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