Time to Ditch 120V AC? How a Low-Voltage, DC-Powered Home Might Work
Has the 240/120V AC residential infrastructure outlived its usefulness? Inspired by Tesla Powerwall, an industry veteran explains how we could get to the the old 'Smart House' vision of a low voltage, DC-powered home.
November 24, 2015
The hysteria over Tesla's Powerwall home battery has reinvigorated the discussion about low-voltage, DC-powered homes, and the potential for ditching traditional 240/120V AC infrastructures. I say reinvigorated because it's a topic that has been debated -- and even tried -- since the early 1980s.
Remember "Smart House?" Below, we revisit that bold (and ultimately unsuccessful) power-distribution initiative, and consider new models that just might work today.
Three events got me re-thinking DC power recently:
1. An In-Home Experiment Last month, my family took a three-week vacation to the coast. Before I left, I made sure everything I could think of was off: lights, computers, A/V gear, etc. But I didn’t go around unplugging everything. About two weeks after we returned, I got the electric bill that covered the time we were gone. It was about a third of our regular trying-to-be-thrifty bill.
I figured something was malfunctioning and eating power, so I went around and turned everything off again, same as before, and put a current clamp on the line into the house.
Surprise. I was reading about 1.8 amps. We have 220V power where I live (Turkey) so that’s about 400 watts. I have a small house (about 1,600 square feet), so that seemed like a lot.
What the heck is drawing that power?
It became obvious that evening when I turned out the lights. The house is full of little LED lights, the ones on the TV, amplifier, PlayStation, DVD player, etc. We all know about these. Devices that don’t really turn OFF. While they are not supposed to eat much power when off, they add up.
I also counted 12 plug-in-the-wall power supplies (wall warts): computer chargers, power supplies for phones, computer accessories, and other “low-power” gadgets. Then there is the modem, router, switch, and two WAPs.
But the big power-eater I forgot was the low-voltage lighting, mostly LED. We have several LED circuits that switch the secondary of the power supply to turn it on/off. The transformers are always warm to the touch. All this junk is ON all the time.
The reason it’s there is to transform the legacy 120V AC power down to a low-voltage that modern electrical and electronic equipment use. Almost everything outside the kitchen runs on low-voltage: TV’s, computers, LED lighting, network equipment, games, etc. The most recent big low-voltage change-over is lighting. Where I live, you can’t easily buy an incandescent light. Everything is LED—inherently low current, low-voltage devices that have to live in a 220 or 120V AC environment. Most of the cost of LED lighting is in the built-in power conversion electronics.
2. Tesla’s Powerwall Debut The second motivation was the announcement of Tesla’s Powerwall (PW) for the home. Like a lot of you, I was totally impressed. This could be the most innovative device for the home since the flush toilet.
After watching how this thing works, I immediately thought about the possibility of using it to go low-voltage in the home. Maybe it could be the catalyst to make that happen. After doing more research on the thing I haven’t been able to find enough technical details about what’s behind the cool facade.
The specs state “voltage: 350-400 volts.” I assume this is because most of the internal Li-ion batteries are wired in series to keep internal currents to a minimum. While the Powerwall is probably not the change-over technology needed in its present form (Version 1.0), the most interesting thing about the device is not the technology. It’s the cool factor. Elon Musk managed to pull off a “Steve Jobs”!
There is no economic model that saves money for the Tesla Powerwall (Solar City can’t even make an argument for it), yet the thing is sold-out for the next two years and there isn’t enough capacity in Tesla’s new $5 billion factory to keep up with expected demand.
This thing has definitely hit a nerve.
It’s a “game changer” that makes a whole lot of things possible that no one had the incentive to do before.
3. That CEPro Article The third motivation was the May 2015 article by Jason Knott, “Will Tesla Batteries Force Home Wiring to Go Low Voltage?”
The article was inspired by a blog post by Paul Self on Buildz.com that promotes the change-over. The article raised the question of switching over the power wiring infrastructure of the home to something like 12 volts, based on the idea that solar panels and storage devices like the Powerwall are low-voltage (by low-voltage I mean anything under about 48V, and DC). I wondered: Why convert this up to 120V AC when, as I described, most things are low-voltage?
Of course, the high-voltage electrical distribution system is a necessity if power is generated at some central location, miles from the end user, and distributed over a large network of cables. You can’t do that practically with low-voltage. But if power is generated where it is used—the house—high-voltage is not really needed; however, as the NEC constantly reminds us, it can be very dangerous. The infrastructure is burdened by tons of codes and regulations to keep us safe.
It Almost Worked with NAHB’s Smart House
As many people pointed out in the comments to Jason’s article, for many reasons you can’t just scrap our beloved 120V AC house wiring starting with the next new-construction housing development. Or...can you? Believe it or not, this very thing has been tried before, big time, and almost succeeded. Those of you over 40 or so might remember the Smart House Project.
The Smart House Project was initiated in the early 1980’s by the National Research Center of the National Association of Home Builders (NAHB) with the cooperation of a collection of major appliance and electrical equipment partners.
The project proposed to completely replace the current home electrical infrastructure with a single multi-conductor ribbon-style cable that included electric power wires, communications cables for telephone and video, and other conductors that connect appliances and lamps with electronic devices that control the supply and switching of power. And it proposed to do this, like, next month.
The system had a central “computer” and power distribution equipment in a rack that provided power only to outlets that had appliances plugged in and turned on. (See Smart House patents in the gallery below.)
An electrical device could “request” either AC or DC, high- or low-voltage, down to 6V. It required ALL new everything. The cost was somewhere in the denial category.
The most amazing part is they almost pulled it off. The project was real enough to convince Amp Inc. to make spools of the special foil-backed ribbon cable, complete with termination tools, outlets, tie-down gadgets, connectors, and so on. They even convinced Whirlpool and other major appliance manufacturers to build the electronics into prototype products. You have to remember this is back when a microprocessor was a new innovation and home automation was in its infancy.
Obviously it didn’t succeed, mostly because the technology was not really ready for prime time and it was way too big a “gulp” for most builders and home owners. Once you installed it (and got it working), no going back.
But despite being too early, there are a few less-obvious lessons from the project.
- With the right backing, it could actually be done.
- At least some manufacturers were quite willing, enthusiastic really, to support it and make stuff to implement it.
- They were able to get cautionary support from the National Electrical Contractors Association.
Keep all this in mind.
Why Low-Voltage Architecture Makes Sense
I got a saying in my business, “Just because you can do something, doesn’t mean you should do something.”
This keeps me out of a lot of trouble in home automation. Any big change, like a low-voltage infrastructure, requires a really good reason before going to all the hassle.
Some obvious reasons to me to implement a low-voltage infrastructure:
- Gets rid of all the wall warts and charges.
- Gets rid of the shock hazards and burdensome code restrictions.
- Could easily be combined with data network, so one set of cables providing data and power where I need it.
- Certainly could save money both in installation and operation.
- Takes advantage of the public’s interest in home power generation and storage.
Let’s assume for the moment that this is enough to move forward. I’m sure there are other advantages I haven’t thought of. The only question then is: What is the best form to implement the system?
The rest of this article describes a workable solution.
Reality Today: How a Low-Voltage Infrastructure Might Work
I think we can make a couple of assumptions about technology trends that will help make a low-voltage infrastructure possible.
- Appliances available in the U.S. in the future (say 5 years) will be just as efficient and innovative as the appliances I can now buy (and I own) in Europe. (I have one, yes one, 25-amp main circuit breaker for my house, yet can run the stove, oven, clothes washer, microwave, lights, and all electronics, at the same time). This means that a future power distribution system (high- or low-voltage) only needs to supply a fraction of the power currently needed by appliances.
- Manufacturers of electronic equipment can easily adapt to whatever the house low-voltage is. Given how fast they change plugs, chargers, adaptors, this is a no brainer.
Based on these assumptions, a low-voltage house is feasible, powered directly from batteries that are charged either from solar panels (a la Tesla Powerwall) or from the legacy power grid while waiting on the backlog of solar panel installation.
This model is a perfect match for the direction power generation is going in the world and will speed up the development of more energy-efficient appliances, more efficient use of electricity in the home, and more innovation around the platform.
What a DC-Powered House Looks Like
The infrastructure can evolve in three phases. Each phase is easily upgradable to the next. Let me jump to the end result first to show how this might work.
Figure 1 shows the infrastructure of a fully functional low-voltage house. I am using the Powerwall as a possible example of low-cost storage, but there are others such as the Energy Hub from Rosewater Energy.
Figure 1. Example fully implemented low-voltage infrastructure (image: Grayson Evans)
Power from either solar panels or the grid, if necessary, is stored in the Powerwall (PW) or equivalent device. DC is then supplied from the PW to a low-voltage load center. This device provides the same function as a regular 240V load center. It changes the voltage from the storage device to the voltage for lighting circuits (12V) and outlets (48V). It has low-voltage, high-current solid-state “circuit breakers.”
Ground wiring is no longer necessary. Branch circuits are wired exactly the way they are now, even using the same cable if desired (less the ground wire, now obsolete).
There will be several types of outlets. A typical outlet—now free of code requirements intended to prevent shock—contains a DC power jack (perhaps in USB form factor), data jack, cable TV jack, etc., as necessary. Lighting circuits work exactly the same except the LED fixtures, now free of the AC to DC conversion circuit, are cheaper and use even less power.
Since most things that plug into low-voltage are electronic and probably are network-capable, the low-voltage outlet should also attach the device to the home network.
Actually, I wrote an article for CE Pro years ago proposing that the Ethernet PoE standard could be significantly enhanced to provide power to devices in the home. Too early.
The necessity for two different output voltages is based on the different requirements for LED lighting and other LV devices. 12V is a very common LED light fixture standard, especially for strip lights. The power required is low, so 14 AWG cable could easily meet lighting circuit requirements of 120 watts. 48V is a good compromise of voltage vs. current. It is the standard used for power PoE devices such as cameras, WAPs, etc., but PoE circuits are limited to 15 watts per device due to the very small gauge CAT5 cable. It can easily be regulated down for target voltages of 24V, 12V, even 5V.
I don’t think it would be necessary to do home-run wiring except for kitchen/laundry outlets. If the LV is wired with standard 14 AWG copper, a 100-foot 48V branch would have a voltage drop of 1 volt at 100 watts and 5 volts at 500 watts. Very acceptable.
To test the concept, a first step might be to simply add an additional low-voltage distribution infrastructure to new or existing homes (Figure 2). It’s just an added component to the structured cabling infrastructure that low-voltage integrators are installing now. The LV cabling needs to be installed where there are other low-voltage jacks (Ethernet, cable TV, sat), plus a few where standard electrical outlets are located.
Figure 2. Initial installation of low-voltage power infrastructure (image: Grayson Evans)
The next phase would introduce energy storage from solar panels or the grid. The added components are shown in Figure 3.
Here we still have the traditional HV infrastructure, but this time the Powerwall is doing double duty by supplying the LV infrastructure and converting DC to AC for high-voltage power.
As more and more devices directly connect to the LV infrastructure, we can ditch the HV infrastructure. In this phase, something like Tesla’s 7 kW model that can output 2 kW continuous is more than enough for LED lighting and a house full of electronics. Traditional 120V would handle larger legacy kitchen appliances, HVAC, other high-voltage loads.
Figure 3. Phase 2 typical installation showing power storage/generation options (image: Grayson Evans)
Obviously, there are a number of details that need to be worked out such as connector types, current limits, max voltage drop, etc., but this are the easy part once the commitment to building a new infrastructure has been made.
The trend to solar power and energy storage in the home is a reality—despite electric utility resistance—and will only increase as panels and devices like the PW become less expensive and more versatile.
Lyndon Rive, CEO of Solar City, is quoted in the New Yorker magazine as saying his company completes a new home solar panel conversion every three minutes, and expects to get that to one every 3 seconds in the near future.
There is definitely an opportunity for home technology integration businesses in adapting the residential infrastructure to the changes taking place in power utilization.
Our industry has the knowledge, tools, and a foot in the door to “ground zero.”
The implementation of a low-voltage infrastructure can “boot-strap” itself like structured cabling did decades ago and how solar conversion is going today. All it will take is a few innovative companies to make the parts and pieces, adaptors, etc.
Perhaps it will be your company?
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