Assuming you mean 5V or 3V DC, there will have to be a baseline level, then it will have to be upconverted or downconverted from the wall AC is (to whatever their baseline standard is in DC), then converted back to AC by X-converting then converting to something in AC, then downconverting to DC OR using semiconductors that used to be expensive (don't know if they still are). Seems complicated. Why not just stick with (in the US AC @ 110V - it's not like it isn't wide spread) and let all the device power adapters take care of themselves?

Or, you know, have better silicon components give us SDVC (Software Defined Voltage Control). I'm joking, but I also don't think you will save what you thik you will in TCO - Energy Savings + Device Cost differential.

I could be wrong. I have been before. And I am operating on almost zero sleep for over a day, so odds are, I could be doing to again at greater than my average error rate.