Horsepower-hour to gigajoules converter above calculates how many gigajoules are in 'X' horsepower-hour (where 'X' is the number of horsepower-hour to convert to gigajoules). In order to convert a value from horsepower-hour to gigajoules (from hph to GJ) type the number of hph to be converted to GJ and then click on the 'convert' button.