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You sure it's not just you tired/powerful legs?
Don't thinks so - 2x20s are pretty much bang on usually.
IIRC the calibration value is 7000ish - I gather that's not good.
I'll be doing the test in the link you posted, once I have a known weight / or once I find my luggage scales.
NotThamesWater-
7000? On which head unit? You'd be more than 20W off if that was the case.
Is it reporting higher or lower? 20W could maybe be unit heat or I once had a tyre go down mid-interval on a Computrainer - wondered why I was creeping up to 45oW when the trainer was set to half that! -
IIRC the calibration value is 7000ish - I gather that's not good.
If it's an old-style Powertap then the 'calibration' number is actually the serial number. My Bromton's calibration number is always -6029.
hippy
Brommers
You sure it's not just you tired/powerful legs?
What is it's calibration value? Is it within the correct range?
The acceptable range is 500-524
http://www.cyclepowermeters.com/powertap-garmin-calibration-check-76-c.asp