ASK THE EXPERTS: 12-Volt Power

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12-volt DC outlet
12-volt DC outlet.

How much savings result from converting your house from 120-volt AC power to 12-volt DC power?

Lenny Grissom • via email

Shifting to a low-voltage (12 VDC) system might sound more energy-efficient, but unless you have a strictly conservationist lifestyle, it will more likely be complicated, unconventional, and expensive. And 12-volt loads are not inherently more efficient; in fact, line losses at this low voltage require larger (read “more expensive”) wire to carry the same amount of energy. Low-voltage systems can make sense in boats, RVs, and small cabins where the wire runs are short, but not in conventional homes.

A better question might be, “How much savings could you attain by switching to energy-efficient loads?” And the answer could be, “25% to 75% of your present energy usage.” Energy savings come from reducing energy (kilowatt-hour) usage, which means doing less, or doing the same job using less energy. A typically wasteful North American home uses 25 to 30 kWh per day for nonheating loads. After an energy-efficiency upgrade—including installing efficient appliances and lighting, eliminating phantom loads (appliances that are on even while not operating, like a TV or microwave oven), and possibly some load shifting to solar or other sources—you may be able to get to as low as 6 to 10 kWh per day. That’s real savings!

It is sometimes suggested that DC is inherently “more efficient,” but for conventional homes, this is rarely the case. On-grid homes are supplied with high-voltage alternating current (AC) from the utility, and converting to low-voltage DC incurs a loss, plus a cost for the conversion equipment. Sometimes, kitchen and other lighting is low-voltage DC, but that requires a step-down converter. If you feel that little black box, it will be warm—wasted energy.

If your energy supply is DC—from solar-electric modules and batteries—you might make a case for having your loads DC as well. I made this case to myself more than 30 years ago, and have lived with some DC loads since. But the reality of modern life is that most available appliances are AC, and very few people (including me) are happy to live with only DC. So even if you can run some loads on DC, you still need conversion equipment and/or wiring infrastructure for some AC loads. DC appliances are less available—they are made mostly for RVs and boats­—so manufacturers are less likely to strive for high-efficiency standards. And the DC appliances, since they are designed for part-time use, may not be as robust as AC appliances that are used daily in homes.

Overall, higher voltage systems will actually be more efficient and more cost-effective, whether DC or AC. Losses in electrical circuits are based on the amperage (electron flow). You can move more energy (kWh) with lower amperage by running it at higher voltage. This is why utility transmission lines run in the thousands of volts, and homes at 120/240 volts. Higher voltage means lower losses with the same size wire, or the same losses with much smaller wire. Copper wire is costly, so higher voltage saves you dollars up front, as well as energy losses throughout the life of the system.

The real savings—in dollars and kWh—is in reducing energy usage. Figure out what you actually need in lights, appliances, and other loads. Then find the highest-efficiency (lowest kWh) equipment to do the job.

Ian Woofenden • Home Power senior editor

Comments (2)

william walton's picture

Best usage for DC is non-motor, non-heat uses that already use DC-some electronics and marine LED lighting are the primary great uses-all else generally requires inversion and other ineffeciencies. 142 house wiring will work well in non-moving applications without much voltage drop in smaller homes. Look into Nigel Calder's book about boat systems to get a great handle on 12v. Cheers, Will

Kalos's picture

Are there loads that can usefully be run directly from a DC photovoltaic system prior to conversion to AC? Perhaps DC fans, pumps, etc?

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