I remember reading an article a few years ago about how the average UK home could save a lot of money in wasted electricity if the houses were wired with 2 electrical circuits – standard 240v and 9v – so that we would only have the 1 step down transformer.
time has moved on since then, and the 2v usb connector
For the same reasons as the National Grid uses high voltage transmisson, with local step down transformers; as has already been discussed here with regard to losses within short power leads from SMPS to Pi: distribution at low voltage within a house would probably be inefficient - as would a traditional 50Hz step-down transformer: so a series of modern energy efficient SMPS, used as required, to appropriate voltages - and my railway uses 4x 18Vdc @0-3.5A, 16Vac @0-5A, 12Vdc@0-4A and 5V@1A, for example shows that imposing builders fitting a single value low voltage socket would be inapproriate and inefficient ... and trake a long time to recover the cost!!!
I also find myself wondering how these 'statisitical comparison' houses are found: apparently using only 500W valve TVs and tungsten bulb lighting with no insulation..
TVs have had 1W standby for years, and 'equipment left on' is usefully heating the house in winter - resulting in less heat requirement from 'heaters' - assuming the home is well insulated. [MOST USEFUL would be the adoption of DYNAMIC DEMAND CONTROL - with fridges, freezers and underfloor heating etc using power when the frequency rises, but deferring consumption when the frequency falls, under load]
this has given me an interesting idea during my current home improvements i wonder what size transformer i can install in the garage and wire to key locations where i want to run a raspberry and charge my phone )
We have 2kW and 1.2kW Grid-connected inverters in our garage, running off the solar panels on our roof. But the wrong place for a lump of copper transformer!
The lower currents required by LED lighting might make the low-voltage-ring more viable (buit copper prices are high!) - but these are usually based around a well-regulated 12V.