Useful energy refers to energy that is transferred in the desired form and location, while wasted energy is energy that is not usefully transferred or transformed during a conversion process. According to the first law of thermodynamics, the total energy input must equal the total energy output, but some energy is always wasted due to imperfect conversions. Useful energy output is what is desired for a particular application, while wasted energy is dissipated in unwanted forms like heat.