Combined heat and power units are systems which produce electric current and heat. To do so, they employ the principle of power- heat coupling.
 
The higher overall efficiency from the fact that the waste heat from producing electricity is used directly where it is produced. This localised use of waste heat means that 80 to over 90% of the primary energy is used. Using this kind of power plant can save um to 40% of the primary energy.

The electrical capacity of these units ranges from five kilowatts to five megawatts. They can be driven with diesel or gas engines or also gas turbines.