Puttering around at 20mph with an extra 30lbs will cost you some power when you encounter even a modest grade.
Agreed, for then there's a net gravitational vector tugging on the vehicle.
According to the calculator, 30lbs cargo will need +70 more watts on a 5% grade. A 10% grade will require +132W to maintain 20mph. (A typical curb-cut or driveway apron can be 7% grade.)
No argument here.
Point: my real world does not have a rocket ship, it has has topography. When I see someone's route that has no grades and no variations in throttle, then maybe I'll believe that weight is a non-issue.
Again, agreed. But since the point of this thread seemed to be comparing efficiency values, hills and acceleration have to be standardized too. In practice this means "on the level, at a particular speed, no wind, no pedaling," else there's no comparing numbers derived from rides up varying grades at varying speeds. So, if we're doing the best we can reasonably do and comparing numbers observed "on the level, at a particular speed, no wind, no pedaling", weight becomes a non-issue.
Like Lowell notes above, we ride through very different terrains, and so there's no comparing efficiency numbers so derived, unless we all agree to ride on a "standardized track".
For this exercise, we can either agree that track is "on the level, at a particular speed, no wind, no pedaling", or we can all fly over to your house and ride around your area (Pay my plane ticket and I'd be happy to....)