Fireurza, some have used the flower when they had more than 0 energy and claimed it didn't work (Belarion posted this somewhere in the forum a couple of months ago). It has already been stated that it completely fills your energy if used when it is at zero.
My energy level is 3050. During the cleanup quest for King Algadon's celebration (and other times) I used 1 or 2 of the flowers after depleting my energy from 3 to an exact ZERO. It filled ALL 2900+ of my energy at that time! Save your flowers please! When you're my level you'll be glad you did.
Don't hesitate to contact me for some guidance tips in PM anytime. My clan is neutral and will assist any nice individual