Why are my power meters reading differently?

We frequently get questions from customers who want to know why one power meter is reading a few watts higher or lower than another. This is always a tricky question to answer as there are several factors that can be associated with the difference in wattages. So, let’s dive in!

Manufacturer Difference

The first is the manufacturer difference. Each power meter company uses a proprietary formula to get to final power numbers. All are a combination of torque x cadence x a constant value. This is sometimes referred to as “power meter slope”. Because each manufacturer uses a slightly different formula, the way your watts are calculated is slightly different.

Power Meter Type

The next possibility for differences is the type of power meter. This is specifically referring to where power is being measured. The most common power meters are pedals, spiders and crank arms. Each of these measure your torque at a different place on the bike. For example, when using pedal-based power, you will see head units ask for crank arm length. The length of the crank arm is critical because of where the torque is being applied. Spider and crank-based power meters usually do not require this parameter.

Drivetrain Loss

The more complicated examples tend to be when we compare stationary trainers. Since power is most often being measured at the cassette, the flex and friction of all the components leading to the freehub body can affect power reading. This is referred to as drivetrain loss. This combination of chainring flex, drag of each roller and chain plate, drag of chain on cassette teeth and flex of the freehub body add up markedly.

Single-Sided vs. Dual-Sided

Power meters are available at a variety of price points and have benefits and drawbacks depending on the features available at a certain price. Many more affordable power meters are single-sided power meters. Dual-sided power meters are often more expensive than single-sided power meters, but they can provide more accurate power numbers than a single sided power meter.

This is because single-sided power meters measure power from a single leg (most often the left) and then double your power in order to estimate your total power. However, by doubling your left leg power, these power meters assume that you are producing the same power from both legs. The power you produce from each leg is referred to as power balance. Because single-sided power meters are doubling a single number instead of measuring from both sides, power can be under or over reports. This is because it is rare that a rider has perfect 50/50 power distribution, especially on every ride. Most of us have an imbalance – some larger than others.

Therefore, if your left leg produces less than 50% of your power, your left-sided power meter will take that number, double it, and will therefore be under-reporting your true power. The opposite is the case if your left leg produces more than 50% of your power. As you can see, power comparisons with a single-sided  power meter versus a dual-sided power meter can produce discrepancies.

Although a single-sided power meter may provide less accurate power readings, they should not be misconstrued as lower quality. They often feature a similar accuracy rating and are just as reliable as dual-sided power meters. Many manufacturers offer both left only and two-sided options. However, the two-sided power meters will give you more detailed and accurate power numbers.

Bike Computers

If you use different head units or apps to track your rides, they may provide different power readings. This usually comes down to how “zeros” are recorded. Depending on the sampling rate of your power meter and the communication protocol being used, head units can differ in how quickly they pick up coasting or spikes in power. While this can affect the data you see from your power meter, this is usually not an enormous consideration and can be smoothed by using an average power in your data fields versus an instantaneous snapshot.