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For a given use case power meter accuracy may or may not matter.
Here are two use cases:

BikeRumor - Power Meters Explained

No matter which power meter you land on, the most important aspect is consistency. The best power meter is the one you use day in and day out. Switching from unit to unit or brand to brand introduces variability that makes it harder to accurately compare training sessions and track progress.

This author is thinking about power meters as a training tool to judge if a rider is getting stronger. In this case consistency is more important than accuracy.

Bicycleguider - Power Meters Explained

When it comes to indoor racing, a high-end power meter is a necessity. The Zwift Premier League requires racers to compete with a direct drive smart trainer, which almost always includes an internal power meter.
[later in the article]
Accuracy is everything, and you can even get disqualified for inaccurate or questionable power data. If you’re seriously into indoor racing, a high-end power meter or direct drive smart trainer is well worth the investment.

This author is thinking about power meters as part of a racing league where one riders numbers are compared to another rider and used to determine a winner.

Summary:
Use Case 1 - Personal training, consistency is more important than accuracy.
Use Case 2 - Competing in the Zwift Premier League, accuracy is everything

What other use cases are there where accuracy does not does not matter?

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  • The use case determines whether accuracy matters, as you've identified. There's a third common use case: when you add or replace a power meter; that's because power meters fail over time (or, in my case, my bike and power meter were stolen). In that case, if you want your data to be comparable across time or across meters, it's good to know if they've been accurate. This is related to a question just recently asked, bicycles.stackexchange.com/questions/91831/…
    – R. Chung
    Dec 26, 2023 at 22:15
  • How many other use-cases for power meters (other than training and virtual competitions) are there at all?
    – DavidW
    Dec 28, 2023 at 2:44

3 Answers 3

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As your question astutely identifies, the need for accuracy depends on the rider's use case. In addition to the two you've already pointed out, there's a third common use case: when you get an additional, or replace an existing, power meter. In that case, you typically want to compare across two power meters either concurrently or over time. For example, power meters can fail over time; or, sadly in my case, my bike and power meter were stolen. It's easiest to compare power data over time or over different units if both are accurate.

That said, some use cases, even if you're not adding or replacing a power meter, are less demanding of accuracy than others. Training FTP (functional threshold power), for example, is arguably one of the least demanding uses for power data. Racers trained their fitness and performance for a century before the introduction of affordable on-bike power meters by riding known courses or known hills and keeping track of elapsed time. Starting in the 1970's, riders used heart rate monitors. For this purpose, one doesn't need much accuracy at all--all one really needs is a power meter that is "ordinally" consistent (that is, that maintains consistency in the order of the level of power). A deeper requirement of accuracy is what is called "interval" consistency, so that the difference between 100 watts and 110 watts is the same interval as between 200 and 210, or 500 and 510, or 1000 and 1010. Certain kinds of training for specific purposes (for example, for sprint training, or for determining training load) can require interval consistency.

In general, we've long known how to go faster on a bicycle: more power, less drag, or better tactics. Properly used, a high quality accurate power meter can help you achieve all three. An ordinally consistent power meter can help you with the first, but not always the other two, and the finish line and timing clock don't care which of those three you used.

Besides online racing (your use case #2) and continuity of data fidelity, two other use cases that demand power accuracy are drag estimation, and training load estimation. In both cases, you use the interval relationship between different powers to calculate the quantity of interest. You may be familiar with the common training metric "training stress" or similar metrics. The most common (but not the only) training stress metric is TSS, which is based on "normalized power" (NP). NP is a way to take variable power data from different rides and "normalize" them so they can be compared to a putative "steady state" power. NP uses the L4 vector norm (hence the name) in which the wattage is raised to the 4th power, its mean is taken, and then shrunk by the 4th root. This algorithm depends on interval fidelity.

For drag estimation, we are interested in both aerodynamic and rolling drag. Rolling resistance drag varies linearly with speed while aerodynamic drag varies with the cube of speed, so to be able to properly allocate the power to these (plus the power dissipated in acceleration and climbing) you need accuracy in power measurement.

All of the above being said, at least at the moment, it appears that most riders use their power meters only for training FTP. A handful of riders race in on-line leagues (for example, Zwift; but there are others) and they need to use approved power measuring devices and dual record their data to ensure accuracy and fidelity. Fewer still do drag estimation, though it is quite common among pro riders and especially among those who attempt world records or Olympic medals. Since the dominant use case is training FTP, and training FTP is one of the least demanding use cases, most riders can get by without much need for data fidelity. This is a good thing, and has democratized access to power measurement tools.

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Obviously, any non-handicapped competition where you are competing, using power meter results, with others, accuracy matters (Unless you came from the Lance Armstrong camp or ethics). Any other time, especially when you are comparing results to previous results, consistency matters.

For most people, even those competing, I would argue consistency matters most.

For indoor racing, unless you at the very top of the class, there are always riders faster and stronger than you. If you are not looking for a podium finish, your goal when competing is to compete against people who are close to you in ability. If everyone uses a consistent but not accurate power meter you will all be competing against those in your 'league'. If you all had accurate and consistent meters, your 'league' might consist of different individuals. If the competition uses handicapping to even things out, accuracy has no value, it is consistency you need. An accurate and inconsistent power meter makes no sense (Possible if the power meter has courser graduations then is inconsistency - e.g. power scale is 1-10, by varies by 5%, but no one would build one like that)

For normal outdoor racing, power meter means nothing - its first over the line is what counts. I suppose in theory you could see competitions outdoors where power meter output is used for results - maybe they will do something with power meters in non-draft races to enforce the rules, who knows.

I do put a cavate on this - if 'winning' (whatever that looks like to the individual) occasionally due to power meter variations stokes peoples egos enough to motivate more than loosing due to inconsistent power meters demotivates, maybe a bit of inaccuracy is a good thing. Thats a more a question for "pych.stackexchange".

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  • See this Bikes.SE answer (bicycles.stackexchange.com/a/7862/2195) for a situation where a power measuring device was consistent (over many years) but inaccurate. Although consistent, its inaccuracy meant that it couldn't be relied upon for training.
    – R. Chung
    Dec 27, 2023 at 2:32
  • Not sure how you draw the conclusion that it couldn't be relied upon for training. Training plan requires consistency (from you and your tools) and 'calibration' to your needs. Perhaps the difference in our thinking is based on the idea you should be able take a number from one machine and put it on another. I am not convinced this ever works well outside of medical grade equipment with calibration certificates. Consumer grade stuff where cost if a bigger driver is not built to the same standard. Pro level training vs weekend warrior training also comes to play as to what is important.
    – mattnz
    Dec 27, 2023 at 3:51
  • You're right that it could still be used for training -- it just couldn't be used for training with power using any of the "common" power training guidance metrics like FTP, NP, or TSS. This is because NP (and thus TSS) uses the L4 norm (that is, it uses a calculation that includes raising power to the 4th power). You could still use it for "Zone 2" training, especially if your training were based on HR. So you could absolutely continue to train with HR--but then you wouldn't need power data, whether accurate or inaccurate.
    – R. Chung
    Dec 28, 2023 at 3:29
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Some terminology first. I think we are all thinking about accuracy as bias. That is, if we put precisely 100W into the crank (e.g. with some sort of electric motor), we expect the average reading to be within, say, 2% of 100W. Accuracy is usually contrasted with precision. Say you put 100W into the crank with an electric motor and the power meter reads an average of 100W with a standard deviation of 10W. That's less precise than a PM that reads an average of 110W with a standard deviation of 3W, but the latter PM is biased. When we say consistent, we probably mean precise - but you can have inconsistency in other ways.

With that out of the way, I'd look it at this way. There are a bunch of power meters that are thought to be both accurate and precise. They don't cost a fortune. You should probably just get one of these if you can. All the SRAM/Quarq PMs, Favero Assioma pedals, likely the Garmin and Wahoo pedals, and Stages/4iiii power meters that are not mounted to Shimano 7000/8000/9100 cranks are well regarded. SRM is probably also in this category, but given their premium price and the existence of good competitors, I don't see a reason to get them.

Left-only PMs are cheaper than dual-sided PMs. The issue here is that we know that our power output isn't perfectly balanced, and our left-right balance might vary systematically by power output, e.g. maybe I am less balanced at VO2max power, but I'm roughly balanced at tempo pace. This is inaccurate. Because the bias isn't constant, I think that the concept of inaccurate but precise is potentially relevant. That said, I don't know what systematic research has been done. My current stance is to avoid single-leg PMs if you can avoid it; you could consider running a smart trainer for structured training and no PM on the bike.

Shimano's native power meters (the R9100P, R9200P, and R8100P) are not accurate. The 9100/8000/7000 generation of crankarms are asymmetric, and all power meters mounted to them (including 3rd party meters) will mis-report power. In addition, we don't know if the bias is constant. If you don't know if the bias is constant, it doesn't matter if the PM is inaccurate but precise.

More disappointingly, the R8100P and R9200P power meters appear to have even worse problems, per testing by DC Rainmaker and GP Lama (both have YouTube channels, the former has a website). One problem observed was that the bias differed by which chainring you were in, which was a thing with early PMs but that everyone else has corrected. Another problem was that the zero offset drifted - that is, the bias may have started off around 0, but it increased during a ride.

Basically, I don't know that there are cases where a PM has a consistent bias, but has acceptable accuracy and thus is usable for training. As I observed, it is 2023, electronics and software have come a long way, and power meters that are both accurate and reliable are easy enough to buy. Some left-only PM arrangements will let you add the drive-side PM later on if you want, so that's another option. In addition, as I mentioned, if you're a smart trainer user, you may find good results foregoing power on your outdoor bike - you may lack certain bragging rights, and if you use a heart rate monitor to track calories, you'll have a (possibly large) overestimate, but those may be things you can live with.

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