What is Energy ?
Energy is (noun) 1. the force or strength of a person He used up a lot of energy rushing around doing the Christmas shopping. She put all her energies into her art gallery. (NOTE: The plural is energies.) 2. a power which makes something work the use of atomic energy or nuclear energy to make electricity We try to save energy by switching off the lights when the rooms are empty. Trams are an energy-efficient method of public transport. (NOTE: no plural)
source: Easier English, Student Dictionary Upper Intermediate Level