Harnessed Lightning

{E} is the energy symbol. It represents one energy counter.

If an effect says you get one or more {E}, you get that many energy counters. To pay one or more {E}, you lose that many energy counters. You can't pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Some triggered abilities state that you "may pay" a certain amount of {E}. You can't pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of {E} as the ability resolves.

Energy counters are a kind of counter that a player may have. They're not associated with any specific permanents.

Some triggered abilities that state that you "may pay" a certain amount of {E} describe an effect that happens "If you do." In that case, no player may take actions to try to stop the ability's effect after you make your choice. If the payment is followed by the phrase "When you do," then you'll choose any targets for that reflexive triggered ability and put it on the stack before players can take actions.

If the target creature becomes an illegal target, Harnessed Lightning doesn't resolve and none of its effects happen. You won't get {E}{E}{E} or be able to pay any {E}.

You choose the target creature as you cast Harnessed Lightning. You don't choose how much {E} to pay until Harnessed Lightning is resolving. No player may take actions between the time you choose how much {E} to pay and the time the creature takes damage equal to the {E} paid this way.