Energy describes the ability to obtain a given result or amount of output while using less energy input. energy describes the practice of reducing wasteful or unnecessary energy use.

Energy efficiency describes the ability to obtain a given result or amount of output while using less energy input. energy conservation describes the practice of reducing wasteful or unnecessary energy use.  

  Energy efficiency represents the ratio between the useful output and input of an energy.

Energy conservation represents a process which is used to reduce the quantity of energy that is used.

Ithink that its like 15 mph.