Pizzaiolo Americano 

That was what I was thinking. The only reason the actual number would ever matter for me is if I was swapping power meters. Even then, I think it would really only take a few rides to figure out the variance between the two.

Seattle Forrest

If you have a consistent but not accurate power meter, what happens when you use a different one? Like for example what this thread is about? It causes a lot of confusion, you don't know your FTP, and you can't use your CP numbers from one to pace yourself with the other.

Seattle Forrest

It's a prediction of how much power you would have put out doing that ride at a steady output. The difference between the average power you actually did, and the NP for that ride is the "variability index" which can be useful in specific contexts.

Anyway, a lot of people think NP is basically avg pwr without the zeros, that idea came up in this thread. The zeros affect your NP in two ways: they bring the number down, but they also allow you to make more power afterwards because they're rest, if you're doing zeros because you stopped you have to start again which means bigger than normal numbers, and those are weighted heavily in NP. The details don't really matter that much, the moral of the story is NP is a prediction, and it's not just ignoring the zeros.

rubiksoval

Normalized power is a rolling 30 second average raised to the 4th power. Typically not that useful for efforts under 20 minutes.

Can be gamed significantly depending on the rider. For example, 6-7 max one minute efforts with easy pedaling in between for an hour can result in an NP significantly higher than FTP for some.

Conversely, long, very steady, high power efforts (30 minutes at threshold, for example) can result in an NP a few watts lower than AP.

TSS/ATL/CTL are all based on NP.

burnthesheep

I do TT. Having the meter say "250w" and it is actually 250w matters. I use the meter with a speed sensor and pitot to do aero testing. If the power reading is garbage, the aero testing is garbage.

So, consistently seeing the wrong number doesn't help me one bit.

I went to a Quarq on the TT bike and now will ask for one for Christmas for my road bike also to replace the abhorrent Stages left-only.

asgelle

NP is not a prediction of anything. It is the fourth norm of 30 second average power. That means you average power readings from the previous 30 seconds, raise that to the fourth power, calculate the average of those values, then take the fourth root of that average. People can choose to use NP however they want. Andy Coggan developed the concept as a measure of the metabolic cost of a workout. In other words, he considered the metabolic cost of all workouts with the same duration and NP equal independent of how that NP was achieved.

colnago62

Those numbers could possibly be a little higher than riding on a flat road in a straight line. To maintain speed in the banking, power goes up. I doubt the difference is much, though.

Seattle Forrest

Thanks for the correction. That explains why above threshold efforts are weighted so heavily.

But as to your first sentence, NP is really a prediction of the metabolic cost. It isn't the actual cost, that's unknown. The take home point for the OP who is learning to use power data is that NP can be a useful metric, but it's not the end all be all and it's not better than avg for a lot of things.

bbbean 

Two possibilities:
1) The Peloton, like many exercise devices, over-reports power.
2) When you ride the Peloton, you are pedaling at all times. You are riding to your power numbers. There is no coasting, no downhills, no tail winds, etc. When you are riding outside, you are riding to speed, and all those things allow you to maintain speed at lower power during parts of your ride.

kosmo886

Adding fuel to the fire...why do different devices show power differently. My wahoo, strava and rwgps showed average power as 206, 213, and 237 respectively for the same ride...

WhyFi

Depends. You might be looking at Average Power or Normalized Power or Weighted Average Power (Strava). There's also the possibility that the highest (rwgps) isn't factoring in the 0s.

WhyFi

I recently stopped using Wahoo, but I looked at a ride from a couple weeks ago that I had on the Bolt, Strava and RwGPS.

Wahoo and Strava both agreed on Average Power, as is typical: 238w.
Wahoo's NP was 271w while Strava's Weighted Average was 257w. Strava's WA being a little lower than NP is also typical.
RwGPS was all messed up - Average Power was 299w. They must not be counting 0s (coasting).

rubiksoval

Strava should take power directly from the Wahoo file.

I've never had Strava report a different average power than my Garmin.

guadzilla

Despite being a devoted power meter user, I agree that power-related jackassery bores me to tears (and I am a guy who likes geeking out on numbers and such). People that talk of rides in terms of TSS, and their weekly CTLs... dudebros, no one cares. Show it on raceday.

The only thing i disagree with you on, Rob, is your statement that powermeters are not that accurate. Looking at DCRainmaker's tests, various meters all tend to be fairly close to each minor (minor spikes excepted). So either they are all inaccurate by the same degree, or in fact they are close enough - within a couple of percents, as claimed.

Psimet2001 

Most are generally accepted to be within +-5% or less. Computrainers are +-2-3%. Infocrank was something like +/-1%. That means they will all "seem" close to each other at lower power numbers. The reality is that they are all "close enough". From that point on as long as they are consistent then that's all that maters.

As for switching between devices - very common to hear athletes say, "Oh I switched over to my cross bike and the power meter on that one seems to read about 5-10W higher than the one on my road bike so I just adjust." Many times when riding at a constant effort even the 3s averaged power readings can vary to a degree that is larger than the PM's tolerance range. I honestly think a lot of riders just can't tell the small differences between devices.

Atlas Shrugged

You missed the point that if numerous power meters are overlaid like DC Rainmaker does you will see that most meters track very accurately to each other indicting they are accurate both in power measurement and power readings.

asgelle

Which is fine until they get home and want to use their power/duration curve for post hoc analysis.

colnago62

If you look at those analytics, you will notice that while they do give similar numbers under steady out put, they can vary greatly under acceleration and short high intensity efforts.

Atlas Shrugged

I assume vary greatly means different things to different people. This is the first graph I found on his site and I am sure you can find a graph with flyer readings. The point is most meters track very closely considering all the variabilities involved.

Psimet2001 

Without zooming in it looks like that last peak in there the power meters were giving difference of up to 30W. That's varying greatly.

billridesbikes

Here is a power meter study (and references to previous studies). After testing 54 meters they claim an accuracy of -0.9% to true value (slight under reporting of true effort) and coefficient of variance (relative standard deviation) of 1.2% for all meters. This was surprising as I thought the accuracy would be worse, also this is a snap shot of meters built around 2015 so they could be better or worse now.

If you believe this study you can repeat the same effort at exactly 100w one hundred times on one hundred different meters and 95% of your values should be between 97W and 103W. And for a bigger effort of a 1100W sprint values between 1075W and 1125W are essentially “the same”, making the meter appear to vary more, but the relative variation can still be the same.

Also, means if your FTP is 250W and you do a new test and it spits out 253W it’s probably not significant.

Accuracy of Cycling Power Meters against a Mathematical Model of Treadmill Cycling

Psimet2001 

I see you have never visited a Zwift forum....

Psimet2001 

I've watched triathletes climb off of bikes and quit their workout because one meter was giving them a 5W different reading.

Metallifan33

This. To be practical, you can think of your power number as a datum. It doesn't matter what number you start at (or even what that number is), it only matters if you can increase it relative to where you started. There are a lot of other benefits. The key is to get used to your meter and know how it relates to your rate of perceived exertion.
In general, equipment that "estimates" your power (and even calories burned) is almost always higher than the actual numbers. It's like they err on the high side (like car dealerships estimate MPG).
To see what your average power is during pedaling, you can look at the power graph and only select the time periods where you are pedaling (the lap feature on your bike computer is useful here too). You can use the lap feature to mark specific portions of your route and then compare the portions.

colnago62

For those people who are using a power meter like a hrm, then having numbers close enough is probably fine. If a rider is trying to improve on their flying 200, sprint or standing start, They will probably want something more accurate.