The question of how many watts equal 1 horsepower depends on how you define horsepower. One mechanical horsepower unit is approximately equal to 745.7 watts. One electrical horsepower, used for rating electrical motors, is equal to 746 watts.
Horse power was originally defined as the power output of a single common draft animal. One horse could sustain the work of 1 horsepower. As steam engines began to supplant horses during the Industrial Revolution, the unit "horsepower" came in to use to define the power output of mechanical and then electrical engines. James Watt, an inventor of the late 18th and early 19th century, originally developed the horsepower unit; in 1882, the SI unit of power was named the watt in his honor.