A power meter (PM) can be one of the most valuable tools for a cyclist. Think about it - all bias uncontrolled variables are thrown out the window, there's no more lying to yourself. For all (well, most) intents and purposes, a watt is a watt is a watt. The same can't be said for heart rate and Rate of Perceived Exertion (RPE). Add to this the current market trends in power meter availability and pricing (the former rapidly rising and the latter rapidly dropping) and a power meter has almost become a de facto part of not just racers but enthusiast and recreational cyclists.
A power meter is a very powerful tool when use correctly and essentially a very expensive bike computer when not. To get the best value out of your investment it is important to understand what the data is and how to use it. The "what" of the data is well documented, so I'm not going to focus on it. What instead I want to talk about is the use and misuse of the data.
Setting your zones
A PM biggest and most basic function is to provide you with your training baseline - your FTP and your training zones. FTP (i.e. the power you can sustain all out for 1 hour when highly motivated and rested) is most commonly estimated using some version of a shorter test (usually 20 minutes) and taking a percentage (95% for a 20 minute test). Many other tests also exist, including doing an actual 60 minute effort (however, I think this approach, while it is the definition of FTP, is a worse approach for most cyclists*).. The point here is to test and use the same test so that it is repeatable. I'm personally less concerned with which test you use or which one provides the best estimate of FTP. Why? Because a few % doesn't really matter. Training zones are wide and a few watts in either direction will not make or break your workout. Add to that your own feeling, and it's easy to tell if your workouts are slightly too hard or too easy and adjust. Additionally, you would never want to pace a 1 hour TT race based solely on a short test. The test will get you in the ballpark and then through training at that FTP intensity you will dial in your race day goal watts.
Tracking your Stress
Tracking training by miles or by duration does not cut it. Neither has any indication of your intensity. The best metric to track is time AND intensity - a combo neatly combined in the Training Stress Score (TSS) metric. Daily and weekly goals for example are much better tracked by TSS than miles or hours. It is a more true sense of the stress done to your body. Additionally, watching TSS is very useful in a race situation. I have a good sense of how my fatigue tracks with TSS during a ride/race. I can see my current TSS load and make smart decisions during a race and know whether I can take some risks attacking or if I'm better off sitting in and conserving as much as possible.
Tracking your Stress - Part 2
Now that you have your total stress, TSS, for each ride calculated, people smarter than I developed a way to match the training responses of your body. The result is the Performance Management Chart (PMC) in Training Peaks (TP). Strava also implements it as their Fitness and Freshness (F&F) chart, however there are major flaws with their implementation so I do not recommend its use**. Chronic Training Load (CTL), aka fitness, is a weighted measure of you TSS/day over a long period, typically 42 days. Acute Training Load, aka fatigue, is also a weighted measure of TSS/day, but over a short time scale, usually a week. The differing time scales represent how our bodies react to training stress - fatigue builds faster than fitness and this is why we get tired after periods of building stress. However, fatigue also goes away quicker than fitness leaving you stronger than you were after a rest or recovery. The PMC models how your body naturally reacts and adapts to the training stress. The final concept to track is your Training Stress Balance (TSB), which is just CTL-ATL, your fitness minus your fatigue. When negative you are more fatigued (typical during training and build phases) and positive means you are more rested (typical for executing peak performance, e.g. a race). Tracking these numbers and learning how your body reacts to the inputs is a huge training advantage (e.g. what TSB is optimal for your race? How far negative can you get and for how long before needing recovery? What rate of building CTL is sustainable in the short and long term.)
What Type of Racer are You?
Testing shouldn't be limited to just FTP. With a PM you can test short and long duration (and also collect the data naturally through normal riding and racing). Typical durations to track are 5 second, 1 minute, 5 minute, and 20 minute (or FTP). Then, a cyclist can use what's called a Power Profile Chart (developed by Andrew Coggan, who also developed most, if not all, the power metrics I've mentioned) to see how their efforts compare. Coggan sampled many different cyclists of all levels to make the chart and determine how to relate a 5 second through 60 minute power. Training peaks has this feature built in for premium users and coaches to track. Using this chart, and comparing your 5 second through 60 minute powers, you can determine if you are a sprinter (higher short duration power), TTer (higher long duration power) or all-arounder (even through all durations) phenotype (or some combination there-of). It can also identify strengths and weaknesses to know how best to win races and where to train more. The chart is not done in pure watts, but Watts/KG, that is power divided by weight, which is typically used to compare efforts when weights vary greatly, however there are some caveats***.
With PMs becoming much more available and affordable (but by no means cheap), they are seeing increasing proliferation among all levels of cyclists and triathletes, from weekend warriors to seasoned racers. However, simply buying the PM is only the first step in the process of using the tool. Understanding the data and how to apply it to your training is a critical next step. As a coach, one of the most valuable services we can provide is taking care of the application for the cyclist so there is no learning curve and there is an immediate return on your investment. Through working with a good coach, they will explain the concepts and the athlete will see them in action and learn themselves.
* A true all out 20 minute effort takes good pacing, strong motivation and practice. Most cyclists will have sub-optimal (i.e. less that a true all out effort) results their first few tries and even when more experienced. They are not fun and take a lot of mental and physical effort to accomplish. If you now try to extend that to a 60 minute test, multiply all the difficulties by a factor of 5. A slight miscalculation in minute 1-30 might break you 50 minutes in. The margin for error is so small. Additionally, a cyclist cannot simply go out any day and ride an hour at FTP - they must be highly motivated (e.g. in a race situation) and optimally rested and tapered. Both of those are VERY hard to nail down on a regular basis in training. FTP testing does not usually create a lot of excitement. Because of this, I trust what will probably be a more accurate 20 minute test with the inaccuracy of using a generic model over a less accurate 60 min test.
** The TSS of a ride is a function of your FTP at the time you did the ride. However, when you update your FTP in Strava, they recalculate all of your old ride data with the new FTP, which changes the TSS! Now you have incorrect TSS values feeding the F&F chart, making the F&F chart also wrong.
*** Watts/kg does not always tell you who the stronger cyclist is or who will be faster. On hills, it is a very good measure - a higher W/kg will usually climb faster than a lower W/kg. If a 60 kg rider and 80 kg rider both ride up a hill at the same W/kg, they will be very close in speed. However, for less steep climbs and flats, weight becomes a much less important variable. Aerodynamic drag becomes more meaningful as well. A better measurement might be Watts/drag (also called CdA). It will be likely that the 60 kg and 80 kg riders from before on a flat road riding equal W/kg will NOT ride the same speed because the 80 KG rider will be doing more watts and the increase in drag will not negate the larger watts