The Power Meter Handbook: A User’s Guide for Cyclists and Triathletes

ride but also is used to adjust the altimeter on the head unit if it relies on a barometer for altitude display. Older power meters used to be affected by big changes in temperature, but recent self-calibrating models have reduced this inaccuracy.
Other
    There are a multitude of other data fields your head unit may be capable of displaying based on how it’s set up and the power meter you are using. For example, it may provide such options as watts per kilogram of body weight (“W/KG”), the current power zone you are in (“ZONE”), Normalized Power™ (“NORM PWR”), Training Stress Score (“TSS”), and Intensity Factor™ (“IF”). I’ll explain each of these in the following chapters. They are critical bits of data that reflect your performance. You can wait until you know more about them before deciding whether you want to display one or more of them on your head unit.
    There are even more options, again depending on the power system you have. Other data fields could be dedicated to vertical ascent in meters per hour (“VAM”), kiloJoules per hour (“KJ/HR”), and left-right pedal balance (“L-R”). GPS-based head units often provide maps and directions much like those you may use when driving your car.
    With all of these possibilities, setting up your handlebar display can be a daunting task. For now, you may want to keep the settings on the unit’s default, just as it came out of the box. Later on, as you get the hangof what all this means, you can customize the display to better fit your needs and interests.
KILOJOULES, AVERAGE POWER, AND NORMALIZED POWER
    There are several less obvious fields on your head unit that are fundamentally important for some of the workouts and analysis you’ll be doing. I’ll describe in later chapters how these are applied to training and racing on a bike. If you are a bit confused by any of these or other terms when you encounter them in this book, on your head unit, or in software, you can refer to the Glossary for help.
KiloJoules
    In Chapter 2 , I told you that watts (the unit of measure for power) is an indicator of how much energy you’re expending during a ride. That may have seemed a strange way to explain something that deals with how much force you are generating and how fast your cadence is. But they are really one and the same. The greater your power is owing to the combination of force and cadence, the more energy it takes to pedal the bike.
    As humans we usually think of energy expended in Calories. A Calorie (with a capital “C”) is the same as 1,000 calories, and the scientific term for the Calories we burn is “kiloCalories.” The distinction is pointless in general conversation, but it is useful for training. You see, mechanical energy, the kind you create on your bike and that your power device senses, is expressed in “kiloJoules.” This is what your power meter is measuring and what shows up on the head unit. And the relationship between kiloCalories and kiloJoules gives you a good picture of how much fuel you burn, which in turn can help you plan your nutrition.
    Here’s how it works: 1 kiloCalorie equals about 4 kiloJoules (actually, it’s 4.184, but we don’t need to be nearly that precise here). Humans pedaling a bike are roughly 25 percent efficient—and that’s probably a bit high but okay for our purpose. This means that only about one-fourth of the biological energy you generate (kiloCalories) during a ride is converted into the mechanical energy that drives the bike (kiloJoules). The rest is mostly lost to the heat your body gives off, no matter whether it’s a hot or a cold day. So if you are 25 percent efficient in terms of mechanical energy generated, and 1 kiloCalorie is about 4 kiloJoules, then only about 1 kiloJoule is actually realized as mechanical energy for every 1 kiloCalorie of biological energy burned. What all of this means is that when your head unit shows 500 kiloJoules at the end of a ride, you have used about 500 kiloCalories. That may be around 10 percent higher than the actual number, but individual riders vary so much that this number is close enough for training purposes. And it’s very useful, as we’ll see later on.
Average Power
    If you’ve been using a speedometer or heart rate monitor when riding, you’re used to dealing with average speed and average heart rate. Average power is a similar metric: It is the total of all the watts generated during a ride divided by the number of time