The [Kusita-Arambhavatthu Sutta (AN8.80)][1] provides eight grounds for the arousal of energy. In the [original Pali version][2], it appears that the word used is "[viriya][3]".

> "There are these eight grounds for the arousal of energy. Which eight?
> 
> "There is the case where a monk has some work to do. The thought
> occurs to him: 'I will have to do this work. But when I am doing this
> work, it will not be easy to attend to the Buddha's message. Why don't
> I make an effort beforehand for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized?' So he makes an effort for the
> attaining of the as-yet-unattained, the reaching of the
> as-yet-unreached, the realization of the as-yet-unrealized. This is
> the first grounds for the arousal of energy.
> 
> "Then there is the case where a monk has done some work. The thought
> occurs to him: 'I have done some work. While I was doing work, I
> couldn't attend to the Buddha's message. Why don't I make an effort
> for the attaining of the as-yet-unattained, the reaching of the
> as-yet-unreached, the realization of the as-yet-unrealized?' So he
> makes an effort for the attaining of the as-yet-unattained, the
> reaching of the as-yet-unreached, the realization of the
> as-yet-unrealized. This is the second grounds for the arousal of
> energy.
> 
> "Then there is the case where a monk has to go on a journey. The
> thought occurs to him: 'I will have to go on this journey. But when I
> am going on the journey, it will not be easy to attend to the Buddha's
> message. Why don't I make an effort beforehand for the attaining of
> the as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized?' So he makes an effort for the
> attaining of the as-yet-unattained, the reaching of the
> as-yet-unreached, the realization of the as-yet-unrealized. This is
> the third grounds for the arousal of energy.
> 
> "Then there is the case where a monk has gone on a journey. The
> thought occurs to him: 'I have gone on a journey. While I was going on
> the journey, I couldn't attend to the Buddha's message. Why don't I
> make an effort for the attaining of the as-yet-unattained, the
> reaching of the as-yet-unreached, the realization of the
> as-yet-unrealized?' So he makes an effort for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized. This is the fourth grounds for
> the arousal of energy.
> 
> "Then there is the case where a monk, having gone for alms in a
> village or town, does not get as much coarse or refined food as he
> needs to fill himself up. The thought occurs to him: 'I, having gone
> for alms in a village or town, have not gotten as much coarse or
> refined food as I need to fill myself up. This body of mine is light &
> suitable for work. Why don't I make an effort for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized?' So he makes an effort for the
> attaining of the as-yet-unattained, the reaching of the
> as-yet-unreached, the realization of the as-yet-unrealized. This is
> the fifth grounds for the arousal of energy.
> 
> "Then there is the case where a monk, having gone for alms in a
> village or town, does get as much coarse or refined food as he needs
> to fill himself up. The thought occurs to him: 'I, having gone for
> alms in a village or town, have gotten as much coarse or refined food
> as I need to fill myself up. This body of mine is light & suitable for
> work. Why don't I make an effort for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized?'[1] So he makes an effort for
> the attaining of the as-yet-unattained, the reaching of the
> as-yet-unreached, the realization of the as-yet-unrealized. This is
> the sixth grounds for the arousal of energy.
> 
> "Then there is the case where a monk comes down with a slight illness.
> The thought occurs to him: 'I have come down with a slight illness.
> Now, there's the possibility that it could get worse. Why don't I make
> an effort beforehand for the attaining of the as-yet-unattained, the
> reaching of the as-yet-unreached, the realization of the
> as-yet-unrealized?' So he makes an effort for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized. This is the seventh grounds for
> the arousal of energy.
> 
> "Then there is the case where a monk has recovered from his illness,
> not long after his recovery. The thought occurs to him: 'I have
> recovered from my illness. It's not long after my recovery. Now,
> there's the possibility that the illness could come back. Why don't I
> make an effort beforehand for the attaining of the as-yet-unattained,
> the reaching of the as-yet-unreached, the realization of the
> as-yet-unrealized?' So he makes an effort for the attaining of the
> as-yet-unattained, the reaching of the as-yet-unreached, the
> realization of the as-yet-unrealized. This is the eighth grounds for
> the arousal of energy.
> 
> "These are the eight grounds for the arousal of energy."

If you consider the ability to concentrate the mind, as a kind of mental energy, then according to the [Mettanisamsa Sutta (AN11.16)][4], metta or loving kindness is one way to gain mental concentration easily:

> "Monks, for one whose awareness-release through good will is
> cultivated, developed, pursued, handed the reins and taken as a basis,
> given a grounding, steadied, consolidated, and well-undertaken, eleven
> benefits can be expected. Which eleven?
> 
> "One sleeps easily, wakes easily, dreams no evil dreams. One is dear
> to human beings, dear to non-human beings. The devas protect one.
> Neither fire, poison, nor weapons can touch one. **One's mind gains
> concentration quickly**. One's complexion is bright. One dies unconfused
> and — if penetrating no higher — is headed for the Brahma worlds.


  [1]: https://www.accesstoinsight.org/tipitaka/an/an08/an08.080.than.html
  [2]: http://obo.genaud.net/dhamma-vinaya/pali/an/08_eights/an08.080.pali.bd.htm
  [3]: https://www.wisdomlib.org/definition/viriya
  [4]: https://www.accesstoinsight.org/tipitaka/an/an11/an11.016.than.html