Voltage is an electrical potential difference, the difference in electric potential between two places. The unit for electrical potential difference, or voltage, is the volt. The volt is named in memory of Alessandro Volta. One volt equals one joule per coulomb. The symbol for the unit volt is written with an uppercase V as in (9V). According to the rules of the International System of Units, the symbol for a unit with a name derived from the proper name of a person is uppercase.