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If you harvest all the power output from the cranks you get a lot more than 3 watts.

Advanced sports cyclists actually generally measure their output in terms of watts, because of how independent that variable is from other conditions affecting the performance of a cyclist (wind/temperature/aerodynamic drag all have high impacts on final speed).

I think producing 57 watts is probably realistic for a long time for even an untrained cyclist, actually. 57 watts seems kind of low to me, though, and is likely an example of a particularly low energy consumption device.