In physics and engineering, including mechanical and electrical engineering, energy efficiency is a dimensionless number, with a value between 0 and 1. The energy efficiency of a process is defined as:
Energy efficiency can also refer to the use of techniques or practices that minimize the use of energy.
An efficiency of 1 (100%) is impossible - the second law of thermodynamics prohibits it. Theoretically, the mutual annihlation of matter and antimatter would be completely efficient, but the collection of the released energy would not be.