DC 12V UPS for your whole house – 12V Socket!

So there’s a reason (or several) why your house isn’t wired for DC electrical operation. But there are a variety of situations in which you may want to add DC capability to your house — for example, off grid situations or if you want a house-wide 12V UPS backup.

A 12V DC NEMAish plug with receptacle.
A 12V DC NEMAish plug with receptacle.

There are some problems here:

  • There is no US electrical code standard for DC wiring at low DC voltages.
  • There is no standard plug for DC. The best we have are cigarette lighter plugs in cars (which are generally terrible for anything except lighting cigarettes).
  • DC at 12V will require 10 times the current as AC at 120V RMS to deliver an equivalent wattage. That means your wires have to have 10 times the cross sectional area to carry the same amount of power. This makes 12V impractical for powering large devices far away from the source, and by the same vain a 12V grid delivery system unfeasible. Whereas AC can easily be stepped up/down in voltage by transformers, DC does not share the same luxury.

So, put in layman’s terms, an exceptionally large 20 amp circuit (the kind with the special flat left neutral plug that you may see around your house if you have new-fangled dual style receptacles) would carry 2400 watts at 120V RMS, whereas it will carry only 240 watts at 12V. 240 watts is roughly equivalent to a flatscreen TV and a DVD player (if that is one of those blasted overburdened HDCP riddled BluRay players). Don’t expect to run a hair drier off a DC outlet any time soon. Maybe a hair luke-warmer…

But, there is hope! If you want to run a couple of LED lightbulbs and you don’t mind custom fabricating your cords, I give you our fair 6-15 NEMA plug. Made by ABS Alaskan, you can now grab your very own DC electrical outlet. I will admit, this standard is technically reserved for 240V power, but the amount of consumer appliances available for this plug will probably limit the probability of cross-plugging to nearly zero. This is the lower amperage cousin to the Nema 6-20 plugs, which we in the US call “drier outlets.”

Bear in mind, there are a tremendous amount of inherent limitations here. But, in the event that you want to be adventurous and wire your house for DC, this is the solution for you. The bottom line is: make sure you never wire anything to a standard NEMA 15 Edison connector that takes non-standard power. I’ve heard the second-hand story of a man who used Edison connectors for speaker connections to his custom stereo system. His girlfriend then remarked, “Hey John, you haven’t plugged your speakers in!” and the sound that came next sounded like “zzzZZZZBOOM” as the speaker coils fused into rings of metal and tripped the breaker.

I want my house to have some quantity of battery backup. A 120V grid tied inverter requires far more infrastructure than I’d want, a standby generator is probably overkill (and furthermore is very much interruptible), and I don’t need all my appliances to continue operation. This is a basic idea for UPS system which will allow me to run a set of 12V appliances through a power outage, limited by the capacity of the battery and the amperage rating of batteries and UPSs:

  1. Power input source DC ~14V (typical automotive charging voltage). Can either be PV (solar, through some ridiculous system) or AC supply.
  2. A large backup battery.
  3. This 12V DC UPS which can supply 10A of DC power. Multiple systems can be run in parallel to increase this capacity, and furthermore help ensure that higher priority systems last longer when placed on a lower amperage circuit. Alternatively, your network can be run off of the UPS to keep the amperage low and the remainder of the house can be a manual switch (jumper cables) from supply to battery (which would be acceptable for things like lightbulbs that don’t strictly need to be uninterruptible).
  4. The 12V can then be wired directly out of the UPS to cable modems, consumer switches, and using 12V-24V DC-DC step ups, it can also run Ubiquiti Unify APs using hokey injection connectors.
  5. Also wire this system through some fashion to your household alarm system. It may complain about running off DC indefinitely, but it’s better than a sharp stick in the eye.
  6. Wire a DC system to your preferred rooms, and then use this to run LED light strips, 12V CFL bulbs, a car stereo system, or anything else you can think of. Using these connectors, you can fashion RV appliances to run off of this system.
  7. Run this to a 12V computer power supply for a Mini-ATX computer at low wattage.
  8. Wire up DC light switches to runs of 12V LED strips around rooms off a separate light switch.

Here you have a potentially off-grid and backed up system for running your house off of DC. Interesting, at a minimum. I’m planning on trying to build something to this affect for my new house in Maine, after the realization that going from 12V batteries to 120V AC and then back to 12V DC represented a somewhat useless inefficiency of transform.

You can start with just a deep cycle lead acid battery and slowly move towards adding things like alarm systems and lights. I will probably get the wiring put in immediately because the walls are not installed. After that I’ll work on connecting it together. It also makes sense to unify all the backup systems in the house.

Anyways, let me know what you think.

3 thoughts on “DC 12V UPS for your whole house – 12V Socket!”

  1. > There is no standard plug for DC. The best we have are cigarette lighter plugs in cars (which are generally terrible for anything except lighting cigarettes).
    There’s lots of “standards” for DC connectors! To quote, Andy Tanenbaum in another context: “The nice thing about standards is that you have so many to choose from; furthermore, if you do not like any of them, you can just wait for next year’s model.”

    My ham radio mates swear by Anderson Powerpoles for DC connectors, but it looks it might be more effort/expense to wrangle these into a standard wall plate.

    The other nice thing about having DC through your house is being able to get rid of wall warts. Not only are they ugly and space consuming they are wasteful. At 240V they draw 1W or 2W or more even when they’re not connected to an appliance – my old CRT TV only draws 0.5W on standby. But not all wall warts output 12V so what to do about those?

    So do you run multiple voltage buses through your house, or just one? And what voltages should they be? As you pointed out the higher the voltage, the thinner (and cheaper) the cable can be, which also means less transmission losses.

    i suspect higher voltages are probably a better idea, but 12V will probably have the edge in pricing.

    1. Yes I’m basically talking about running a 12V DC bus through the house as an extension of my server closet and household alarm DC backup. And yes I am somewhat crazy. Though, a large quantity of the appliances I use do run on 12V. This would not be to power anything significant, but it will at least provide a few LED lightbulbs some juice independent of the grid. More of a pet project and to be contrarian.

      1. Sorry I was a bit unclear. I was just trying to generate some discussions on the pros and cons of using higher voltages and whether multiple buses might be a good idea – not question your sanity. My motivation is hatred of wall warts rather than surviving off-grid.

        Another thought is whether a home run topology might be a good idea for power as well making it easy to change the voltage to any socket, or vary the sockets/lights on each circuit.

        BTW, while not very convenient, I find it hard to beat the low cost/long standby/low maintenance capability of candles for the occasional outage we face in suburbia. Ok, the light isn’t very good and it isn’t very safe, but in principle, I prefer the idea of solid or liquid fuels that provided they can be stored safely can be ignored until the rare occasion when they are needed rather than the maintenance and periodic replacement requirement of batteries. Since we also have piped gas that could also be used for the majority of outages – if we invested in gas lights – and you think you are mad?

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