It's probably dependent on the amount that needs to be washed. From the point of view of manpower and the amount of water that a human thinks they need, a machine would probably be more efficient in both the quality of work and the lesser amount of water used as a resource. There is however the element of energy used which does have a cost against the human input, which is probably costed in terms of time.
On the other hand, if the human was to provide the machine's energy need directly - eg. pedal-power - would that be the 'ideal efficient' way to go, I wonder? The human would have the added benefit of 'exercise' of limbs so needed in our very machine ordered world.
Just some thoughts anyway and thanks for the question.