Energy units are measurements of energy used to quantify the amount of work done or energy expended. Joules, rads, and ergs are all units of energy. Joules (6 letters) is a unit of energy equal to the work done when a force of one newton is applied over a distance of one meter. Rads (4 letters) is a unit of absorbed radiation dose, equal to an energy of 100 ergs per gram of irradiated material. Ergs (4 letters) is a unit of energy equal to the work done by a force of one dyne acting over a distance of one centimeter.