Should a given power output on flat ground be equally draining at the same power output on an incline? Like 200 watts on flat vs. 200 watts on a 10% grade? Obviously gearing comes in to play... eventually you'll drop into harder to maintain cadences as it gets steeper.

About the same for me. I recently had better 20min power on a downwind section than a 20min hill climb. Some other factors at play but I've never noticed a big difference other than for short 30S sprints uphill. I find uphill sprints easier to maintain power as there is less shifting.

I don't have a power meter but am considering getting one... I just dunno why I suck ass at climbing. Maybe it's a mental thing. I'm in the top 1% of most of the flat segments on strava but lucky to crack top 50% on climbs.

My guess is you could lose a few pounds. Extra weight won't hurt you much on the flats but has a proportional impact on your climbing performance. In other words, lose 10% and you'll be 10% faster.

i find climbing a bit more tiring, the pedaling dynamics when climbing feels different than on flat road, where you apply force over a larger portion of the pedaling circle. i think it uses your muscles differently and it takes more training to be good at it.

I think you're onto something there. When climbing a hill the inertial load at the pedals is low much like on a trainer. In contrast riding downwind results in a high inertial load.

Solution: Climb lots.

I'm 6'6" and 235-240... I'm about as skinny as I can get without looking sickly

ya you are a pretty tall dude, but at the same time fairly heavy for your height too. i cant say you are fat without seeing a picture, maybe you have too much upper body muscle mass, which doesnt help on the hills.

Came from a powerlifting background... was 280 when I started cycling, have lost quite a bit of upper body muscle since started cycling. Lean enough to see raised veins in my obliques, but yeah I could lose more muscle if the climbing mattered a ton to me.

powerlifter...yep that makes sense, i guess the definition for "thin" is different when you talk about cycling.

At 235 pounds you're probably having to haul at least 100 pounds more uphill than the top 5% hill climbers on Strava. It's all power to weight ratio for climbing and with your weight you'll require a lot more power to match the fastest times.

trade offs i guess. some people are just more built for the flats, and others are built for the hills.

Ride crits.

Putting 380 into a climb feels easier than 350 on the flat. 6'3", 185 so not a lightweight climber.

How much headwind are we talking?

Flats for me, mainly based on w/kg.

Neither, ever, for anyone. If it's really the same power output or effort, exhaustion will come at the same time regardless of what one is doing.
I will say, however, that it can be a challenge to maintain a lower level of effort going up hill. When on flats, one can from time to time drop to zero effort with no perceived ill effect. That's nearly impossible on inclines, so exhaustion may come sooner, as a matter of time, simply because the effort uphill is unrelieved, while on the flats, there may be a bit of soft pedaling, if not coasting from time to time (averaging out to the same level of effort exerted at a constant rate uphill) - and even fractions of seconds of such relief can accumulate make a difference.

On the flats you're going faster and have significantly more cooling breeze so fatigue is less.

I still enjoy climbing more though.

Build & position could be huge factors. You need to train at either to be good. & train a lot! But there is a reason that even some pros can be great tt guys & not be good climbers, & vise versa. Very few riders are true all arounders

No difference if you don't run out of gears, where a somewhat lower cadence feels right on climbs due to the lower inertial load. If you do the climb can be much more fatiguing - Training and Racing with a Power Meter mentions a racer who got dropped every time he spent more than five minutes at his one-hour power but a cadence below 70 RPM.

In hot weather, I think you have a point.

If its not hot it should essentially be equal. Watts are watts. From a mental standpoint, its easier for the vast majority of people to hold a high wattage level for a sustained period on hills than flats.

Go up a 10% grade, it may take 300 watts just to maintain a decent pace. It feels natural to be working that hard.
The hill pushes you.

Now try to maintain 300 watts on flat ground for a sustained period, and you have to push yourself to keep working that hard and going that fast.

This is why the wattage targets for climbing intervals are typically set a little bit higher than the wattage targets for steady states of the same duration on the flats.

And cadence also is a factor in how tired someone may become. The faster cadence required on a flat could wear out the cardiovascular and aerobic system before the legs tire. It all depends on the person.

I think climbing is easier. Could be that I weigh less than you guys....

Climbing is easier because you're looking forward to resting on the descent. That's why riders agree that Paris Roubaix is the hardest race. There's no letup.

I prepped for a century ride with a hair under 6k ft of climbing by doing a bunch of 60-80 mile flat rides (I live in a valley) and the century was surprisingly easy. I was ready to pig out at the end for sure but it was easier than I thought, even with the last 30 miles being into a headwind.