Is your power meter giving you useful information?

Some power meters are more aero, less weight, more convenient, less expensive, more pro, and the list goes on. If you’re in the market for one I would recommend the most accurate and precise one that you can get. I have been guilty in the past of buying a single sided crank arm power meter because it was the most aero, only added twenty grams to my bike, and allowed me to use a fancy chainring. The downside was that I found it wasn’t precise enough to use it to pace myself in time trials or to test my aerodynamic position. Its imprecision meant that although it was still very beneficial, I had to interpret the data a little differently and I couldn’t get all the benefits of training and racing with power. Below you’ll see how you can make a reasonable guess about a power meter’s accuracy and precision. After that are some thoughts on how to best use an inaccurate or imprecise power meter.

Accuracy and Precision

A scale is accurate when you put a one pound weight on it and it reads one pound. One time I averaged 515 watts in a forty-five minute Equinox cycle class. That power meter was not accurate! Having an accurate power meter is important so we can compare our power outputs from ride to ride even if and when we change power meters. We can also compare our numbers with other people’s.

The precision of a power meter is its ability to read the same from day to day. If you went out two days in a row for a maximum sprint you probably shouldn’t see 1000 watts one day and 700 the next. Unless you have a heart problem or are taking medication you shouldn’t see a zone 2 heart rate one day and a zone 4 heart rate the next at the same power output. Imprecision is obviously problematic because it could mean we are pedaling too hard or too easy depending on what kind of day our power meter is having.

Testing Your Power Meter Accuracy and Precision

If you have a second power source, like a smart trainer, you can make a dual recording on www.zwiftpower.com. A dual recording compares two fit files and will show you how close two power meters are reading to each other. The two power readings and your heart rate can be used to make some assumptions about the accuracy and precision of either power source. This is a video explanation on how to do this.

What’s all this mean?

Hopefully you find evidence that your power meter is both accurate and precise 🤞🤞. That is ideal and allows you to take full advantage of training and racing with a power meter. If it isn’t then you have two choices. You can get a new power meter or learn how to live with it.

Inaccurate but Precise

If I had to choose I would pick precision over accuracy. If you know you’re getting the same reading from day to day you’ll be able to target specific power for training or pacing in a race. You’ll also be able to measure progress both short and long term while using that specific power meter.

The main downside of an inaccurate power meter is that it will be difficult to compare today’s fitness with your future fitness on a different power meter. It will also be more difficult to compare your fitness with other people’s. Another negative is that you wouldn’t know your actual TSS, CTL, or power profile. The 515 watt Equinox bike would say that my TSS and CTL were way higher than they actually were and that I was on my way to winning the Tour de France. However, if it was precise, I could compare my effort from week to week. For example, suppose using the Equinox bike data that Trainingpeaks showed I had 1000 TSS this week and 900 TSS last week. I know that I didn’t actually do 1000 TSS this week but at least I also know that I did more TSS than last week. So even though there are downsides to an inaccurate but precise power meter it will still be very beneficial for training and racing. You might not be able to compare your fitness with other people but you can compare your numbers from day to day, until you get a new power meter.

Accurate but Imprecise

As I mentioned above I have experienced this. From day to day my heart rate and power would stay stable on my Tacx Neo but the crank arm power meter reading would be higher and lower. The Tacx Neo is a very fine, accurate, and precise piece of machinery. Knowing that and also using my heart rate data I knew my power meter was imprecise.

It may be confusing as to how something can be imprecise but accurate. To use an analogy my power meter was reading like an archer that never missed the target but also never hit the bullseye. The power meter was always telling me a reading that was not exactly accurate but it was never far away. If you looked at a week’s worth of rides and averaged the power it would have been very similar to the Tacx Neo’s week worth of rides. So zooming out you can say that it was accurate.

Its imprecision meant it was reading fifteen watts too high or low depending on the day. This meant that I would either go too easy and get a slow time or too hard and blow up at the turnaround point in my time trials. I could never know for sure that I was holding the same power compared to an interval from last week. Before I found this problem if I had a lower result in a power test I would think I was getting slower when in reality it could have just been the power meter reading lower that day.

Having a power meter that isn’t precise can still be useful but you shouldn’t use it to hit specific power targets. You have to use a range. For example, as we know VO2 max intervals are done at 106-120% of FTP. The first time you do these intervals you should try to get in that range. Say you found that you were able to complete the intervals around 106% of FTP. If you have an imprecise power meter just know that next time you might be a little over or under that 106%. As long as you know you were pushing yourself hard you should assume that you were truly at VO2 max intensity even if you dipped below the 106% lower threshold. Next time you may be above 106%. You would definitely assume you were in the VO2 max intensity, but you couldn’t assume you were riding stronger than last time.

You’ll also need to use a power range rather than target to pace yourself in races. If you have a high heart rate and low power you should believe your heart rate monitor and assume your power meter is reading low. You may even stop using power for pacing in a race, like I did with that power meter.

On a positive note because an accurate but imprecise power meter regularly gets near the target (but does not hit the bullseye) your TSS and CTL numbers over longer periods of time would be pretty accurate and very useful to plan your meso and macrocycle. Zooming out over weeks you would be able to see on average your power outputs increasing or not. You’d be able to see where your fitness was trending.

Inaccurate and Imprecise

You’ll be training with heart rate and the RPE scale.