the extent that just a single quill was scanned to have limitless power and able to power Robotnik's robots and armaments well beyond anything else in modern society.
Using Food Immersion, Toriko (Toriko) can absorb and save more nutrients from food inside his body than normally possible, allowing him to store almost unlimited amounts of energy and survive for abnormal time spans without food or water.
After Clear Note obtained his strongest form, he altered his bookkeeper Vino (Zatch Bell!) to have an unlimited supply of heart energy.
After acquiring Fairy Heart, Zeref Dragneel (Fairy Tail) now possesses Infinite Magic Power
The Infinity Gems (Marvel Comics)
The Legendary Silver Crystal (Sailor Moon) possesses tremendous amounts of energy to perform spectacular feats, such as reviving an entire world and all of its inhabitants from death and ruin.
The Eye of Dashi (Xiaolin Showdown) can generate a limitless amount of energy to power up machines.