I’ve got quite OK indoor power stats to get an outdoors result; I’ll never get one though as I simply don’t have the skill/nerve 
Are you burning too many matches outdoors whereas indoors you pace your effort better constantly looking at computer screen target and not traffic etc. I’m guilty of that.
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Having good success with indoor dumb trainers (rollers and Kurt Kinetic), outside riding and now a smart trainer (Wahoo Kickr) I feel like making power inside is slightly different over the top. IMO riding outside to get the same “feel” over the top or keep power on the pedals I need to shift to a harder gear. It’s just easier outside to let the pedal fall away w/o much power. While physiological I also think there is a mental/concentration aspect to riding outside that you need to get used to. RPE is underrated…Probably has nothing to do with you but, food for thought.
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It’s a fair question.
I think you need to compare apples to apples, so look at your power only (first). Compare the average and NP of your 15min intervals indoors to outdoors - ideally load the same workout as an outdoor version onto your bike computer and execute it out on the hills you are talking about.
I would do this instead of working out indoors so you are in the same routine as last weeks successful indoor intervals. @toribath has some good points on what to look for.
Then you will have an idea of your difference in execution and a better idea what’s going on.
Bear in mind that a 10% increase in FTP May mean that you can express 10% more power in your intervals but that won’t mean 10% faster.
Oh and calibrate your power meter before your outdoor workout.
I have read and really appreciate all of your answers.
@ibaldwin 's answer stuck with me the most and the
answer is so simple and so logical. I guess I got carried away thinking people mostly do indoor workouts that bring the most benefits and only go out “Just for fun” or for “long easy coffee rides”, but in reality, indoor work = outdoor work and even if one does 90% of work indoors, those 10% done outside is very well though out and focused on the specifics of outside riding.
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It’s really easy to get caught up in the hype of indoor training making you faster. It’s efficient and for those of us that are time crunched it really helps.
Having said that there is a lot that you miss indoors. Outdoors the terrain is ever changing, you have wind to contend with, mental fatigue from traffic and the elements, and lastly muscle recruitment on varying terrain, all of these are factors indoor training doesn’t touch on. I like indoor training and use it a lot but I realize you need to supplement with outdoors rides. We ride indoors to get faster outdoors so it’s only logical that some time is spent outdoors.
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This is why I’ll never give up my Kurt Kinetic dumb trainer. It actually feels like riding on the road.
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Theres almost no variance when riding on a smart trainer. If you use ERG mode, it sets the power and you sustain it. Simple as that. Youre in an AC’d room, a nice fan and all the water you could ever drink.
Outside, nothing is constant. Not the gradient, wind, surface, traffic, debris in the road, you name it. When I look back at my power for any given climb, I may vary +/- 150 Watts at any given spot. Youre probably cooked much faster outside bc you spend way more time above threshold, without even realizing it. When youre climbing outside, your brain says “get over this hill as efficiently as possible”…meaning you dont say “hold 250w for the remainder of this climb regardless of what speed that translates to”. When you have 15 minutes of sweet spot on the trainer, your mind says “ehh…keep doin the same thing for 14 more minutes”
Theres no substitute for putting on the miles outside, but Im a firm believer that theres no better way to fill an hours workout than on the trainer.
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Two points…
The best solution to all the problems stated is to do TR outside workouts. Then you get the changing gradients, wind, watt spikes, micro-rests, heat…etc all while getting the adaptations of structured interval work.
To the posters poo-pooing on indoor training/erg mode because it’s TOO constant, perhaps another answer instead of dropping inside training and riding around outside is to become more consistent with your wattage use outside. Meaning you try to eliminate all the big spikes and surges and whatnot and ride in a more constant zone of ~20 watts, basically simulating erg mode when you ride outside. Fundamentally that’s going to be faster anyways, that’s why all the triathlon/tt specialists work on it. Constant high power, that varys very little, never drops too low except on steep descents and rarely spikes to an unsustainable level.
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I find this really interesting. My simple logic would say that muscles wouldn’t know the difference between ERG or non-ERG. But like you said, you notice the difference so there must be something there, or it’s all in our head (but hé, if it works, it works).
It is all in your head but it is very real. Spiky power is harder than steady power. If you spend 80% of your training with no need to regulate spiky power (e.g. Riding in ERG mode), it stands to reason you are going to suck at regulating power the 20% of the time you’re outside and be spiky. And it will be harder.
I don’t think one needs to totally ditch ERG mode but if you use it 100% of the time indoors and you do most of your training indoors you are compromising your training if outdoor performance is your goal. With ERG mode you do hit your target power but with that external regulation you are getting the double whammy of not perfecting power regulation AND not training to handle short term variations in power.
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It was also my logic and that is why I was telling this with some reserve…but experience is different. Level mode feels like it has more inertia and you have full control on the power. It is especially visible when riding on flats and level 0 very nicely simulates the experience. ERG feels a little bit more…flat, without up and downs with power? Hard to describe it but, like I said, this is the only real variable for me and outdoor riding feels completely different this year than the last one.
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Well, if it works for you, it works.
Maybe it also has something to do with what trainer you use. I’m on a Kickr Core and I can’t really relate to this thread anyway because i feel the opposite. A 20min outdoor SS effort feels way easier then and indoor 20min SS effort on ERG. Average power over 20mins is the same. But for me i think it’s more a mental thing. Outside the time flies, you get “distracted” by other road users, traffic etc. While indoor those 20mins just creep by, even with something on Netflix to keep me distracted.
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Do you think that some of it is flywheel speed, especially when using ERG, I own a Neo, and a lot of the posts about it are “it feels amazing”, but I do wonder if ERG mode you get the flywheel going to fast, it lacks muscle recuitment, and leads you to indoor specifc “spinning”, I have a big difference between indoors and out, but it’s been getting better since I’ve been trying to slow the flywheel on my trainer (if that makes sense)
I have the same view as you. I find outside no problem. I use a Tacx neo 2 indoors. I only use Erg mode indoors. The benefit for erg is really the power is constant and if my mind wanders I am still doing the same work.
Outside riding I have found for me has continued to improve year over year. Outside you need to be more aware of your power…it can be easy to go too hard in some areas and at times you forget to watch the power as the speed is the same but your power is dropping ie slight downward slope.
Outside the terrain varies and part of improving to ride is learning how to deal with these variations.
I personally found more work on longer sweet spot sessions and doing Vo2 sessions of 30/15s have really helped my ability to ride outside. I just find I am better aerobically and have increased muscular endurance for longer rides outside.
The orginal post…I would suspect that the power on his first two intervals was not as consistent as it is when inside. Would suspect it isnt an apples to apples comparison.
I also have a kickr core and my experience is exact opposite;) during longer threshold like 20 minutes time fly quite fast but outside 20 min at threshold is quite a long distance and time runs slower for me. Maybe this is the case where I ride and as there is absolutely nothing exciting during my rides.
100% if you have access to an hour long climb. I don’t have anything even close to that which means climb and then freewheeling
I guess the take home here is whether it’s an hour on the trainer or an hour on the road, it’s better than an hour on the couch!
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Still have mine and it was far more road like than my KICKR Snap. In fact, the KICKR Snap feels nothing like the road.
100%. Don’t let people convince you that quality work can’t be done outdoors. It couldn’t be further from the truth. Indoors is just a lot more time efficient and “easier” to execute if you don’t have suitable words for certain types of workouts, but it has its drawbacks.
Honestly, I think too much indoor riding makes you an inferior overall cyclist, unless all you care about is raising your FTP and doing Zwift races.
I don’t know. But I doubt there’s some phenomenon that’s preventing you from making the same power indoors vs outdoors unless you’re trainer is dirtying up your power. Maybe it’s psychological due to the perception of speed? Headwinds?
Personally speaking, my HR tends to be a little higher outdoors at a given effort, mainly due to heat and more muscle activation due to not being locked into a stationery position. Otherwise, I make just as much, if not more power outdoors.
Time for a test: Load a bike and trainer in the back of a pickup or flatbed trailer, have a driver pull you along at typical riding speed 30-45kph and see if you can match power output on the trainer. Gets you plenty of air flow, true outside conditions and such so the only variable is the trainer… right? 
Just kidding and not suggesting anyone actually take on such a silly and potentially risky proposition.
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For those of you who use ERG mode, Im not surprised you cannot easily replicate the power outdoors. When I ride the trainer and do intervals, I dont use ERG because it is alot different (at least mentally) to hold a power level than to just focus on pedaling and have the trainer lock you into some wattage.
If OP doesnt have some discrepancy in power meters (like estimated power indoors and a real meter outdoors that makes it so much harder), then I imagine it is harder to focus and sustain the particular wattage outdoors than indoors and this is the nature of their problem.
It could also be the fact that indoors lets you control variables so tightly that you can have much more energy for the actual interval than outdoors. For example, I like doing proper structured short intervals indoors because I can almost perfectly control my rest intervals.
If you really care about speed, unless you are in some constant gradient climb, going harder on steeper grades and relatively less hard on lesser grades leads to a faster overall time on the climb. However, having the sum of this effort average your wattage range in order to call it threshold doesn’t have quite the same meaning as setting your ERG mode to x-watts and slogging along for 15mins. I think you need to learn/focus on pacing real life climbs if you want improved times using your now higher wattage. Pacing as in knowing when to push harder and when to ease off.