How accurate are power meters?

Or should I ask how inaccurate?

According to http://www.youtube.com/watch?v=5DRQwKREgvI it appears a 3%
increase in bike+rider mass results in a 14% increase in power to
maintain the same speed.

Unless I missed something, the rider+bike mass for the pro rider was
78.4kg, and at 16km/h on an 8% gradient the power meter recorded 279W
average.

When they added 2.6kg, to achieve 81kg total and approximately a 3%
increase in mass, the power required to maintain the same speed of
16km/h was 40W higher, at 319W. A 14% increase!

I thought the power increase would be about proportional to the mass
increase, i.e. 3%.

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JS