Allot of DC devices at home run at various voltages from 22Volts down to 5Volts. So just running a 12Volt wire is impractical because you would have to regulate the voltage up or down - Just like with 110/220Volts now. That is why you cannot find a receptacle for 12 Volts.
You can use a 5Volt receptacle :)
OK- But what has that got to do with 12Volts?
Everything! You can run an Ethernet cable to transport your 12Volts throughout out the house which typically uses 24AWG. To be safe lets use 26AWG specification for current calculations.
26AWG by standard should handle 0.3Amperes. At 12 Volts that is no more than 3.6Watts per pair. If you decide to use an Ethernet cable you will have 4 pairs you could use giving you a total of 14.4Watts per etrher cable. That is enough for basic applications that need 12volt! But if you find thicker ether cable like 22awg you can raise this to 10watt per pair.
Then at each receptacle you can buy a 12v - 5v DC-DC converter (£1.50) and your USB plugs will be standardised and power via your 12Volt supply.
You may ask why not just run 5v directly from the batteries? The same reason you have 110v/220v in your socket. Over distance you loose power and the lower the voltage the thicker the cable you need to transport more power. So step down regulators guarantee near constant 5v at the source while you can supply several places with the same cable guaranteeing more power on smaller core cables.
But LED lights do not run at 5v! - Only at 12v!
No they do not run at 5v. So the only way to properly run devices that require 12volts in your home is NOT using a receptacle but wired in for good. Exactly the same way your 110/220v lights are wired in(all this is just downscaling) . You should then buy the properly colour coded wires and a larger core copper cable like
Generally for consumer wiring the standard should be,
- 220v brown/blue (yellow+green earth)
- 110v white/black ?
- 12Volt red/black
- 5Volt yellow/black
Many people will comment that above statement is wrong. Well, consumer wiring is the wires we get with TV's, kettles etc and that is what many countries follow. But standards vary from country to country and really old buildings pre 1950 in most countries do not follow any standard unless they were specifically rewired.
You would then just run insulted cable at a suitable gauge directly to your static LED lighting, even in your existing conduits! Just make sure you everything colour codded so the next
fella can SEE the difference. Even labelling 12v 110/220 is good practice but not necessary.
But I really want 12v at my reciptacle
This is an updated part thanks to another answer
You can buy these 12volt standardised plugs and connect them up directly to 12volt. It will require a bit of DIY but you can buy blanking plates, drill the correct size hole and fit these in. But i'll tell you what. They sure are ugly and I personally would avoid them.
It is good idea to make a Distribution Board just like the ones used for high voltage and use circuit breakers. With cat Cable (26AWG) you want 0.3A to be safe - the rest is up to what cable you decide to run.
But even a DIY one like this will work very well and ensure safety against short circuit, over current with the ease of monitoring consumption. You can even run 110/220 into this DB and incorporate a fail over, in case your batteries run down, so you could then have a simple sensor, relay and a 12volt 2~5a power converter.
Just one thing to remember - If you are using acid batteries do not leave them on concrete! Always place them on top of timber or elevated. A strange thing happens that acid batteries discharge when placed on concrete/ground and should not be close to earth. Not sure about dry cells.
This also looks nice and easy and uses car type fuses.
But using proper circuit breakers rated for Direct Current should be used as they offer reliable protection for various things, but cost allot more.
Not following standards
You might think to yourself, that you can just run 12volts directly into a USB receptacle. Sure, it will work. But what if your sons/daughter/wife friend comes round and plugs in their iPhone without asking. Ooops, its getting hot in there.