The Kusita-Arambhavatthu Sutta (AN8.80) provides eight 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, eleven advantages are to be expected from the release
(deliverance) of heart by familiarizing oneself with thoughts of
loving-kindness (metta), by the cultivation of loving-kindness, by
constantly increasing these thoughts, by regarding loving-kindness as
a vehicle (of expression), and also as something to be treasured, by
living in conformity with these thoughts, by putting these ideas into
practice, and by establishing them. What are the eleven?
. 1. "He sleeps in comfort. 2. He awakes in comfort. 3. He sees no evil dreams. 4. He is dear to human beings. 5. He is dear to non-human
beings. 6. Devas (gods) protect him. 7. Fire, poison, and sword cannot
touch him. 8. His mind can concentrate quickly. 9. His countenance is
serene. 10. He dies without being confused in mind. 11. If he fails to
attain arahantship (the highest sanctity) here and now, he will be
reborn in the brahma-world.
