Well, one potential issue that springs to mind is that DC 120V might overstress the diodes in the full bridge rectifier. With 120V AC only 2 of the 4 diodes are active at any one time and on average each of them is on only half the time - with DC only 2 of the diodes would be active, but they would be active 100% of time. Say the power supply needs to draw 1A of current, a reasonable good estimate for the voltage drop of a forward biased diode is 0.7V, so each diode that is on will dissipate 1A*0.7V = 0.7W. With AC, each diode would only be on half the time, meaning it would dissipate an average of 0.35W. With DC, 2 of the 4 diodes would be on constantly dissipating the full 0.7W each - this will result in the two diodes getting much hotter. Ask yourself how likely it is that most power supply manufacturers are designing in a safety margin of *2X*...

And that's just dealing with power supplies for electronics (which mostly just want low voltage DC) - if you get into appliances, while almost all will have a power supply that converts to DC, they usually have some circuits that use AC directly (think heating elements, safety critical sensors that use the 60Hz AC waveform as part of their feedback to make sure they haven't locked up, etc). Even appliances that convert the AC input straight to DC (think anything using an inverter to drive a motor or compressor) will often use a voltage doubler circuit to get a DC bus that is ~340V from the 120V AC input (it doubles the peak voltage, which is actually ~170V and not 120V in US) ... The voltage doubler circuit will not infact double a DC input, and the appliance will likely be most unhappy when it's DC bus is a lot lower than it was designed for.

Diodes may not like a constant DC load like that. In any case you're now definily running that equipment out of spec and totally at fault for any damages to the equipment, buildings and people stemming from that, so you better know for sure that that specific power supply can handle it.

I am not saying there isn't anything that can handle this out there, what I am saying is nothing is rated for handling it and this is experimental territory.

I don't think it's that dangerous to work with 120v. There's arguably a greater risk with DC starting fires and killing you that way, or burning your house down.

IANAEE, but DC does most of its work with current instead of voltage. So direct shorts are more likely to turn into heating elements (you have to run thicker wire anyway to do the same amount of work). AC can detect and trip faster.

Also, adding/removing loads is more finicky with DC. It affects all the other things on the line much more dramatically than AC.

If I had the time and money to work on this, I would focus on powering heavy appliances with solar and just paying the power co for all the wall wart electronics. Getting a heat pump working with a thermal battery (water tank with heat exchanger), etc. Separate circuit for electric space heaters (with extra consideration for fire safety), and smart thermostat that runs extra heating/cooling beyond what I requested when the sun is shining, to try using the house's thermal storage.

You can run your lights over ethernet. :)