Definition (care of Wikipedia):
One watt is one joule of energy per second.
1 W = 1 J/s = 1 newton meter per second = 1 kg·m2·s?3
Watts is watts, but there is no formula for correlating watts on an elliptical or watts on a bike (or a rowing erg, for that matter.) Doesn’t mean you can’t train by watts in another discipline. You can use the theory of wattage training from cycling and apply it to other machines.