  Giving a universal upper bound on the power output of heat engines is a
long-standing open problem. We tackle this problem for generic quantum machines
in self-contained formulation by carefully including the switching process of
the interaction. In this way, we show a fundamental upper bound on the power
associated with the energy-time uncertainty principle. As a result, the energy
fluctuation of the external controller is verified as a necessary resource for
producing the power. This bound implies a trade-off between the power and
`noise' for work extraction, which yields an estimation on the time scale to
obtain detectable work extraction. Ideal clock-driven model of autonomous
quantum machine gives a concrete demonstration of our bound.
