In general, my experience is that 40 km in 1 hour needs an average in the low 200’s Watts on a Wattbike.
A Wattbike is probably one of the few calibrated machines, other devices use algorithms etc.
However, in the real world watts/kg is what really matters. A 100kg rider could ride 40 km on a Wattbike, but would be slower than a 60 kg rider. In hilly country the gap increases of course.