There are two types of energy: Mental energy and physical energy. Although we need a certain amount of physical energy in our practice, if we know how to bring forth mental energy, often the physical energy will follow. Energy comes from interest, and interest comes from Right View, Right Intention, Right Motivation, and also from confidence.
Confidence is a spiritual power, it's the first of the Five Spiritual Powers. Unless we have a certain amount of Saddha or confidence it's difficult to get energy because we always believe that it's not possible. Or we don't have enough faith or confidence in the Path to walk it and let go of some of the things that block us from directing our energy towards things that are important for our spiritual development. So if we are lacking energy in the practice it may be very helpful to increase our confidence.
Also another thing that develops energy is Generosity. It's amazing how much energy arises from being generous. Taking joy with our good generosity brings more joy and happiness. Another thing that brings energy is if we have good Morality. Because a lot of energy is dissipated by feeling bad about ourselves and what we did. So if we are more careful about our Morality and live by the good values, trying not to harm ourselves and others then we will find we will have less regret. Then less energy is dissipated from worrying about these things, trying to fix them up or criticizing ourselves.
And develop confidence inspiring objects like Sympathetic joy, reflections on the Paramis and ways to develop them further, reflection on the Buddha, Dhamma, Sangha, reflection on one's Generosity and good Kamma. And remember in order to be able to reflect on our Generosity and good Kamma we have to make some. All of these help us to develop more Energy.