Energy may change form many times on its way to doing work in your home, such as toasting bread.  In the process, some energy is converted into unwanted thermal energy or waste heat.  In fact, it is impossible to convert one form of energy into another without wasting some energy.
 
In many energy conversions, more energy is wasted than is used for work.  For example, automobile engines typically waste more than two-thirds of the total energy used, primarily through heat.

On the other hand, electrical motors convert electrical energy into mechanical energy with only about 10 percent of the energy wasted via heat.  Energy efficiency means converting energy from one form to another with the least possible waste.law of nature is called the law of conservation of energy.