Power meters spit out a lot of numbers: current power, average power, max power, and all manner of derivatives. But what matters more, says Joe Friel, longtime coach and author of The Power Meter Handbook, are the metrics that are based on these data points. Here are five of the most important ways to use your power meter.

Functional Threshold Power

Essentially your highest sustainable power output for 60 minutes, functional threshold power (FTP) is your north star when it comes to power-based training. “It’s the reference point for your training zones, which is the basis for prescribing structured workouts and recovery,” says Friel. A simple, painful way to find yours: Warm up for 15 to 20 minutes, then do a 20-minute all-out time-trial effort. Multiply the average power for that interval by .95—that’s a close approximation of your FTP.

Related: Rotor Releases INPower, a Less-Expensive Power Meter

Note: Choose a safe course you can do without coasting or stopping. Ideally it’s a subtle climb with a gradient of no more than a few percent. “Steeper hills will skew the data high,” says Friel. Also redo the test periodically, preferably on the same course and under similar conditions. As you get fitter, your FTP will rise and you’ll need to adapt your training intensity by resetting your zones.

How to Use It

FTP forms the basis for all your training, since zones are expressed as a percentage of FTP. Without it, you simply can’t improve as quickly. Effective structured training prescribes interval efforts according to your zones, and zones also help enforce real recovery rides—with current power displayed, you’ll know if you’re “cheating” on a recovery day by riding harder than you should.

Average vs. Normalized Power and Variability Index

Average power is what it sounds like: your average power output over the duration of any ride. But if you do two rides, and one is steady while the other includes a lot of surging and coasting, the latter will be more tiring even if the two result in the same average power. That’s why normalized power is a more accurate representation of how physiologically hard your ride was; it’s an algorithm that adjusts for times when you’re either not pedaling or putting out much more power than average—accelerating up a hill, for example. “It tells you what the metabolic cost of your ride was,” says Friel—and how hard a ride actually feels.

How to Use It

For long, steady-state efforts like a triathlon, you want to minimize power-sapping surges. A lot of riders simply start out too hard and fade later in the event. Keep a lid on your enthusiasm by monitoring Variability Index (VI), which is nothing more than normalized power divided by average power. In something like a time trial, your VI should be as close to a 1:1 ratio as possible. “More than 1:1 and you’re surging,” says Friel. “Metabolic cost is way up and you’ll struggle later in the race.”

Intensity Factor

Intensity Factor (IF) is a simple measurement of how intense your workout was. Divide your normalized power by your FTP, then multiply by 100 to get a percentage of FTP, which is the IF of the ride.

There’s an inverse relationship between time and IF. For very short workouts, IF could be close to or even more than 100 percent (possible if the ride is shorter than an hour). For very long events—say, a century—it will be much lower, such as 60 percent.

How to Use It

Most athletes use duration as a measure of difficulty, says Friel, counting hours of activity. IF lets you focus on what you’re doing with those hours, especially as your training relates to a particular event. Since you know that, say, a cyclocross race will have a very high IF, you can model your training to mimic that effort.

Extra Credit: TrainingPeaks, the online power analysis software that is the overwhelming favorite of coaches and enthusiasts, has a metric called TSS, or Training Stress Score. TSS incorporates IF, normalized power, and ride length to give you one simple number to show how hard your total workout was. TSS is a TrainingPeaks trademark, but it’s widely used and is a more complete reflection of how hard you’re riding than IF alone.

Efficiency Factor

Don’t chuck that heart rate-monitor strap just because you bought a power meter! Power measurement doesn’t replace heart rate, says Friel; in fact, the relationship between power and heart rate is the key to getting fitter. EF is nothing more than normalized power divided by average heart rate. It directly measures aerobic fitness, the key thing we’re all trying to improve. As you gain aerobic fitness, you can generate more watts at the same heart rate.

How to Use It

Get to know your lap timer. Hit the lap button for a given portion of your workout; afterward, calculate your EF. “As that ratio rises, you’re improving your aerobic fitness,” says Friel.

Kilojoules

kJs, as they’re called for short, are a simple measurement of actual work done. By a happy accident of nature, they’re also basically equal to calories. In reality, one kJ is worth four to five dietary calories. But the human body is only 20 to 25 percent efficient at converting energy to power output, as measured at the power meter. The rest is lost, mostly as body heat. Power meters capture the rest, very accurately. So at the end of your ride, the power meter readout of kJs of work done is roughly equivalent to the number of calories burned.

How to Use It

If you’re riding to lose or maintain weight, monitoring kJs can help you meet your weekly calorie loss goals, since you can add kJs from rides to your basal metabolic rate to get an idea of how many calories you burn a day, or week. It’s also a great way to figure out what you really need to eat on a ride. If you do a two-hour ride that burns 800 kJs, and eat a 200-calorie energy bar plus two bottles of energy drink at 100 calories per bottle, your net outcome is 400 calories burned.

Best Practices for Using a Power Meter

Here are some no-kidding basics to getting the most out of your power meter.

Enter the data

You can set up a head unit with personalized data (many have multiple profiles for different bikes). Enter all your information, like weight and FTP; it helps provide more accurate, real-time training information. One important element: When asked if you want to “include zeroes in data” select “yes.” You’ll get more accurate info, says Friel.

Simplify your screen

While all the above data points make great info to analyze after a ride, they’re not as useful during a ride. The only real power metric you need on your computer’s home screen is current power. Other “home screen” fields can be used for staples like speed, distance, ride time, cadence, and heart rate. Put metrics like average and normalized power on your second screen, an easy click away but not constantly staring at you. Pro tip: For your current power display, select three-second smoothing to even out spikes in power output. You can also choose to have your current training zone displayed, so you can target an intensity without constantly staring at the wattage and working out zone calculations in your head.

Don’t fixate

It’s easy to get caught up in playing around with your power output. But you can get so involved you aren’t paying attention to traffic. If you want to focus on the screen, pick a quiet spot to spin around, like a large, empty parking lot.

Zero out your power meter

Power meter accuracy relies on a properly calibrated unit. While power meters are getting better at compensating for things like temperature shifts, there’s no harm in a simple zero reset at the start of every ride. Think of it like checking your tire pressure.

Take care when cleaning

You know not to spray high-pressure water at things like bearings. That goes doubly for your power meter. It’s weather-sealed and can put up with a normal bike wash, but if you use a power washer, don’t be surprised if you break your new toy. And no, it’s probably not covered by warranty in that scenario.

Check the firmware

Every now and then, it’s a good idea to check to see if your power meter manufacturer has released updated firmware, essentially the OS for the power meter. A firmware update can often fix problems like dropped data or a power meter that won’t pair to the head unit.

Use in events

Power meter data from an event, whether a race or a gran fondo, can be valuable to analyze later. You can see how your training matched up with the demands of the ride and what you need to do more, or less, of. But it’s also easy to “race to the numbers” and let those dictate your abilities. If that’s a problem for you, try taping over the computer screen where the power readout is (many pros use this trick) or simply tuck it securely in a jersey pocket. It will still get the signal, but will be out of sight

Joe Lindsey Joe Lindsey is a longtime freelance journalist who writes about sports and outdoors, health and fitness, and science and tech, especially where the three elements in that Venn diagram overlap.

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