The general meaning of energy is the ability to be active. If you have a lot of energy, it means you like to be active. If you plan a low-energy day, it means a day of lounging around.
Energy is from Greek energeia "activity, operation," from energos "active, effective," from en "in" plus ergon "work." Specialized senses of the word energy include the power that comes from sources such as heat or electricity, and the ability of sources such as heat or light to do work. In physics, an erg is the centimeter-gram-second unit used to measure the amount of energy or the amount of work done.