I think you are focusing on a small fact about power conversion efficiency, and ignoring all the more important factors:
- extra cost of copper in house wiring to carry significant DC power
(this is partly why Tesla's AC beat Edison's DC system),
- the lack of a single standard DC voltage so there is still conversion
- the requirement for higher-voltage AC anyway for existing and likely
future home devices,
- the reliability, availability and economy of low-power AC-DC
- the flexibility of AC-DC conversion techniques
- the flexibility and scalability of converting from a single AC
standard to the specific requirement of the device
- the extra complexity of house wiring if two 'standards' have to be
routed, e.g. the need for new plug standards, extra cables routed
- there will still be a period of transition when both systems must be
supported, and IMHO no sane democratic government would mandate the change-over,
- I also believe as power increases the cost of components for DC-DC increases quite
quickly above modest power conversion requirements
Copper is expensive, London Metal Exchange - copper lists one tonne of copper (cash) is $7050, and that hasn't been processed into cable.
By comparison, London Metal Exchange - steel billet lists one tonne as $430, i.e. copper is 16x more expensive than steel.
So how thick must the 50A 12V cable be so as not to waste more energy as heat than AC? (Remember most of the benefit sited is efficiency, so it seems reasonable to ensure the losses due to heat in the DC cable are not worse than 230V AC.)
Lets compare AC mains cable rated at say 15A (Europe 230V), to 12V 50A DC cable.
Power loss = I^2 R
15^2 Rac = 50^2 Rdc
Rdc = Rac x 15^2/50^2 = Rac x 0.09,
i.e. Rdc must be ~11x lower resistance than Rac to achieve a similar power loss
DC cable would need to have over 11x greater cross sectional area than AC cable to reach the same power loss in heat. Put another way, DC house wiring would need 12x more copper so that the losses in the DC cables were no worse than AC mains.
Further, while the 230V 15A AC cable could carry 3.6kW, the 12V 50A DC cable carries 600W for the same losses.
Yes, for 11x more copper (to maintain comparable energy losses to AC) the DC cable carries 1/6th the power. Even at the scale of a house, the proposed low voltage DC cabling is not viable vs 230V AC.
The significant and important economic, practical, political and transition costs seem to dwarf any notional benefit.