# Efficiency

## Usefulnes of a process

Efficiency is the measure of "usefulness" of an operation, process or machine - and can be expressed on the generic form

*μ = W _{o }/ W_{i} (1)*

*where *

*μ = efficiency *

*W _{o} = output from the operation - can be work, power, produced products ... *

*W _{i }= input to the operation - can work, *

*power*

*, input products ...*

Efficiency can be expressed as a percentage or as a per-unit decimal fraction of *1.*

### Example - Efficiency of an Electric Motor

The input power to an electric motor is *2000 W*. The output power to the shaft is *1800 W*. The efficiency of the motor can be calculated as

*μ = (1800 W) / (2000 W)*

* = 0.9*

* = 90%*

### Example - Efficiency of a Wind Mill

The teoretical energy potential for a wind mill is *300 kWh* at given conditions. The power produced is *70 kWh*. The efficiency of the wind mill can be calculated as

*μ = (70 kWh) / (300 kWh)*

* = 0.23*

* = 23%*