The problem is that nothing wants the same DC voltage. It wants 3.3V, or 5V, or 12V, or 48V, or 18.7V, or whatever. You end up with layers of conversion and save nothing in the end.
Let’s say you start with solar photovoltaic panels, a DC source. It feeds into a DC-DC inverter that puts out a steady 48V. Existing DC-AC microinverters can get 95% efficiency, and DC-DC conversion can be around the same.
In an AC system, we convert to DC at point of use (be it through a wall wort, a floor wort, a USB power port integrated into traditional AC plugs, or something else). This conversion can likewise be around 95% efficient.
In a DC system, we still have to do a DC-DC conversion to get the voltage you actually want at point of use. This is around 95% efficient, as well.
In the end, we stack the same number of conversions at around 95% efficiency no matter if we run AC or DC. Except that assumes we’re coming from a DC source in the first place, like solar photovoltaic. If we come from an AC source, like wind or hydro or pretty much anything besides a solar panel, then we only have transformer losses of converting the higher line AC voltage to what your house uses. Those aren’t 95% efficient; they’re closer to 98% efficient, so we’re better off.
Not only that, but we would still prefer AC for basically anything with a high draw motor, like air con or refrigerators. Now we’re doing DC-AC conversion, and that’s closer to 85% efficient. These are some of the highest draw items in a house–and they’re going to be used more as heat pumps for HVAC, water heaters, and clothes dryers become more common–so that drop in efficiency hits that much harder.
So DC home power sounds like a good idea until you break down how conversion efficiency hits things in practice. If we’re just going to get to the same place, why bother ripping out our current system?
That said, I would like to see PoE get used for residential more. There’s lots of devices that can run off 48V and can also use networking (like smart LED lights). Why not put it over the same plug and skip having to put those devices on a wireless network? Also, you don’t need a licensed electrician to run it. You can’t be an idiot about how to run and terminate it, but you don’t need a license. This would likely be alongside our existing AC wiring, though.