Energy, we can't do much without Energy. We have to know how to cause Energy to arise. Energy comes from confidence. So in order to develop a great deal of energy, we have to develop confidence. When the Buddha talked about the Five Spiritual Powers, he put Confidence before Energy because without Confidence and believing in something, we don't muster the energy to do it. So confidence in ourselves and confidence in the path will bring forth Energy. How do we sustain this energy? Wisdom and Compassion help with sustaining Energy.