I got this article from Joe Friels blog. He doesn't post on his blog anymore, but there are still some great articles on it.
Why You Need a Power Meter
Should you buy a power meter? After all, they aren’t cheap and sport is already expensive. You’ve spent a small fortune on bicycles and all of their assorted and costly components. And don’t forget the entry fees, travel to races, special foods and supplements, and on, and on, and on.
And why get a power meter since you already have a perfectly good heart rate monitor? It’s just one more gizmo to have to figure out.
So why should you get a power meter? The short answer is that you simply are more likely to achieve your race goals by training—and racing—with a power meter than without. It is the most affective tool you can get to go faster on a bike.
Don’t get me wrong, heart rate monitors are great intensity-measuring devices, also. But heart rate by itself actually doesn’t tell you much. It’s like the tachometer on a car—it tells you how hard the engine is working. Nothing more.
For example, what if your heart rate is 10 beats higher than usual? What does that mean? Is it good or bad? The only way to answer that question is to know if you were putting out more power or less than usual.
Input data such as heart rate isn’t meaningful until it is compared with some measure of output. Output is critical to success; input isn’t. After all, they don’t give awards at races to those who worked the hardest or had the highest heart rates (input), but rather to those who had the fastest time which results from high power (output).
Let’s get back to why you should get a power meter.
No More Guessing
Should you buy a power meter or fast wheels? Given the choice I’d recommend a power meter every time. When it comes to speed the engine is always the most important part. A power meter will help you develop a bigger one. With sleek wheels you still have a small engine.
How do they make your engine bigger? Power meters remove most of the guesswork that goes into training and racing. For example, I’ve known athletes who when doing intervals with heart rate monitors don’t call the work interval “started” until their heart rates reach the targeted level which could take several minutes. During that time they are guessing how hard to work. With a power meter you soon learn that the interval starts as soon as the power hits the targeted zone—which means right away. You get the intensity correct immediately with no guesswork. The intervals don’t taper off near the ends any more either. This means no wasted training time and precise intensity.
Also, realize that you’re not trying to train the heart solely when doing intervals or any workout, for that matter. In fact, what happens in the muscles during workouts, not the heart, is really the key to your success. Heart rate monitors, while quite valuable to training, have many believing that training is just about the heart. It isn’t. Power meters allow you to focus more on muscle.
Cheating With Power
Using a power meter in a long steady-state race such as a triathlon or long time trial is almost like cheating. When everyone else is fighting a head wind, excitedly going too fast down wind or guessing how hard to push when going up hill, the athlete with a power meter is just rolling along at the prescribed power. He or she will produce the fastest possible ride given the conditions so long as the optimal target power has been determined through training and observed closely during the race. While something similar can be done with heart rate there are some confounding factors such as the excitement of a race, cardiac drift, the acute effect of diet and the slow response of pulse on hills, accelerating out of corners or when passing others.
Power meters also provide highly accurate details about how your fitness is changing throughout the season. I test the athletes I coach regularly using a combination of heart rate and power. Without this information I really wouldn’t know for sure if they are making progress. I’d just be guessing. Now I can precisely compare output with input by dividing the average (or, preferably, “normalized”) power for a workout by the average heart rate. An increasing value for similar workouts tells me fitness is improving.
Moving On Up
There are many benefits of training with power. But perhaps the best indicator of their value for performance is the elite athletes who use them. Power meters are common with pro road cyclists and they are becoming increasingly popular with pro triathletes. Cyclists are increasingly using them. Age group triathletes have been slow to adopt this technology, which is unusual. Over the past twenty years triathletes were the first to adopt such innovations as aero bars, beam bikes, deep-dish rims, clipless pedals and gels.
The trend is definitely toward the adoption of power meters in road racing, triathlon and mountain biking. Many are leaning that a power meter will help them race faster. Start setting aside a few bucks aweek so that some day you can get one. It will definitely change how well you train and race.