Efficiency is the fraction of the input power which reaches the output load. Typically, the more efficient a system is, the better. Any (non-superconducting) circuit with current flowing through it loses some energy to heat. As such, no closed system is 100% efficient.

The lost energy is not destroyed, merely transformed into a less-useful form. This is typically waste heat. If this waste heat is large enough, heat sinks or other cooling means may be required.

Efficiency is the fraction of a quantity that can be used to perform a desired task. For example, in power conversion and transmisison systems it would be the fraction of the input power that reaches the output load. Typically, the more efficient a system is, the better.

The term can also be applied in other ways. In information processing and communications, efficiency can refer to the fraction of physical bits or symbols that actually convey application data, as opposed to "overhead" bits used for synchronization or error detection/correction.

Efficiency is the fraction of the input power which reaches the output load. Typically, the more efficient a system is, the better. Any (non-superconducting) circuit with current flowing through it loses some energy to heat. As such, no closed system is 100% efficient.

The lost energy is not destroyed, merely transformed into a less-useful form. This is typically waste heat. If this waste heat is large enough, heat sinks or other cooling means may be required.