The watt (symbol: W) is a unit of power or radiant flux. In the International System of Units (SI), it is defined as a derived unit of (in SI base units) 1 kg⋅m2⋅s−3 or, equivalently, 1 joule per second. It is used to quantify the rate of energy transfer. The watt is named after James Watt (1736-1819), an 18th-century Scottish inventor.