"He gives strength to the weary, and increases the energy to him that has no strength." Isaiah 40. 29
"Renewable energy" is an expression that speaks to the Christian! There are also in his life energies that are exhausted. Many of us, in terms of physical abilities, have to say : "I can not do what I did at the age of twenty." Even the intellectual abilities themselves diminish markedly with age. So we need "renewable energy" on a spiritual level. The Christian knows the sources :
• God Himself
"Blessed is the man whose strength is in you" (Psalm 84: 5).
"The Lord is the strength of my life" (Psalm 27: 1).
"The salvation of the righteous comes from the Lord ; it is their strength in the time of trouble" (Psalm 37:39).
• The Holy Spirit, the inner energy of the believer
"God hath not given us a spirit of fear, but of power, and of love, and of common sense" (2 Tim. 1: 7).
"I am full of power by the spirit of the Lord" (Micah 3: 8).
"... so that, according to the riches of his glory, he gives you to be strengthened in power by his Spirit as to the inner man" (Ephesians 3:16).
• The Word of God
"As he spoke with me, I took strength" (Dan 10:19)
• Prayer
"In the day that I cried, thou hast answered me ; thou hast increased the strength of my soul" (Ps 138: 3).
These are inexhaustible sources of energy even as our human energy weakens (Neh. 4:10) or runs out (Job 3. 17).
But let us not forget our role towards one another : "Straighten your weary hands and your failing knees" (Heb.12: 12).
Source (To pleasing the Lord)