We consider the theoretical maximum extractable average power from an energy harvesting device attached to a vibrating table which provides a unidirectional displacement Asin (ωt). The total mass of moving components in the device is m. The device is assembled in a container of dimension L. The masses in the device may be interconnected in arbitrary ways. The maximum extractable average power is bounded by mLAω3π, for motions in 1, 2, or 3 dimensions; with both rectilinear and rotary motions as special cases; and with either single or multiple degrees of freedom. The limiting displacement profile of the moving masses for extracting maximum power is discontinuous, and not physically realizable. But smooth approximations can be nearly as good: With 15 terms in a Fourier approximation, the upper limit is 99% of the theoretical maximum. For both single-degree-of-freedom linear resonant devices and nonresonant devices where the energy extraction mimics a linear torsional damper, the maximum average power output is mLAω34. Thirty-six experimental energy harvesting devices in the literature are found to extract power amounts ranging from 0.0036 to 29% of the theoretical maximum. Of these thirty-six, twenty achieve less than 2% and three achieve more than 20%. We suggest, as a figure of merit, that energy extraction above 0.2mLAω3π may be considered excellent, and extraction above 0.3mLAω3π may be considered challenging. © 2021, The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature.