A power transfer or energy transfer is the process of passing working energy from one system or device to another. Such transfers can be between energy types, as from electricity to mechanical motion, or within a type, from one mechanical motion to another.

For instance, a coal-fired power plant involves these power transfers:

1. Potential energy in the coal converted to thermal energy
2. Thermal energy converted to kinetic energy in steam
3. Steam energy converted to mechanical energy in the turbine
4. Mechanical energy of the turbine converted to electrical energy, which is the ultimate output

In such a system, the last step is almost perfectly efficient, the first and second steps are fairly efficient, but the third step is relatively inefficient.

In a conventional automobile, these power transfers are involved:

1. Potential energy in the fuel converted to kinetic energy of expanding gas via combustion
2. Kinetic energy of expanding gas converted to linear piston movement
3. Linear piston movement converted to rotary crankshaft movement
4. Rotary crankshaft movement passed into transmission assembly
5. Rotary movement passed out of transmission assembly in second step
6. Rotary movement passed through differential
7. Rotary movement passed out of differential to drive wheels
8. Rotary movement of drive wheels converted to linear motion of the vehicle