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Can home appliances be directly powered by direct current eliminating AC-DC conversion unit from the power unit. What is the economical value of this if possible.


  • You need to look at the 1885 - 1900 history of commercial electric utility development. Thomas Edison was a proponent of DC. He built the Edison Electric Utility in Manhattan based on a DC distribution. The challenge was the location of generation points and the very limited radius of distribution. On the order of a mile not 10s of miles.

    Nicola Tesla developed the AC model which allowed distant generation and distribution. George Westinghouse partnered with him to build and design a hydro electric plant in the Niagra Falls area and bring the power to NYC.

    So in short the answer is yes. Practical use is more on the order of not really. Too much has been invested in AC. Appliance design, light bulb design,and distribution infrastructure are 3. To add PV and not convert the power to AC would require two distribution systems in a home.

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  • Everyone on this list has heard about ECM and PSC motors. We have heard about the variable speed fans and variable refrigeration is looming.

    The units using these types of technology have to stay AC. Think furnace blower, ASHP blower, ac or hp compressor, refrigerator compressor, ceiling fans, etc All of these utilize the sine wave characteristics of AC. Change that you have to change the efficient design of the appliance.

    Do I see it happening? No? Could it happen with economic value, probably not.
  • David ButlerDavid Butler Posts: 3,889
    @John, ECM motors and variable compressors are in fact natively DC, but the problem, as you said, is finding products that will work with DC out of the box.

    I'm currently working on an off-grid project near Phoenix. The owner wanted to use DC lights and appliances wherever possible, against my advice as well as his solar installer. We're using a small chiller that's designed for off-grid applications and even that product has an AC power supply. The internal components pull different voltages off the power supply, some DC and some AC, so we would have to redesign the power supply for his 48VDC PV system. The same would apply to washing machine, refrigerator, entertainment system, etc.

    12VDC appliances and lights are available for RV and boat industry, but choices are very limited and typically not the sort of products you'd want to have in a home.

    The main thing is that today's inverters are around 95% efficient so there's not much to gain here.
  • Brad CookBrad Cook Posts: 153
    From a practical view, the advantage of the AC is that a changing electromagnetic field will induce a current in a nearby conductor, which is how we change the voltage of AC. That can't be done with DC, so you are stuck with low voltage DC (unless you put each panel in series, which presents its own problems). Electrical conductors all have some resistance, and with low voltage ("pressure") you need to have rather large conductors to minimize voltage drop when carrying any useful amount of electric current.
  • David ButlerDavid Butler Posts: 3,889
    @Brad, in a purely DC system with batteries, the battery configuration determines the distribution voltage, not the panels. The panels are only used to charge the batteries, so the panel configuration must be compatible with the charge controller's inputs.

    Some irrigation pumps for example are designed to run directly off of PV without batteries. In that case, what you said is exactly correct.

    But your point about electrical conductors is spot on. Even a 48VDC system (say, 24 x 2V batteries) requires MUCH larger wires to service higher power appliances that normally would be powered with 240VAC, assuming you could even find 48VDC products.
  • @David what of HVDC. HVDC lines experience less loss than equivalent AC lines over extremely long distances. Additionally, HVDC allows different AC systems (e.g. 50 Hz and 60 Hz) to be connected. Despite its advantages, HVDC systems are more costly and less reliable than the common AC systems.
  • David ButlerDavid Butler Posts: 3,889
    HVDC, a long distance transmission standard, has low loss (and doesn't require ridiculously large conductors) only because voltages are very high, typically hundreds of thousands of volts (e.g., 600kV). The conversion equipment is very costly (and maintenance prone as you mentioned) relative to conventional AC transmission. But in the right application, HVDC can be more economical. In any case, it has no relevance in the present discussion of powering appliances, presumably in a residence.

    Interestingly, the National Association of Home Builders Research Center (now Home Innovation Research Labs) spent over $60 million in the mid-to-late 80's trying to redesign the residential power system to include both AC & DC power as a part of the now infamous SMART HOUSE project. I did some consulting for them earlier in my career as I was involved in developing network protocols for inter-product communications. The SMART HOUSE power scheme looked good on paper, but had no practical basis.
  • I would like to see appliance mfgrs change to DC motors, that way we could wire home for both lighting and appliances to use DC current supplied by Photovoltaic panels, which would eliminate the heat loss from converting DC to AC and back.
  • I would like to thank the authors of each comment in this discussion. As a layperson I was unable to understand all of it, but enough got through to generate (no pun intended) the thought process and for that I thank each of you.
  • David ButlerDavid Butler Posts: 3,889
    @Craig, many appliances already have DC motors. But the manufacturers must provide an AC power supply in order to sell their products. The market for DC (e.g., off-grid) is way too small to make separate versions, and in fact, the cost of supporting both AC and DC in an off-grid home is considerably more than the conversion losses in terms of the additional capacity required to make up for those losses.

    In a grid-tied home, if you had a separate wiring system for DC loads, you would have to install a battery and charging system to power those loads at night. The cost of doing that would be rather ridiculous compared to the conversion losses.
  • David ButlerDavid Butler Posts: 3,889
    @Mike, feel free to ask questions about anything you didn't understand and are curious about.
  • Now this is a super interesting conversation. Thanks.

    Also, isn't saying ECM Motor like saying ATM Machine :)
  • David ButlerDavid Butler Posts: 3,889
    @Matt, true, but in the former case, I think the redundancy is useful since many people may not be familiar with the abbreviation.
  • I get it, I have seen very little in the way of advertising the availability of DC motor appliances; however, I would have to assume it is a small market, and AC motors to replace current appliances and the household wiring will keep AC appliances in demand
  • David ButlerDavid Butler Posts: 3,889
    @Craig, you're conflating the type of motor with the power supply of appliance. As the price of appliance compatible DC motors has dropped, more and more appliances include them for their benefits (primarily controllability and efficiency). But the marketing literature typically doesn't mention "DC motor" as a feature, but rather will describe the benefits e.g., higher efficiency. Even in hvac, you have to learn the jargon (ECM, X-13, variable speed, variable capacity, inverter drive, etc.) to interpret the documentation. But with a few notable exceptions such as RV and small boat market, products with DC motors are going to have AC power supplies for the foreseeable future. There's just no upside to doing anything different, given the cost & complexity of supporting a DC power distribution system in the home.
  • David, Thanks, I appreciate the comments and overall conversation.
  • Thanks so much for the contributions, more conversation are still welcome. Kudos
  • There is also a safety issue with DC distribution. Once an arc is initiated, it will not self extinguish until the conductors are separated by a significant distance. AC goes to zero voltage 60 times per second. At higher voltages all relays and breakers need to have a special design to blow out the arc. Think of DC welder. DC transformers are being developed for distribution systems, as they are now cheaper than the iron/copper transformer costs. But AC is still the final delivery product.
  • Bosch is working on a DC building-scale DC microgrid, and they've demonstrated it in some places. Enter "Bosch DC microgrid" in your search engine of choice.
  • David ButlerDavid Butler Posts: 3,889
    The Bosch DC power system co-exists with conventional AC power distribution. It necessarily includes storage capacity to power the DC loads at night. The Bosch system gets around some of the inherent problems with DC by distributing @ 380VDC. But this system is hardly practical for residential.

    One of the obvious applications for dual power systems is where lighting is the dominant electrical load. DC LED lights are readily available. Longer term, in certain types of large commercial, industrial and institutional buildings, especially where demand charges make up the largest portion of power cost, this type of power system has the potential to be cost effective.

    NREL published a report last year on the Bosch system. Here's the key take-away:

    "the Bosch DC microgrid uses generated PV energy 6%-8% more efficiently than traditional AC systems." (http://www.nrel.gov/docs/fy15osti/63959.pdf)
  • Interesting to know if the takeaway refer to microgrid dealing mainly with lighting or with other loads? I'll dive into that! Thanks for the link to the report, David!
  • David ButlerDavid Butler Posts: 3,889
    @Shawna, the report only references lighting and ventilation. Here's an excerpt:

    "Lighting and ventilation are the preferred load types because their profiles are predictable and align well with PV generation profiles"