Katie Power is the youngest child of Dr. James Power, a physicist who was working for the United States government on a means of producing large quantities of energy cheaply through combining matter and anti-matter. (The sub-atomic particles composing anti-matter have opposite charges to those of the equivalent particles of matter itself. When matter contacts anti-matter, both are converted entirely into vast quantities of energy).
An alien Kymellian named Aelfyre Whitemane learned of Power's discovery and intended to stop it from being used. Whitemane knew that a device such as Power's would, despite Power's good intentions, trigger a chain reaction that would destroy the Earth. Moreover, he knew that Power's device was sought by the alien Zn'rx, nicknamed "Snarks," who intended to use it to conquer inhabited worlds.
Snarks captured Dr. Power and his wife Margaret, intending to force him to reveal the secret of his device. Whitemane saved the Powers' children. Alex, Julia, Jack, and Katie, from being captured, but he himself was fatally wounded in the process. Dying, Whitemane transferred each of his superhuman powers into one of the four children, and asked them to use their new abilities to save their world.
Katie named herself the Energizer, after her own power, and the four children called themselves the Power Pack. Being only five years old at the time, Katie thus became one of the youngest superheroes ever. Power Pack thwarted the Snarks, rescued their parents, and destroyed Dr. Power's dangerous matter/anti-matter converter, thereby saving the Earth from destruction. The four children have since continued to have adventures as Power Pack.
Recently Katie was asked by Doorman to join the Great Lakes Avengers during their recruitment drive after the death of Dinah Soar and the apparent mental breakdown of Mister Immortal. Like many other superheroes, Katie rejected the offer.
At some point, Energizer briefly rejoined the Power Pack and teamed-up with Devil Dinosaur, Gwenpool, and Moon Girl.