Electron-volts
An electron-volt (eV) is used in the fields of atomic and particle physics. It is the amount of energy gained or lost by a single electron when it is accelerated through an electric potential difference of one volt.
Btu
The British Thermal Unit (Btu) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is primarily used in the United States to describe the energy content of fuels and the power of heating and cooling systems.