And how much more expensive is the twice as efficient power system that hasn't been developed yet? Nearly all robots in actual use have tethers, it's really no

And how much more expensive is the twice as efficient power system that hasn't been developed yet? Nearly all robots in actual use have tethers, it's really not a big concern. Further there are other methods of providing power, such as induction. For any situation where long range mobility is really a concern, you probably don't want a humanoid robot to begin with.