Modern switching transformers are pretty darn efficient. I think it's very unlikely that the losses from converting AC to DC would outweigh the transmission losses you'd get from using low voltage DC (even over the short distances inside a house).
I would love to see a Fermi estimate of this, I'm afraid it's outside my bailiwick.
The proposal would be 48VDC (pretty standard for panels) to 120VAC 50Hz, back to 5 and 12 volt at the application, versus sending 12 and 5 volt around a house, call it 10x10 meter perimeter and pessimize the layout (some outlets go all the way around the house).
My strong sense is that the single-step conversion beats the three-step, but losses transmitting low voltage DC are substantial.
Sending 12V around a 22' boat leads to voltage drops that are beyond the tolerances of many of the devices that consume 5 and 12 volt DC. Sending it around a whole house would require stupidly thick wires to stay within ATX spec (5%).
Of course, if you're running off a battery it's DC to start with, so you're avoiding using the inverter.
If houses started coming with standardized low-voltage DC alongside the regular 110 AC, manufacturers could perhaps start selling CFLs and LEDs without builtin transformers. Although I'm not what voltage they actually run on internally.
CFLs require a switching power supply because they operate at far higher voltages than line voltage. LEDs require some sort of current regulator, and a switching power supply is far more efficient than a series resistor. So, you don't gain anything for them by using a different voltage.
Yeah, I wondered about that, but my electronics-fu is not up to analyzing it. Running low voltage DC from one end of a large house to the other might simply not be practical. I'd still like to see a low-voltage plug standard other than USB and cigarette sockets, even if the conversion was being done inside the electrical box in my wall. I see the huge number of wall warts I have as a source of needless duplication and waste. (They are also, I understand, a revenue source for vendors selling replacements at inflated prices, so there is an economic incentive for the status quo.)
