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