Watts to mechanic / hydraulic horsepower: 1 hp(I) = 745.699872 W so P(hp) = P(W) / 745.699872
Watts to electrical horsepower: 1 hp(E) = 746 W so P(hp) = P(W) / 746
Watts to metric horsepower: 1 hp(M) = 735.49875 W so P(hp) = P(W) / 735.49875