How to get the most out of using your power meter in both training and racing situations.

By Coach Jerry Jacobs
 

Power meters are super valuable training tools. They have become mandatory tools to be able to work optimally with a coach. We can dial in training zones and targets precisely. We can evaluate training progression. We can detect potential problems such as too rapid an increase in training load(leading to a setback)well before an athlete may detect this on their own.
Racing with a Power Meter is a completely different matter. There are situations whereby knowing your watts can be helpful. There are other situations whereby the Power Meter can be a detriment to success.  I will explain.
For Crits and other variable races under 2.5 hours, I never look at my power meter.  I advise athletes to do the same.  The data during the race is not helpful and can actually limit a potential result.  Early in my racing career with a power meter, I raced a circuit race with a kicker 60 second hill in an elite masters 40+ field. I watched my power rise each lap over a 9 lap race...425...450....475.  At some point in the last few laps, my brain said "you just can't go this hard". Boom...dropped on the next lap. 
The truth is, that I could have stayed with the field and had a better result. Athletes can do amazing things during a race situation.  I should have turned the power meter off. Just "compartmentalizing" and going as hard as I needed to go could have allowed me to stay with the bunch. You do whatever it takes. Focus.  Then "reset" and prepare for the next part of the race.
Turn off the power meter (or don't look at that screen) during the race! We will look at the power as a diagnostic tool, after the race. That can be helpful. That way we can target the watts that you will need to do in your training.  I always tell athletes that they will not produce a number in a race that they never do in training. If you need 350 watts for 3 minutes to stay with the leaders, you will need to train to those demands.
During the race itself, the idea is just to race and compartmentalize. What do I need to do now? Where do I need to be?  During finishing laps, I am just focused on where I need to be positioned and to give it everything that I have at that moment. 
Time Trials:
Here is the opposite extreme, whereby a power meter is super helpful. Knowing your target number and riding at that target can improve results materially. Without a power meter, the vast majority of athletes will go way, way too hard in the early minutes of a typical 20 minute TT.  That "over limit" effort will cost them many seconds, even minutes. The reason is that an early over threshold effort will put the rider deep into the anaerobic "red zone".  To recover from going way too hard, they will need to now ride well under threshold to recover.  This is highly inefficient. More importantly, it is ineffective both physically and mentally.  At worst, riders will be in a "negative feedback loop", whereby they see power dropping and are getting passed. They can become convinced they are riding poorly.
Alternatively, riding the first quartile of a TT at your target number, and training yourself to crush the last half and last quartile of the TT, will result in a faster time. Ideally you are in a positive feedback loop. You know you are at your target and riding well.  You will focus and compartmentalize in a positive manner.  From about the halfway mark, give it everything that you have.  Focus on how hard to ride. What power can you sustain to the end? 
Long races and long climbs:
For races over 4 hours and longer training rides, the key number to focus on is your average "normalized power".  This will tell you accurately how hard the effort has been. We can calculate what the optimal number is for a long race(let's say 5 hours). Going above that number can, and probably will, result in blowing-up later in the race.  That can be expensive and painful.
This has helped me numerous times in the various races such as NYGF and L'Etape.  Knowing when you need to drop back to a more sensible group will result in a better finish.  Knowing exactly where you are, and what you can do, will also result in the ability to ride with the fastest group that you can stay with later in the race.  I know for athletes that I coach almost exactly what the maximum number can be that will enable the rider to maintain pace later in a long race.
This can also be critically important for climbs over 20 minutes(and especially long HC climbs of 60 minutes, or more).  Here, knowing your target number, and riding efficiently, is mandatory for success. Going too hard early in these climbs will result in a slower finishing time.
For example, a well conditioned athlete should be able to ride between about 80% and a maximum of 90% of FTP for very long climbs of over 60 minutes. Of course, this requires that your FTP is calculated correctly(most riders do not do this correctly), and that your "fatigue resistance" is dialed in.  Other factors such as heat and elevation must also be factored in.  For example, watts will be about 10% lower at 8,000 feet of elevation, all other factors being equal.  
Summary 
As with many other things in cycling, using your power meter correctly is both a science and an art.  In some situations such as a Crit, just race. Turn the power meter off. We will look at the numbers, later, as a diagnostic tool. In TT's, long climbs and long races, knowing your targets and staying at targets can help greatly!