This is the key value in cycling. Whether your a climber who can do 400w a GC rider who can win grand tours; 450w, or a rouleur who can do 430w, it's the key value, along with weight of course, which together determine what type of rider you are, or indeed, if you're a good rider at all.
It's measured by going flat out for twenty minute and having lactate samples taken at five minute intervals.
All it is is a tool to prescribe training zones accurately. And there is no prize for going fastest in the lab, but it is interesting.
Different people accumulate different quantities of lactic acid and many things about performance and physiology, I accumulate very little, which I am told means I am more endurance and less explosive. My power output was down a little on last year, not sure why. But I am fatter also so I presume I am less trained, or haven't trained as well as previously. It's negatively encouraging, but encouraging all the same. I will apply myself more now, so that I am that bit faster.
I will also practice pacing. So I know how to go the very fastest possible, as apparently it's possible to try too hard and go slower as a result.
Amazing, three years cycling and I'm still so ignorant about it!