But the vast vast majority of that wasted energy is not recoverable using any known technology, whether batteries or BTC mining.
Those numbers refer to the fact that, say, transportation is 21% efficient. That's because of things like the heat that the motor gives off, the energy that goes into moving molecules around due to poor aerodynamics, rolling friction and so forth. There is already incentive to use that energy - for example, more efficient cars would increase the range from a give size battery. But people want large, comfy cars with A/C and subwoofers and that means there's a limit to the aerodynamic efficiency of a car. You can't suddenly harness those losses and turn them into BTC.
It's not like 50% of the electricity generated simply goes away. Yes, of course, some of it disappears in line loss and transmission costs, but BTC mining only addresses that issue if the BTC mine is right next to the generation facility. Even then, you have significant losses in efficiency due to transformer loss, CPUs heating up, and so forth.
Anyway, the point is that if you use grid power to mine BTC, you don't suddenly efficiently start using that lost energy, you compound those losses by sending more energy inefficiently down the grid.
Now there are times in California and Germany when renewables produce enough power that there is more juice in the grid than the system can use and they have to export that power to other states/countries. That might be a case were you could BTC mine "for free" (and that's the situation I was thinking of), but I still think that it would be way better to store that energy as energy, not as a hash, and then use that energy later that evening when all the solar goes quiet, instead of holding it until your old age and using it to retire on.
