Hour of Power

Superpowers can be loosely broken down into those that are "on" all the time such as Superman's super-strength, and those that need to be activated in some way by the character such as Superman's heat vision.

Hour of Power, named for the Might and Magic spell, is a special limitation placed on the latter type, in which the power only lasts for a certain time interval after being activated. This time interval is always the same, no matter how many times a character activates it, and it's always an easy-to-remember amount of time, usually a round hour, but occasionally a number with some other significance. Expect lots of Race Against the Clock plots, where the character comes perilously close to running out of time while in the midst of danger.

There's usually no explanation of why the limit exists, or why it's so specific. If it's magically imposed, it's not a big problem, since magic only needs to be internally consistent. It's a little more Egregious when the powers are granted by Applied Phlebotinum, especially if every other time limit has been broken, or if it's clear that the creators don't use "Earth" hours. Real Life technology doesn't tend to have fully predictable running times, but fictional technology, apparently, has it all the time.

Most of the time though, it is just useful for writers to create a sense of danger. The character is forced to be careful about how often to use their powers, and can be put in opposition to a sufficiently prepared villain. This one way to avoid the Sorting Algorithm of Evil.

Compare Super Mode.

Not to be confused with Pastor Robert Schuller's broadcast from the Crystal Cathedral.

Anime and Manga

 * Moldiver is a rare example of a time limit that isn't a round hour, or even a round minute. It's still meaningful, though: 666 seconds.
 * The Evas in Neon Genesis Evangelion technically have exact-time battery limits depending on power usage, but are usually conveniently vague about said usage. Evas can fight much longer while still attached to an umbilical power cord, but they're less agile and much less cool-looking when doing so.
 * Not that running out of power ever particularly hinders them. Shinji is the one who tends to run out of power, but he also tends to go berserk and kill the Monster of the Week regardless. Asuka is not so lucky.
 * Considering why exactly Eva-01 is able to outperform Eva-02 in Berserk Mode is not astonishing, if you understood whose soul resides in it...and that person may be the primary player in the series' massively complex and confusing conspiracies
 * Ichigo of Bleach could initially activate his Super-Powered Evil Side safely for eleven seconds at a time before it started to take control of his personality. Recently, though, this limit seems to have been extended. Should he overdo it, it will go away by itself, apparently severely tiring him out in the process.
 * Eleven seconds was the time he could keep his mask on when the diversion attack happened/he went to Hueco Mundo. When he first started being able to call on his Hollow side/mask at will, he couldn't even keep it on for eleven seconds and had to train. Seeing Orihime and Nel in danger from Grimmjow's attacks was what enabled him to keep it on for longer.
 * The other Visoreds similarly have time limits for their Hollow masks, which vary from one to the next. Two of them have a limit of 3 minutes, which (in a reference to Ultraman) they claim is because they're "superheroes".
 * The effects of Chopper's Rumble Ball last for exactly three minutes. He can try it a second time if the fight lasts longer than that, but he loses control of his transformations if he tries. And if he goes for a third, well... things just get bad.
 * Luffy's Gear Third has a variation of this. While there's no set amount of time it lasts, when Luffy stops the technique, he changes into a chibi version of himself that's pretty much useless in combat; he stays this way for an amount of time equal to that he had just spent using Gear Third.
 * Ban Mido of GetBackers can cause a person to hallucinate for exactly one minute (real time). He nearly always greets a person coming out of a hallucination with, "Exactly one minute. Have any bad dreams?"
 * Tekkaman Blade turns into a mindless, berserk killing machine if he remains in Tekkaman form for thirty minutes.
 * In Tokyo Mew Mew, can only remain as a cat for fifteen minutes or get stuck like that forever.
 * Oolong from Dragon Ball has shapeshifting powers that only work for five minutes before he changes to his default form. This is because he was kicked out of shapeshifting school before he could learn how to shift indefinitely
 * In DBZ, the Fusion Dance introduced in the Buu arc lasts for half an hour, but if the fusion is powerful enough it can shorten the amount of time they have. In GT, for example, the fusion of Super Saiyan 4 Goku and Vegeta, Gogeta, is so immensely powerful that he defuses after only ten minutes.
 * The Japanese anime version of Teenage Mutant Ninja Turtles has the title turtles able to transform into super versions of themselves, but only for 3 minutes.
 * In the Pokémon Special manga, Mewtwo could only fight for three minutes at a time after he was caught by Blaine, thanks to some trouble with his cloning involving some Applied Phlebotinum and Blaine's cells. Any longer risks Blaine's life and tires Mewtwo out considerably.
 * Yellow has a good number of powers, including heightening her Pokemon's level to a frightening point when under emotional stress. Using these powers, however, eats at her stamina like nothing else, and when she uses them too much, she falls into a sleep that cannot be externally disrupted no matter how inopportune the following situation.
 * In the games, Regigigas is an inversion. Only 11 other Pokemon meet or exceed its 680 base stat total, and only two exceed its base Attack, but it has to suffer 5 turns of weakness to get the benefits.
 * Can be negated entirely through the use of Gastro Acid, a move that disables a Pokémon's ability (which, as it turns out is what limits Regigigas).
 * Mobile Suit Gundam 00 has the TransAm system which, when activated, makes the Gundams turn red and go three times faster for a limited time. Afterwards, they operate at reduced power for some time (since TransAm releases all the stored power at once and it takes a while to build the stores back up).
 * In Naruto, In the end.
 * Sage Mode uses up a lot of energy and can only be recharged by standing perfectly still, so it wears off quickly in battle.
 * In Freezing, the girls' superpowered, armored "Pandora" mode has a limit of three minutes, after which they can no longer fight for quite some time, meaning that this is a last resort only.
 * Subverted in Super Dreadnought Girl 4946. Mana is told that it's okay to eat Tobita for a power boost as long as she spits him up within three minutes. In reality Tobita should have died the moment Mana swallowed him, and the three minute rule was a lie to convince Mana that she wasn't killing her boyfriend. The fact that Tobita actually survived was a surprise to everyone except Mana, and ever since then they kept the three minute rule simply because they have no idea about how long he can ACTUALLY survive.
 * In To Aru Majutsu no Index, Accelerator gains this limitation after his Heel Face Turn. Originally, he could use his Story-Breaker Power to change physical vectors as often as he wanted, with no real limitations. Later, however, he can only use it for up to thirty minutes, and relies on a vulnerable transmitter to do it. He later manages to improve the device and make it more energy efficient.
 * The powerful mage Cendrillon automatically loses her powers at midnight or sunrise, similar to her namesake: Cinderella.
 * Wild Tiger's Super Strength in Tiger and Bunny has a 5-minute time limit per activation, as does that of his partner, Barnaby.
 * Rurouni Kenshin: Shishio Makoto was once burned alive, damaging most of his sweat glands. As a result, he cannot fight for more then fifteen minutes, at which point he overheats and dies.
 * Rosario + Vampire: Whenever Tsukune got injured, Moka would inject him with her blood to heal him. At first, this would grant him temporary vampire powers as a side effect. After awhile it gets subverted, because the blood starts to take a toll on his body, and the transformation becomes permanent, with consequences. He recovers somehow and keeps the powers, so this trope becomes redundant.
 * Although, he's still at risk if his limiter is ever broken.
 * In Magical Idol Pastel Yumi, the drawing Yumi brings to life only last for a short time.
 * Although, he's still at risk if his limiter is ever broken.
 * In Magical Idol Pastel Yumi, the drawing Yumi brings to life only last for a short time.

Comic Books

 * As his name implies, Hourman's Super Serum only lasts for an hour; he uses the phrase "Hour of Power" quite often too.
 * The second Hourman Rick Tyler also had the ability to spend an hour in a timeless dimension with his father. This eventually ended when
 * Android Hourman also had an Hour of Power, which gave him access to all sorts of cool time-manipulation abilities (just about the only thing he couldn't do was make his Power Hour last longer than 60 minutes.)
 * The Green Lantern's Green Lantern Ring used to require charging every 24 hours, but this has gradually been retconned to more closely follow a "battery" model, where the charge depletion depends on the use of the ring.
 * That wasn't a Retcon, the Guardians deliberately changed how the rings worked. At one point when Hal Jordan traveled forward in time, his ring still worked the old way (it became a plot point when he and Kyle switched rings because Kyle's didn't have a time limit)
 * The Doom Patrol's Negative Man could separate his energy form from his body for 60 seconds at a time. He had to act fast; If he didn't return to his body in time, he would die.
 * In one Thor story, his power had been halved, and a villain was beating the living daylights out of him. As a test, Odin granted Thor his full power for thirty seconds. Thor won. (Do not screw with someone whose titles include "god", no matter what they are the god of. If they're the god of something like "thunder" or "storms", don't even think it.)
 * Originally, if Thor lost contact with his hammer for more than 60 seconds, he would change back to his human form. This limitation has since been removed, although it made a comeback when Eric Masterson, otherwise a normal human, became a replacement Thor by taking up the hammer.
 * According to one X-Men source Rogue's power drain ability (in the comics) tends to last for about a 1:60 ratio--that is, for every second she holds it, she has what she took for 60 seconds. Life Energy and memories may work on a different scale, of course, and since it's possible for her to take powers permanently, this ratio breaks down the longer she holds on, but, well, A Wizard Did It.
 * It was later revealed that the limits to her powers were psychological, not physical. After she was "reset" and her trauma-induced mental blocks restored, she could call up any previous power or memory she had absorbed at will, as well as be able to touch someone without draining them. That said, the 1:60 ratio was rarely outright stated to begin with, and it's very possible that later writers ever heard of it, so it's more likely just a non-definite exponential ratio.
 * J2 of Marvel Comics 2 has a time limit on his transformed state that gradually gets shorter as his body matures and becomes better at using his powers.
 * All-Star Superman has a formula that gives whoever drinks it Kryptonian superpowers for 24 hours.

Film

 * Darkman, as opposed to Ultraman, needs to get out of light within 99 minutes.

Literature

 * The Animorphs had two hours before they had to "demorph" into human form, or else be stuck as an animal. Used as a major part of the plot (one of the main characters was stuck from the beginning of the series), but suffered somewhat from a Two of Your Earth Minutes Running Gag. It also appears possible for someone with high willpower to demorph just beyond the time limit, but with greater difficulty. Note: There are instances in which the time limit can be reset. One book has Cassie get trapped as a caterpillar, but her natural transformation into a butterfly resets the clock. Also, the Ellimist has the power to restore someone's morphing ability after they have been mode locked.
 * Inverted in The Stormlight Archive by Brandon Sanderson. Shardblades can be summoned out of thin air, but it always takes ten of the owner's heartbeats to do it (meaning you get it faster if you're adrenaline rushing, nice feature for a weapon). In this case, it's not an exact time until the power ends, but an exact time until the power begins.

Live Action TV

 * The Animorphs television series has the extreme Exact Time to Failure version: Ax is close to being trapped in human form, counts down the last few seconds to a nearby teammate, but is finally able to get away and has none of the difficulty changing back that the book versions had when cutting it close. This would suggest that changing back after 1 hour, 59 minutes, and 59 seconds is no different from changing back after five minutes, but changing back at 2:00:00 is completely impossible.
 * Ultraman popularized this trope for live-action Japanese superheroes. His Bizarre Alien Biology makes the atmosphere harmful to him, but - depending on the iteration - he stores solar energy for three minutes, often marked with a "color timer" on his chest that goes from blue to blinking red as this runs out. Just about the right amount of time for an easily-budgeted Monster of the Week fight sequence in a 24 minute show...
 * ...And then you have Ultra Seven, who doesn't seem to have this limit.
 * From the same studio there's also Fireman, who cannot stay in his Agan form for more than 3 minutes due to sunlight being harmful to Agans.
 * There's the 70's superhero Kaiketsu Zubat, who explodes if he wears the Zubat suit for more than 5 minutes. Justified in-show as being a Super Prototype, finished by the guy himself even though it's out of his field.
 * The Makai Knights of Garo have exactly 99.9 seconds to fight in their armored forms before it runs out. They can choose to exceed the time limit if they want, but the results are... not pleasant.
 * A version of this occurs in Stargate SG-1, where barring some Applied Phlebotinum (and for this show exceedingly rare applied phlebotinum--perhaps three or four times in the entire series), no artificial wormhole (i.e. Stargate connection) can be maintained for more than 38 minutes and a few seconds.
 * Silver Ranger/Megasilver from Power Rangers in Space/Denji Sentai Megaranger had a transformation time limit of 2.5 minutes.
 * And then got over it when the bad guys tried to exploit this. With lots and lots of research.
 * The Storm Megazord/Senpuujin in Power Rangers Ninja Storm/Ninpuu Sentai Hurricaneger had a special form that lasted 1 minute before switching back (although as the series went on it went transformed for about 2 mins).
 * The Engines from Engine Sentai Go-onger can maintain their large forms in our universe for 10 minutes before rusting.
 * In Power Rangers SPD, Kat was given an expendable morpher for one episode to help her team which would only work for one hour.
 * Faiz Axel Form in Kamen Rider Faiz only worked for 10 seconds before shutting down. While this seems short compared to some of the other examples on this page, keep in mind that this is 10 seconds of Super Speed.
 * In what is surely a Mythology Gag we have Accel Trial Form from Kamen Rider Double, which must complete it's Maximum Drive within 10 seconds after activating it.

Video Games

 * Metroid Prime 3: Corruption has Samus' Hypermode, which actually has two timers: 10 seconds after activation, the Hypermode turns into Corruption, causing the meter to rise over time (as does being hit by certain attacks), and 25 seconds after activation, the mode is automatically turned off. To be fair, the second time limit actually makes sense as an internal safety measure of her PED suit.
 * True to form, the Corruption can be used as a Dangerous Forbidden Technique by abusing the recharge, at the risk of total corruption.
 * A number of powersets in City Of Heros have abilities along this line. The best example is probably the defensive powersets which frequently include an "uber-mode" power that renders the character nearly invulnerable for a few minutes but has a very long recharge time.

Web Comics

 * In Endstone, Kyri explains the need to recharge.

Web Original

 * At the Whateley Academy in the Whateley Universe, Dash has super-speed, super-strength, and Nigh Invulnerability. But it looks like he only has it for a matter of minutes, and can't call it up without a lot of effort ahead of time.
 * Word of God finally explained it: Dash is some kind of Energizer and can focus to make his body stronger and tougher and faster, but it only holds up for ten minutes or so, and it takes about that long for him to power up. After, he needs hours before he can do it again.
 * Many of the SCP Foundation artifacts have specific hard time limits, although the round units are generally averted. For example, SCP 431 after 22 hours and 17 minutes of being worn.

Western Animation

 * The Omnitrix of Ben 10 has a time limit that is supposed to be ten minutes Earth time, but in practice differs from use to use....
 * Roger Ramjet's proton pills gave him "the power of twenty atom bombs for twenty seconds".