Why is my game using エナジー for "energy" instead of エネルギー?
Basically what I know エナジー & エネルギー both are gairaigo words & interchangeable (shown up as alternative of each other in dictionary), but they have different origins.
1) エナジー originated from English term "energy", commonly used to refer both "mental powers", "spiritual powers" & "physical powers" (broader meaning).
2) エネルギー originated from German term "Energie", specifically means "physical powers" as at the time of its invention, "power" associated to scientific physical activity.
There is another explanation from Japanese loanwords history:
エネルギー is one of the German based loanwords that were imported into Japanese as part of the modernization effort. At that time, German had more presence in science than today.
エナジー was imported from English. I think it's a relatively new katakana word, like a few decades old. As such, it feels newer and therefore cooler than エネルギー.
So, in your case, if the word refers to mental or spiritual powers, "エナジー" is more preferred term. For usage in science (related with physics phenomena), "エネルギー" is more commonly used (you can see in this Japanese Wikipedia of 'energy').