Well it’s not standardized yet to my knowledge, but for example if we used something like the USB-PD protocol it could be a baseline 5 volts, with device negotiated step up to 9, 12, 15, 24, 28, 36, and 48. Higher voltage isn’t out of the question; EV systems safely run closer to 400 and a number of home batteries range up to 600, but I’d be iffy on the idea of the average contractor putting that voltage in the walls of the average home.
It’s true the copper for longer, higher current, or lower voltage DC runs could get very expensive, but even without HV for distance, thoughtful distribution of storage to expected points of delivery would limit the number of heavy lines needed for current spikes.
Long short, I’m not talking about switching entirely from AC, or pumping DC power through existing residential circuits. I’m talking about adding a secondary system that’s a more integrated version of the ubiquitous portable power station / “solar generator” batteries. It would be a home modernization upgrade, similar to running Ethernet to PoE enabled jacks in each room, installing a fancy intercom system, or what have you.