Unit of power

Since $P=W/t$, power is a scalar quantity, with dimensional formula $[ML^2T^{-3}]$ (energy's $[ML^2T^{-2}]$ divided by time's $[T]$). Its SI unit is the watt (W), named for James Watt, the inventor of the practical steam engine: one watt is the power when one joule of work is done in one second, $1\text{ W} = 1\text{ J s}^{-1}$.

Larger multiples are used for bigger machines and power plants: $1\text{ kW}=10^3\text{ W}$, $1\text{ MW}=10^6\text{ W}$, $1\text{ GW}=10^9\text{ W}$. Motors, engines and automobiles still commonly quote an older, non-SI unit, the horsepower (hp), with the conversion $1\text{ hp} = 746\text{ W}$. …