Join Date: Nov 2008
Location: Gagetown, New Brunswick
Bikes: Cervelo S1, Norco Faze 1 SL, Surly Big Dummy
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With a lot of these things, don't concentrate on absolute numbers. Instead, concentrate on deltas (the change of your reading over time).
The problem with absolute numbers not being consistant device to device is not limited to bicycle power meters. Read up on car dynometers and how inertia dynos (like Dynojet) compare with eddy current or water dynos. You can put down one number on a Dynojet, then drive across the street to a Mustang and get a completely different number.
Calibration differences, differences in how the power value is determined... all these things can change the value the meter reads.
BUT, in most cases the meter is internally consistent, meaning that although it may under or over read, it always does the same "error" the same way. That means the deltas are correct. If power is increasing, you are putting out more power, and the amount of increase on the meter is proportional to the amount of power being increased.
My suggestion is to forget that your meter reads in "watts". Pretend that it is an arbitrary unit of power, and all that matters is that a bigger number means more power.