"energy" often has something to do with the circumstances of where you are singing ( particularly when singing unaccompanied ). A room full with people and furnishings will sap the voice ... making it much harder to project. Similarly, if the room is very hot and humid.
Whereas , if a room has good acoustics - singing can be a joy as the voice resounds and grows.

The other important factor is confidence .... in the song and in yourself.
Singing a song you enjoy and are comfortable with will transmit to your audience and make them enjoy it so much more.

It took me a lot of years to realise that and when I did I stopped being nervous.