Energy

In the Revealing the Profound Secrets Sutra the Buddha tells World Voice Perceiver Bodhisattva:

There are three kinds of energy. The first is the energy that covers one like armor. The second is the energy to turn towards and developing wholesome dharmas and putting them into practice. The third is the energy to put into practice that which benefits sentient beings.

T676, 16.705c18-705c20

The Awakening of Faith in the Mahayana describes the perfection of energy as follows:

How do you practice the gate of energy? It is to have a mind that does not slacken or withdraw from doing good deeds. It is to be firm in one’s resolve and far from timidity. Recall that since the remote past, one has vainly endured immense physical and mental suffering to no benefit. Therefore, one should make efforts to cultivate all that is meritorious, benefiting oneself and benefiting others, and quickly relieving all suffering. Moreover, if a person, despite cultivating faith, has many extreme transgressions and evil karmic obstacles from past lives, they will be disturbed by devils and demons. Or they may be entangled in various worldly matters. Or be troubled by the suffering of illness. There are many such obstacles. Therefore, one should be courageous and energetic by making obeisance to all the buddhas at the six four-hour intervals of the day and night, sincerely repenting [before them], requesting [that they teach the Dharma], rejoicing [in the merits of others], and dedicating one’s merits [to the attainment of] awakening. Always avoid idleness to avoid all obstacles and increase one’s wholesome roots.

T1666, 9.582a01-582a12

In the Bodhicharyavatara (Introduction to the Practice of Awakening) by the Indian Mahayana monk Shantidva (c. 685 - c. 763), the following is said about the importance of the perfection of energy or vigor:

1. Patient in this way one should cultivate vigor, because Awakening depends on vigor. For without vigour, there is no merit, just as there is no movement without wind.

2. What is vigor? The endeavor to do what is skillful. What is its antithesis called? Sloth, clinging to what is vile, despondency,, and self-contempt.

The Bodhicaryavatara 7.1-2 as translated by Kate Crosby and Andrew Skilton (adapted).

Shantidva gives the following guidance for developing the energy to attain buddhahood:

31. The powers of desire, perseverance, delight, and letting go, all lead to the fulfillment of the needs of living beings. Out of the fear of suffering, and while meditating on the praises, one should create desire.

32. Uprooting the opposition in this way, one should endeavor to increase one's exertion through the powers of desire, pride, delight, renunciation, dedication, and self-mastery

Ibid, 7.31-32

What Shantideva means is that to counteract sloth one should cultivate the righteous desire or aspiration to attain buddhahood. To counteract despondency one should develop perseverance. To counteract self-contempt one should be able to take delight in what one is able to accomplish, to make the bodhisattva practice a “labor of love.” To counteract clinging to what is vile, one should develop the ability to let go and renounce all that is holding one back. One should also develop pride or self-respect, continuing dedication of one’s efforts to achieve the goal, and self-mastery.

Among many examples, here is a story in which the bodhisattva who becomes Shakyamuni displayed the perfection of energy :

 In a previous life, Shakyamuni Buddha was practicing energy as Earnest Donor, a prince who gave away all his family’s wealth to help the impoverished. When the wealth ran out, he went in search of the wish-fulfilling gem so he could conjure enough food clothing, medicine, and other items to meet everyone’s needs. He finally received the jewel from the king of the dragons who controlled the waters, but the lesser dragons stole it back. Prince Earnest Donor then tried to empty the oceans with a turtle shell until they returned it. He was helped by gods sympathetic to his cause, who were moved by his earnest efforts to get the jewel back. With the help of the gods the dragons were convinced to return the wish-fulfilling gem, and so Prince Earnest Donor could return home and use the gem for the benefit of all.

A version of the story of Earnest Donor is told in the Great Perfection of Wisdom Treatise, though that version leaves out the part about Earnest Donor trying to empty the ocean. Elsewhere in the Great Perfection of Wisdom Treatise, the attempt to empty the ocean is told. For example:

Moreover, with unceasing exertion the bodhisattva one-pointedly (ekacittena) seeks buddhahood; such effort merits the name of virtue of patience. Thus the bodhisattva Hao che (Mahātyāgavat) seeking the philosopher’s stone (cintamaṇi), filtered the water of the ocean using his nerves and his bones, and did not stop working before having found this philosopher’s stone; he gave it to beings to ease their sufferings. The bodhisattva thus accomplishes difficult things; this is his virtue of exertion.

A version of this story is also told The Three Jewels: A Study and Translation of Minamoto Tamenori's Sanboe.

Nichiren Shonin mentions the story of Earnest Donor (or Prince Dana or Alms-giver) in his writings as an example of energy. (e.g. WND2, p. 136 & WNS7, p. 139)