So personally I find riding hills easier to put out power on the flats, so my question is:
Does it make a difference if I complete a workout on a hill, rather than on a flat.
Eg. I have a workout planned this weekend which is 3 x 20min @ 88-94%. I have a 5km climb nearby and I think it would feel easier to hit the numbers doing 3 x 20 minute repeats, than if I was to do 3 x 20 minute efforts on flat/rolling terrain and hit the same numbers.
Would I get the same benefit? 
Yes, unless your goal is to learn to pace on the flats for TTs or other events. Hitting high power targets on flat roads is a skill, it’s difficult. Much easier to hold the higher power on a hill, if you have a hill available that takes 20 minutes then absolutely use it. A lot of people don’t have a place or hill that affords 20 minutes of effort with zero interruptions (lights, intersections, traffic, etc).
1 Like
We address this question over in our FTP:FAQ article if you want to check it out:
3 Likes
I wish I had a 20 minute uninterrupted hill near me for reps
2 Likes
Try it both ways - one week hills - the next week on the flats.
I like steady state on the flats but if I were pro I’d be a classics type rider. Guys in my club with better power to weight ratios drop me on climbs but then they get dropped on the flats when it gets fast and furious.
Thanks guys. Ended up opting for the rolling terrain today instead of the hill. Worked well 
Must admit I’m lucky to live where I do. At the base of an escarpment with multiple climb options, or flat & rolling roads in other directions. Spoilt for choice! 