Many of the phrases that surround money can be used with other things.
For instance you can waste (money); you can spend (money); you can save (money) or you can invest (money). But you could replace the word money with any number of concepts. The energy that you describe above can also be exchanged with money and it should help people decide how you decide to divide your energy.

Some people are so wrong headed that if you devote energy on them it is a waste. The energy does not benefit you nor the other person.
Some people are almost like a vampire - they thrive on the energy that you devote to them but you receive nothing in return. This is when you spend your energy.
Saving energy is when you put energy into a relationship with a person but receive little return. I think of people who are old acquaintances whom you might meet in the street. Aside from the odd reminiscence there really is little that the two of you have.
The best of all is when you invest your energy in another. An investment is where you get far more out than you put in - and hopefully the same is experienced by the object of your investment.

I use the metaphor that my energy is contained in a bucket that slowly gets depleted as people take sips or cups of my energy. Eventually my bucket becomes empty and I need to return to the well to replenish it.