I already posted that those that CAN do 300W/hour are uncommon.
Revising - hardly anyone ever as measured.
I'm going to revise my post and say those that actually
DO 300W/hour are real rare, hardly anyone
DOES 300W/hour. Take the USA (1:1M would be 350 people) - find a Strava segment of :50-:70min where there are even top 50 doing >300W.
Often those same 30 riders that do it up one climb are seen doing it up others. There are not all that many climbs that take an hour by those that can lay down that power.
When the climbs are epic as some posted here - they lose power due to O2 levels etc. So they didn't really do 300W for an hour.
In a race, they are generally trying to use as little power as they can, so there are some WT pros that have breakaways where they do that, but the normal case is racers do not.
If a national ITT took more than an hour, they would, but it doesn't, so they don't.
No question it is done for the hour record, but it is not measured using a PM, so was it real power? And could they have done 300W for an hour 5 min? Sure. Did they - no.
I was looking at my kids's and others climb segment where he had no PM but was close to those with a PM and published well over 300W/hour. I expect he did about 350W. So...I looked up his hour power for the time he was at college 2016 (light), 2020 (dark) in the pic below . Note again, he does not race with a PM.
No where near 300W ever, although he used a PM infrequently I just don't think he did outside of big climbs. The RR and crits didn't need that average.
I think actually
recording 300W average over an entire hour is very very rare.