The Kusita-Arambhavatthu Sutta (AN8.80) provides 8eight grounds for the arousal of energy:. In the original Pali version, it appears that the word used is "viriya".
"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), 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.
