How Accurate Are Power Meters?

Most of us have been inundated with the beauty of power as the purest and best training variable to monitor. Before jumping on the power bandwagon however, it's important to understand just how accurate and reliable these units are. Fortunately, the Aussie sports scientists have done exactly that.

More: 11 Reason to Buy a Power Meter

Power to the People

It's no secret that the use of power monitors has revolutionized training in cycling, providing quantifiable data that advances and complements heart rate monitoring.

The problem however, even for the most well-heeled amongst us, is justifying the huge financial outlay.

Being a science geek, I'm not here today to sell you on the merits of power training, but suffice to say that power monitors can be a vital cog in maximizing your training.

More: 5 Reasons to Train With a Power Meter

Accuracy and Reliability Defined

What does interest me, however, is whether these tools are accurate and reliable, which is by far the most important parameter in monitoring tools. Accuracy refers to whether the tool is actually measuring what it claims to be measuring. In this case, is the readout of 200 W on the monitor actually 200 W, or is it 210 W in reality?

Reliability refers to the repeatability of measurement. If an identical power (e.g., 200 W) is recorded on this interval, will the same power output read 200 W again the next interval? The next day?

In many senses, reliability is the more important of the two parameters when talking about the utility of a tool, whether it's a power monitor or a bathroom scale for measuring body weight. That's because, the vast majority of the time, the important thing is not to compare your values with anybody else, but to track your individual response over time.

To further the bathroom scale analogy, it's more important that my weight reads the same when I weigh myself twice in a row, and not as important whether my "real" weight is 150 or 151 pounds.

More: Why Is My Brother More Powerful Than Me?

What the Aussies do for fun

Thank goodness for us that the sport scientists at the Australian Institute for Sport have a lot of toys on their hands, along with the time to play with them.

In a 2004 scientific article in the well-respected journal Medicine and Science in Sport and Exercise, Gardner et al.1 systematically tested the accuracy and reliability of two of the dominant power monitors on the market—SRM and PowerTap (PT).

The basic experiments were as follows:

  1. 19 SRM and 5 PTs were tested for accuracy using a CALRIG standardized device at 100 rpm from a power output range of 50 to 1,000 W. The SRMs were "Pro" models (four strain gauges) less than three years old at the time. PTs were less than one year old. Fifteen SRMs were then retested following a full season of use.
  2. To examine the effects of cadence, the most accurate SRM and PT unit from Experiment 1 was tested at 60, 80, 100 and 120 rpm using the same setup and test protocol.
  3. To examine the effects of temperature, the most accurate SRM and PT unit from Experiment 1 was tested during and following exposure to 6 C and 21 C temperatures.
  4. A field simulation was performed with random variations in cadence and power outputs using the most accurate SRM and PT unit from Experiment 1.

More: 5 Simple Ways to Increase Power on the Bike

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