Nouns ending in -ie are usually feminine.
The suffix often names an abstract quality, an activity, or a discipline.
Is énergie masculine or feminine?
l'énergie
The rule behind it
95.7% of -ie nouns are feminine
In context
Il consacra toute son énergie à l'expérience.
All his energies were devoted to the experiment.
La maison est chauffée à l'énergie solaire.
The house is heated by solar energy.
J'investis toute mon énergie dans ce projet.
I am putting all my energy into this project.