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