One watt is 0.0013410221 mechanical horsepower, so one horsepower is 745.7 watts. At this scale the pair belongs to small motors, cycling power meters and anything where a fraction of a horsepower is the useful quantity.
About the units
The watt is the SI unit of power, one joule per second. The mechanical horsepower is 550 foot-pounds per second, 745.6998715822702 W exactly. Small equipment is where the fractional horsepower ratings appear: a bench grinder at half a horsepower is 373 W, a garage door opener at a third is 249 W, and a domestic fan at a twentieth is 37 W. American motor catalogues list these as 1/2 HP, 1/3 HP and 1/20 HP, which is a convention with no metric equivalent — European catalogues simply state the watts.
The exact factor
The exact reciprocal is 1 W = 0.00134102208959503 hp. For human power the numbers are humbling: a fit cyclist sustains about 250 W, which is a third of a horsepower, and a world-class sprinter peaks briefly above 2,000 W, or 2.7 hp. A working horse sustains roughly one horsepower by definition, which was Watt's point — the unit was a sales argument comparing his engines to the animals they replaced.
Where you meet this conversion
Small motors, power tools and pumps, where American products are rated in fractional horsepower and European ones in watts. Cycling and rowing power meters read in watts, and comparing an athlete's output to an engine's requires this conversion. Electric bicycles are regulated by motor power in watts in the EU — 250 W for pedal assist — while American classes are defined differently, which is why the same bicycle can be legal in one market and not the other.