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