12-24V DC domestic power.

Literally everything in my front room right now that's plugged in converts 230V AC down to between 5V and 19V DC. If my home ran on batteries, I'd be wasting up to a third of all the energy they contained on pointlessly shoving it all through an inverter and then back down again through a DC adapter.

(There's also a DC-permitted variant of BS546 plugs and sockets for extra-low voltage DC, but I can't remember right now which one it was or what its max current rating is.)