I'm considering putting a Raspberry Pi in every room of the house.
One issue is whether it is inefficient to have a separate USB power supply running off the mains in every room.
How would one go about providing an adequate DC supply for a whole home? Would any issues of resistance need to be considered given the distances (e.g. 10m to outer reaches of the home)?
My answer to a related question and some of the other discussion on the following may prove instructive. https://raspberrypi.stackexchange.com/a/9561/8697
The simple answer is NO.
The reason domestic power is higher voltage (110/230v) is to reduce losses, and this is only distributed over relatively short distances (a couple of km at most).
In telephone exchanges (at least pre digital) the power was distributed over copper bus bars typically 25mm * 9mm and this was 48v DC. To run 5v over domestic distances something similar would be required.
There are possible solutions. International fibre links have powered repeaters operating over tens of thousands of km, but these are typically fed by ±10kV from both ends, feeding the repeaters in series. Something similar would be viable over shorter distances, but the lack of available hardware would make this infeasible.
As previously noted by other answers it isn't practical to run separate DC wiring inside your home. Given that, a great alternative that I use to avoid having 5v power supplies plugged in everywhere is to change one or more A/C outlet in each of your rooms to an A/C outlet that contains a built-in power adapter such as the ones shown below.
Here's a link to one on Amazon.
To add some numbers to the already stated fact - power loss along the line:
Assume 10 m distance between load (the Pi) and a central power supply - equals 20 m of wiring (back and forth). Further assuming 1 A drawn by the Pi and additional peripherals (note this could be higher if more Pis are added...). Consider the power supply provides 5.25 V (acceptable max. voltage) and at the far end a voltage of 4.75 V is required (acceptable min. voltage). So a total voltage drop of 0.5 V is allowed. So with
U / I = R = 0.5 V / 1 A = 0.5 Ohm
the max. resistance of the wiring is 0.5 Ohm. Or considering the length of the cable (20 m) - 0.025 Ohms/m. With this we find that a cable with at least AWG18 (0.823 mm²) will be required. Expect a typical price of approx. 1 $/m. That's the kind of cable you'd use to wire speakers to an amp.
For comparison, network cables have typical strands as thin as AWG22, 23, or even 24.
