[cesarb]

There's already a somewhat common standard for DC wiring in homes and offices, though it's probably not exactly what you had in mind: it's Power over Ethernet (PoE), which provides around 48V over standard twisted pair Ethernet wiring, while still allowing it to be used for data.

[ianburrell]

PoE is too low power to use for many things that would want DC power. 802.3bt tops out at 71W. You couldn't run a 100W USB-C adapter on it.

Also, PoE is pretty high losses running power over Ethernet cable. The max power at source is 100W. PoE puts in higher voltage for account for the losses which means it isn't a straight 48V input to 48V output.