You target a creature or player when the ability triggers. You decide whether or not to pay as the ability resolves.

Once you decide to pay , it's too late for anyone to respond (such as by activating a regeneration ability the targeted creature has).

If you pay , the creature that entered the battlefield deals damage equal to its current power to the targeted creature or player. If it's no longer on the battlefield, its last known existence on the battlefield is checked to determine its power.

Electropotence is the source of the ability, but the creature is the source of the damage. The ability couldn't target a creature with protection from red, for example. It could target a creature with protection from creatures, but all the damage would be prevented. Since damage is dealt by the creature, lifelink, deathtouch, and wither are taken into account, even if the creature has left the battlefield by the time it deals damage.