The greens will give you the energy boost you're looking for. Actually, when you're fasting, your digestive system shuts down and that is where your body will gain its extra energy... but usually this extra energy is put to work to detox (via autolysis) and to heal. However, there are many veggies, primarily the greens that give fantabulous energy boosts.
Below are a couple links to juice recipes and to various articles pertaining to juice fasting, including articles that discuss the properties of fruits and veggies, so one can tailor their fasts (as you'd like to do) according to the properties of the produce one drinks.