Is maintaining a constant wattage for 20 mins harder than constantly fluctuating between, say, 80% and 120% of that wattage?
The reason I ask - I did a 20min race on zwift and was given an average power of 260w, but I’m sure I couldn’t do 260w for the whole time. I have a new FTP based on approximately 95% of that 20 min average … and I worry it has been overestimated.
No it’s actually the opposite 
Thats the whole reason the idea of NP was concocted to weight the time spent at higher powers more than time spent at lower powers.
The NP of overunder intervals will be higher than the average power. You could argue that the NP would be a better number to base FTP estimations on.
If your average (mean) power was 260w then it’s safe(ish) to take 95% of that as your FTP. I say “ish” because the orginal method to determine FTP from a 20min effort depended on having some form of an effort beforehand to empty anaerobic stores.
I wouldnt worry about it - I’m sure your FTP is valid enough 
In other threads you mentioned your FTP being 244W, 245W, and now 247W… these are all the same FTP, not only because of the measurement accuracy of typical power meters (1-2%). I recommend rounding and sticking with a value until you detect significant changes and ignore the noise.
Also, Zwift has the standard 20-minute FTP test protocol, it’s a good workout, give it a go.
It was an approximation used in order to ask a general question. I’m not actually saying my FTP is changing by one watt.
I’m a relative weekling but on a TT etc, I can hold a certain 20min power constantly at around 92% max HR without drop off, but a few over pushing me to 95% MHR will cause my power to drop off. I may be able to recover a bit in an under period but it certainly isn’t as doable as a constant power.
I think this is an “it depends” question. Since people’s ability varies above threshold a lot 120% for one person might be their 3min power while for another it could be their 8min power. So if you’re doing O/U with 2min over and 2min under. That 2 minutes at 120% will be much harder for the first person than the second.
Also, some people find they struggle with constant power because the constant tension on their muscles causes them to feel like they tighten up. While varying that power can vary which muscles get used and how hard they’re pressing.
Generally though, I’d say the O/U is going to be harder than the constant interval.
Are you sure this is average and not normalized? I’d assume if you could average it in a Zwift race then you could do it straight through. Especially if that race had near all out efforts to close gaps, attack, etc. But motivation might also be an issue in a solo constant effort vs a race.
Based on the 20min FTP test you can’t just take 95% of any 20min average but it should be of a near steady state 20 minute effort. So a Zwift race wouldn’t be a good place to take it from unless it was a flat-ish Zwift time trial.
And that’s not getting into the efficacy of a 20min test and whether it is actually an accurate way to test your FTP. Read here for that explanation and for a better way to test FTP: The FTP Test: Physiology and New Protocols
I forgot to say if you never did the FTP test protocol (a 5 min Anaerobic blow out before a pure Aerobic effort) I’d use 90-92% as an estimate. The 95% is based on a pure aerobic effort and the test protocol is designed to burn off the anaerobic element which can inflate things.
It depends, one’s ability to work above ftp is highly variable and changes throughout the season based on their training.
One person could find that a couple minutes at 120% is not very taxing whereas another athlete with the same ftp could find that to be difficult.
This would impact the RPE of the workout quite significantly between different athletes.
Whereas just riding at or slightly below ftp should be much closer in terms of RPE (assuming a correctly set ftp) up to the point that each person approaches their TTE at ftp.
Maintaining a constant power is always the most efficient way to achieve a specific average power though, surely? 
Physiologically probably, however I have worked with several people who preferred to do their ftp work as over/unders rather than as one long steady state interval.
The variation in power made the workout feel easier for them compared to just riding steady state the whole time. It all averaged out to be basically the same so I don’t worry about it too much.
Ah ok.
Over under intervals will always have higher NP and IF than steady state intervals at the same average…. but I accept that’s not always the full picture when it comes to something “feeling” harder.
How can it be any other way? Over max lactate steady state is not particularly efficient, it’s gonna cost you in the end no matter “ how you feel about it “
Joe