What’s Watt

The less you weight, the faster you'll get up this hill. The less power you generate, the longer it'll take to get up this hill. Like everything in cycling, it's about balance.
The less you weigh, the faster you’ll get up this hill. The less power you generate, the longer it’ll take to get up this hill. Like everything in cycling, it’s about balance.

Start cycling for any significant length of time, and you’ll inevitably hear about “power meters” and “watts per kilogram” and “threshold” (and many other power-related numbers). Now, cyclists are almost by definition numerically obsessed: miles, mph, elevation gain, weight, grade, temp, heat index, weight, wind speed, rolling resistance, weight… the list of metrics a cyclist can follow is staggering. But the more you ride  – especially around the “did you just climb that hill at 32 mph” set, the more unavoidable these “power” terms are.

So the questions are: What is a watt? And why is it so important to cycling?

The Science

Watts = Joules / time = (Newtons * distance) / time = (Weight * distance squared) / time cubed. Say watt?
Watts = Joules / time = (Newtons * distance) / time = (Weight * distance squared) / time cubed.
Say watt?

First, to the science. A watt (W) is a unit of power, named after James Watt. Power is a unit that measures the amount of work done over time. Another way to say it is it’s a force applied to an object across a distance, divided by the time it takes to move that distance.

So what does a watt have to do with cycling? Plenty. Cyclists like to know their power output because it’s a good way to measure their performance over a specific duration. If you can output a huge amount of power over a short time, you’re probably a sprinter. If you can’t output that massive power, but you can hold a high power output for a really long time, you’re probably an endurance rider or time-trialer.

The Usage

On it’s own, power output is helpful to knowing how hard you’re working. Yeah, a heart rate monitor can help, but heart rate is highly variable, and can change for all sorts of reasons. Some days your heart might be racing, and you’re barely pushing. Other days you’re hammering, and your heart looks like it’s flat-lining. Then there’s the problem of heart rate monitors going on the fritz, and giving you the heart rate of a rabbit or a tortoise.

Power is currently measured through one of these methods:

  • At the rear hub
  • In the pedals
  • On the crank

That’s not to say the connection between your power meter and your Garmin (or other device) might not be faulty, it’s just less likely. And unlike heart rate, it’s not prone to be off due to random factors in the body or environment. You can tailor your rides and/or training around a certain power output (instantaneous or average or both).

But when we combine power with other information, such as weight and time, we start seeing some really interesting stuff. You could be of herculean stature and put out enough power to grind granite into dust, but if you weigh as much as a mid-sized rhino, all that power won’t mean much. That power needs to move both your bike and you. The more you and your bike weigh, the longer it’ll take to move a set distance with the same power (see the 3rd formula above/right).

So, three things to consider:

  1. Power: how much can I output?
  2. Time: how long can I hold my power output?
  3. Weight: how much do I have to move with that power?

The Reality

You can get a good sense of who will perform best when you know how much power everyone can output versus how much everyone weighs.
You can get a good sense of who will perform best in a race when you know how much power everyone can output versus how much everyone weighs.

When you start combining these things, you can compare cyclists of entirely different types to one-another. You can take a body-builder clydesdale type powerhouse and compare her to a dainty wisp of a weight-weenie. That body-builder could put out a lot of power, and overcome the fact that they weigh much more. Likewise, that wisp of a rider might be all lean muscle, and can pound the pedals like a jack hammer. Example:

  • Cyclist 1 (200 lb clydesdale): 90 Kg x 5.6 W/Kg = 504 Watts
  • Cyclist 2 (154 lbs weight-weenie):  70 Kg x 5.6 W/Kg = 392 Watts

Assuming these two cyclists have the same power to weight ratio (W/Kg), Cyclist 1 needs to generate almost 29% more power output to keep up with Cyclist 2. Another way to look at this:

  • Cyclist 1 (200 lb clydesdale): 400 W / 90 Kg = 4.44 W/Kg
  • Cyclist 2 (154 lbs weight-weenie):  400 W / 70 Kg = 5.71 W/Kg

For the same power output, cyclist 2 (our weight weenie) would need a power-to-weight ratio nearly 29% higher than cyclist 1 (our clydesdale) to generate the same power.

So what does all this tell you? Well, a few things. First, the more you weigh, the more power you have to generate in order to move at a given speed. This is the one we all know: drop weight, ride faster, right?

But being thin isn’t all it’s cracked up to be. The less mass you have overall, then potentially the less muscle mass you have as well, which translates to less overall power output. Being ultra-light alone won’t get the job done, and losing weight isn’t all about just shaving grams and laying off of bacon.

There’s also what’s called “functional threshold power” (FTP). This is the max power you can sustain over an hour without wearing yourself out. Basically what it means is, “How much power can I put out over long periods of time?” Time trial specialists have the ability to hold very high power output over really long periods of time. They might not have the explosive power of a sprinter, or the massive power of a climber, but they won’t fatigue as quickly as either of those other two over a really long ride. When you know how much power you can put out without getting tired, you can adjust your training to a very fine degree.

The Conclusion

Is any of this useful for the casual rider? Um… eh, not really. Or perhaps I should say, it depends on your cycling goals. Here’s where power applies to cycling goals:

  • Losing weight: No
  • Getting in shape: No
  • Casual riding: No
  • Endurance Riding: Some (this goes more to energy conservation, or holding your power over long periods of time)
  • Getting faster (overall): Some
  • Become a good climber: Yes
  • Become a good sprinter: Yes
  • Racing: Emphatic Yes

So for some riding, it’s not really important at all. For riding farther and/or faster, you can start considering how much power you can sustain over long periods (endurance) or whether you’re generating more power than you used to (more muscle) or are lighter (less weight) and so can ride faster. But until you start focusing on things like climbing, sprinting, and racing overall, don’t spend your money on a power meter. Instead, to keep it simple, add some resistance training (weight lifting) to your biking regime to help build power while losing weight. Until you’re down to the “shaving seconds and grams” stage, that’s all you need.

Semper equitare.