While going through transformation processes, some of the initial energy gets lost i.e a part of it can’t be used.
Energy efficiency is a technical term that shows how much of primary energy can be transformed into useful energy and how much of it gets lost in the process. It is shown as a percentage.
Energy appears in different forms. Potential, kinetic, heat, electrical, chemical and nuclear energy are basic energy forms which allow for explaining all other known natural processes.
Classification of energy types is done based on its level of transformation from energy types that can’t be used directly into those that can. With that in mind we divide energy into: primary, transformed and useful energy.
Primary energy is the energy contained in the energy source itself. The energy sources are usually gained directly from nature without any need for transformation processes.
We divide them into:
Fossil (stone, dark coal, crude oil, natural gas, liquid gas,...)
Nuclear
Renewable (sun, water, wind, biomass,...)
Seeing how only a few of the primary energy types can be used directly, we need to transform them into usable energy forms. Therefore, transformed types of energy are usually created to gain usable energy. For eg pellets, enriched nuclear fuel, fuel oil, electrical energy, heat,...)
Useful energy is energy that is used to help the end users with their energy needs, meaning energy that we can use directly. An end user can use this in a form most suited to their needs for ex. the heat of electrical heating plate on a stove.