I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via th

I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads. When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit would be expensive.