I could take the 12 volts times 9 amps approach to get the max running load amps would be but I know its not accurate I believe gm would not cut it that close or there would be a lot more blown fuses. 
no, GM didn't "cut it close", figuring 9amps gives enough margin that it's highly unlikely the fuse will blow unless there's a major malfunction .... the fuse info is the best you'll find, there is nowhere other than on the motors themselves that the running amperage is specified, so you might try looking there - I've never seen watts referred to anywhere
I'm puzzled that you're adding every possible modern 'amenity' and are concerned about 'overkill' on the alternator, why not get the best you can afford and have that margin of safety??
Bill
