### When Is A Watt Not A Watt?

7.04w/kg …or is it 5.78w/kg…

Aside from the fact that the first of the two numbers above is a prediction (waste of time –see previous post) and the second is just given to us by Team Sky (no data was given, just a number that has apparently been corrected for off round chain ring variations), I thought it might be good to take a quick look at variations between power meters as a watt is not a watt.

Now assuming we are all happy with the fact that two riders can ride the same hill, with the same accuracy power meter, on the same day, doing the same watts per kg, but one can be faster than the other (due to influences talked about in the previous post e.g. rolling resistance, wind, positioning in the group, bike weight, bike efficiency, the path taken up the hill, air density and the list goes on…), we should take a better look at power meter variations.

### Power meter variations

##### Tour de France Watts:

Here’s a list of the numbers released by riders for the climb up La Piere-Saint-Martin on Stage 10 0f the 2015 TdF (15.3km/1100m in approx 41mins – for Froome):

- 7.04w/kg – Chris Froome, not so accurate mathematical calculation
- 5.78w/kg – Chris Froome, Stages (this doubles the power for the left leg and assumes this to be the overall power)
- 5.9w/kg – Robert Gesink, Pioneer
- 5.84w/kg – Adam Yates, SRM
- 5.5w/kg – Ten Dam, Pioneer

##### Fitlab Testing Watts:

Here’s a list of some one off data comparing different power meters in one off rides. The Powertap Pro was used in all the below examples with one of the other power meters on at any one time. The data is from several lactate tests outdoors (apart from the one labelled indoors of course!).

Powertap |
SRM |
SRM Indoors |
Stages 1 |
Stages 2 |
Quarq |
InfoCrank |

180 | 185 | 188 | 184 | 192 | 183 | 190 |

220 | 222 | 232 | 224 | 234 | 225 | 222 |

260 | 263 | 271 | 259 | 260 | 266 | 258 |

300 | 302 | 312 | 302 | 300 | 314 | 298 |

340 | 349 | 356 | 341 | 344 | 340 | |

380 | 384 | 365 | 387 | 380 | ||

420 | 428 |

I don’t think there is enough data here to draw any consistent differences between power meters (the same goes for the Tour de France numbers above), but it does show that in the same situation, doing the same effort on the same terrain, two power meters can be quite different. It is simplistic to simply say there is a percentage difference between two meters as it is often different for different ranges of power e.g. the percentage may be larger at higher or lower power outputs. Once again, there are so many variables that create differences between power meters e.g. what type of sensors or strain gauges, number of gauges, placement of gauges, sample rate, left leg only, location in drive train, etc, that it’s hard to draw solid conclusions from power numbers.

### Conclusion

While I am a big advocate of using power in races and for training (I have used a power meter since 1996 – the great Look Max One and several others since then) we need to be sensitive to the differences and variations between meters. If you test and train with the same unit, there is no issue. It is when we try to compare between two set-ups of meters that the problems arise (or use inadequate maths to make predictions).

For interest, here is the Look Max One being used to transmit power numbers on TV during the 1989 Tour de France…and you thought all this technology showing pro watt numbers was new! This old clip is better in many ways as it shows more than a few seconds of power data and is more helpful than Phil and Paul telling us a rider is pulling 400w (this means nothing if we don’t know for how long!).

So if you are not using a power meter, I suggest you get one as, when used well, it will provide the most gain in performance for your money. Just be cautious comparing your numbers with others!