You would be better off thinking more about how much biking you can realistically do per day or week. Obviously the more you do, the more calories you burn. But you are not going to realistically be able to ride more than maybe an hour or two per day as a beginner - and probably considerably less if you are unfit and overweight.
So I would say just start riding and see how much you can cope with. Along with a sensible diet, you will almost certainly lose weight in the long term.