Basically, nothing. The problem is voltage. USB needs 5V, CPUs/GPUs need 0.8V-1.4V (you feed them 12V, but that gets down-converted), plenty of other chips nee

Basically, nothing. The problem is voltage. USB needs 5V, CPUs/GPUs need 0.8V-1.4V (you feed them 12V, but that gets down-converted), plenty of other chips need 3.3V. You can't wire a home for 5V or even 12V DC because the losses would be unacceptably high. This means a full-home DC grid would need to run more like 100V-200V DC, so you need DC-DC conversion at every point of use. And efficiency-wise AC->DC or DC->DC don't differ much. They're both around 95% in ideal scenarios, or more like 80% in real-world use. It really isn't worth the effort.