Energy is the 5th Mental Perfection:

Energy means enthusiasm, effort, exertion and interest!
Energy means eagerness, endeavor, initiative and intensity!
Energy means action, force, power, drive, and hyper keenness!
Energy means devotion, dedication, determination, and commitment!
Energy means enjoyment :-)

Energy pave the way for success in all, and any project ...
Energy is thereby instrumental for all the perfections!
Without Energy no achievement
can ever be accomplished..
The proximate cause for Energy is a sense of urgency!
The characteristic of Energy is striving, its function is
effort, and the manifestation of Energy is endurance.

The Blessed Buddha said about the perfection of Energy:

Get up! Sit up!

Of what use are your dreams ?
How can you sleep, when mentally sick,
stabbed by the arrow of urge and craving.
Sutta Nipāta 331

Get up! Sit up!
Push on your training, until reaching pure peace!
Don't let the king of death -Mara- see you sloppy and
thus delude, and dominate you like a stringed puppet...
Sutta Nipāta 332

Possessed of Energy and Endurance:
Be always Earnest and Keen in your training.
The clever one is not satisfied before the final
goal of ending all misery is completely achieved.
Theragatha 585

It is too cold, it's too hot, it's too early, it is too late!
Such bad excuses, makes one give up the training and
miss one more precious opportunity to end suffering...
DN 31

This straight and Noble Way has now been clearly shown:
Don't hesitate, keep walking forward, and do not turn around.
Urge yourself to advance further by your own Energy!
Only thus will you obviously approach, and attain Nibbāna!
Theragatha 637

The 2 efforts to prevent and to eliminate all evil.
The 2 efforts to develop and to maintain all good.
These are the 4 right efforts, taught by the Buddha.
AN II 17

And what, friends, is feeding the Energy Link to Enlightenment,
not yet arisen, and food too for boosting of any present Energy ?
1: The element of initial initiative,
2: The element of launching into action,
3: The element of persistent endurance.
Systematic attention to these 3 aspects, is initiating your Energy
and is boosting any Energy
that is already present .
Samyutta Nikāya XLVI 51 Bojjhanga-samyutta

At such times, when the mind is Slow, Sluggish, and Heavy:
Then it is the right occasion: for cultivating:
The Investigation-by-curiosity enlightenment-factor,
The Energy-of-Enthusiasm enlightenment-factor,
The Rapture-of-Joy enlightenment-factor...
Why is it so ? When the mind is slow, sluggish, and heavy,
it is easily raised up by exactly these 3 mental qualities.
Suppose, friends, that a man wants a fire to blaze up,
and he put on dry grass, dry wood sticks, and blow it
with dry hot air, and do not cover it with any dust,
would that man then see his fire blaze up ?
Certainly So, Lord...  SN V, 46. Bojjhanga-samyutta

The Buddha when
Moggallana where sitting nodding in the jungle night:
Well then, Moggallana, whatever experience you had in mind when
drowsiness demoralized you down, don't attend to that experience,
don't follow it. Remember instead the Dhamma, as you have heard and
memorized it, reflect on and examine it! Then raise up and repeat aloud
the details of Dhamma, as you have learnt it! Then pull both your earlobes
and rub your limbs with both your hands.! Then get up from your seat,
and after washing your eyes with cold water, look around and upward
in all directions, and identify the major stars and planets!
Then attend to the experience of inner light, resolve on the clear
perception of daytime, by night as by day, and by day as by night!
By means of an Awareness thus open, unhindered and vivid, develop the
bright mind. Its possible, that by doing all this, you will shake off your
lethargy... But if by doing this you don't shake off your laziness, 
then continually noting what is both in front and behind set of a
distance to meditate walking back and forth, your senses inwardly
settled, while your mind is not getting lost outwards. It is possible
that by doing this, you will shake off all your mental sluggishness...
Anguttara Nikāya VII 58

Born as the Brahmin Mahajanaka, the Bodhisatta was once aboard
sinking ship far from shore. All the crew were in great panic...!
The Bodhisatta though ate his belly full of sugar and ghee, oiled his
clothes and swam continuously for 7 days towards the shore, until
he was rescued by a guarding female devata. Later he remembered:
"Even far out at sea, where many men were lost,  yet still unruffled
by worry was my mind. I just kept swimming for seven days!
This was my ultimate perfection of Energy."
Mahajanaka-Jātaka no. 539

One who exerts effort,
One who is
always alert and aware,
One who is restrained and controlled,
One who is kind and considerate in all dealings,
One who does good deeds voluntarily and deliberately,
One who lives according to the real truth of the Dhamma,
carefully and genuinely, the fame of such one always grows!
Dhammapada Illustration 24 Background Story 24

By arousing and inspiring oneself to put forth energetic effort,
By meticulous carefulness, by well restrained self-control,
The wise and clever ones make themselves a
safe island,
no flood or fever of desire, can ever overwhelm!
Dhammapada Illustration 25 Background Story 25

ore on Energy, Enthusiasm, and Eager Effort: (Viriya):
The_Definition_of_Energy, The_Arising_of_Energy,
The_Chief_Hero, Avoiding_Efforts, Dual_Energy

Energy Viriya, Feeding Energy, Energetic_Effort,
The_Ability_of_Energy, The_Origin_of_Energy,
Power_of_Energy, Ballanced_Energy,

More of the 10 mental perfections (paramis):

Enthusiastic is Energy!