PowerTap versus CompuTrainer Comparison

Living in Iowa, the majority of my biking is done indoors on a bike trainer, specifically a CompuTrainer (CT).  Overall I find it to be a very useful training device, providing simple and reliable operation.  Since the conditions are always the same, I use the CT for all of my Functional Threshold testing as well.

For riding outdoors, I have a PowerTap (PT) Pro+ on Zipp 404 wheels.  I use and Garmin Edge 500 bike computer to provide power stats as I ride, to help effectively gauge my pacing.

I’ve wondered if there were any significant differences between the devices.  I’ve noticed that I “felt” like my power numbers were higher on the PT than the CT, but my perception has come from time trials where adrenaline may have pushed me to achieve higher wattages than my normal FTP testing.

Both are shown by the manufacturers to have 2.5% accuracy.  Comparing then against each other, then the worst case there would be a 5% difference between the two power meters.  I decided to put together a somewhat simple test to vary power over a series of steps and compare the average at each step point.  I created a defined power profile (Erg file) in the CT, which basically keeps a relatively fixed power level independent of cadence.  I then used Training Peaks to look at the average of each point for comparison.

The CT data looked reasonably steady, as you can make out the steps along the way. Although I warmed up the tire during the calibration of the CT, I also started with 3 minutes at 125 watts, then 2 minutes at 150, and the rest of the points were 1 minute on the build (170, 190, 210,230, 250, 260) and then some cool down time.

During the same time, the PT recorded the following graph.

As you can see, there is a lot more variability with the PT, as the strain gauge interprets the power.

Using the same time duration in TrainingPeaks, the average wattage for each interval is shown in the results below:

Target

CT

PT

Diff

% Diff

130

125

128

3

2.40%

150

148

151

3

2.03%

170

167

171

4

2.40%

190

189

192

3

1.59%

210

208

213

5

2.40%

230

229

235

6

2.62%

250

250

258

8

3.20%

260

259

265

6

2.32%

Average

196.875

201.625

4.75

2.37%

The simple test was fairly consistent, with an overall average variance of around 2.4%, which would be well within the level of accuracy stated by the manufacturers.

Keep in mind that as the power goes up, if the percentage stays the same, the larger the disparity in watts.  I’ve attached a graph of the data as well:

Overall I was pleased with the consistency of the data.  Although not an exhaustive test, it did show me there was a relatively small but consistent difference between the two devices.

Is one more accurate than the other?  Probably, but without reference testing equipment, it would be difficult to know for sure which was more accurate.  My simple guess is that the CT is probably the more accurate, simply because it is not seeing the degree of variability that occurs in the PT.  The bottom line is that it really doesn’t matter as the two are relatively close overall.

My one small takeaway is that there are a few watts difference as the power goes up, meaning that I’ll likely add 5 to 10 watts to my targeted average power for short duration races such as time trials or sprint triathlons, when using the PT for pacing purposes.

2 thoughts on “PowerTap versus CompuTrainer Comparison

    • Certainly variance / sensitivity could be a contributing factor, but only to a degree before it’s outside of the manufacturer’s claimed accuracy. Most power meters are supposed to be accurate within plus or minus 2%. If you were riding at around 300 watts, the implied variance would be plus or minus 6 watts total. This would mean if your Kickr was minus 2% and your SRM was plus 2%, you maximum acceptable variance would be 12 watts overall, and both could still be within spec. That being said, most manufacturers are now trying to get power meters down closer to plus or minus 1.5% variance… or better (SRM claims 1%). Wahoo originally stated that they were plus or minus 2%, but believed they could eventually get to plus or minus 1 to 2 watts over a 200 to 400 watt “normal” operating range, which could be much better than 1% accuracy. There are so many variables and variances involved with power meters… it’s hard to pick which one is “correct” overall. It also depends on where you measure power as well. For example, crank based power should theoretically vary from hub or Kickr power by around 2 to 3% due to drive train losses (crank based power should read higher). It is also important to make sure that you do the calibration of any power meter, per the manufacturer’s recommendations, to ensure the unit hasn’t drifted over time. If you are interested in the challenges with direct comparison of power meters, see this post from DC Rainmaker’s site which gets into detailed comparisons http://www.dcrainmaker.com/2013/06/stages-review-update.html). For the most part, I think the key is consistency over time. I do most of my interval work and testing on the Kickr, so I have repeatable data for setting training power levels. Frankly it doesn’t matter if my FTP is 300 or 306 watts in reality, as long as I test and set my training zones based on the same device. It is beneficial to know the differences between devices though, as this gives you the ability to change the targets as appropriate when you use the different devices. For example, if your SRM always shows that it typically reads 12 watts higher than the Kickr (which could simply be power train loss), then you’d want to adjust your targets accordingly when using your SRM for pacing racing targets. The issues people were reporting below were backwards to what you’d expect… Kickr reading higher than an SRM (crank based), and the Kickr should be lower after drive losses.

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