If one considers the number of low voltage devices in a modern home – mobile phone chargers, TVs, PCs etc – and add that to the possibility of using photovoltaics or maybe wind power linked to a 12V battery array, I have often wonedered if efficiency gains could be made by installing a 12V DC system in parallel with our conventional 240V AC wiring. Then there could be 2 systems, one running lighting, consumer electronics and possibly things like fridges, and the other running the high power appliances such as ovens, etc (I would have solar water heating so no big geyser).
So I went to google and this is what I found:
Self-sufficientish has done it for his lights
Halfbakery had some good discussion although there are some reality checks in the comments
Treehugger has an article too
Homepower Magazine looks like a good source of information (a good rule of thumb from the site: every dollar spent on energy efficiency saves $3-5 on providing new power)
So in short… its been done, its not that easy to do it properly, but its worth trying…