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