I haven’t seen any Mennos with gear for measuring watts output, but I did come across the Bike Calculator which is an interesting device for determining how many Watts one would need to put out to average a certain speed over a certain distance with a certain wind and a certain gradient at a certain altitude. To average 40 kph over my usual 40 km TT distance into the usual 50 kph head wind when on the hoods would require a monstrous 1750 watts. On the other hand, to average the same with no wind would require an average of 350 watts. Check it out here at bike calulator