I understand that for transporting electric energy it makes way more sense to have AC. But within my house, I don't think that argument holds any more.

Devices I use (roughly ordered by power consumption):

  • Stove / oven: Isn't connected to the usual power plugs anyway, but a "high power" one
  • Washing Machine: max 900 W
  • Micro Wave: 800 W
  • Hand mixer: 450 W (I was quite surprised by that)
  • Fridge+Freezer: max 110 W
  • Laptop (usually 50 Watt, max 90 Watt)
  • Smartphone: 15 W?
  • electrical shaver: < 10 W
  • LED lights: 3 W
  • alarm clock: ?
  • Charging USB 2/3 devices

I've just seen that the car charger cable for my Notebook is WAY smaller than the one for the usual power plug. Looking at the power supply unit of my laptop, I see that it outputs direct current.

Seeing this huge difference in the size of the charger unit (and the price as well), I wonder why the usual power socket does not provide DC, but AC. What are the advantages within a house to use AC? Why wouldn't it be a good idea to have DC power plugs (e.g. as in cars)? (I've also heard that solar pannels provide DC).

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    \$\begingroup\$ Most of the devices you have listed are "natively" working with AC and do not require conversion. Only the Phone and laptop (and other USB) are inherently DC devices. \$\endgroup\$ – Eugene Sh. Feb 4 '19 at 18:56
  • \$\begingroup\$ What makes a device natively AC? \$\endgroup\$ – Martin Thoma Feb 4 '19 at 18:56
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    \$\begingroup\$ Type of the motor in most of your cases. Devices that heat stuff - do not care. \$\endgroup\$ – Eugene Sh. Feb 4 '19 at 18:57
  • \$\begingroup\$ Maybe something longer about the type of motors might be an answer already :-) (also: aren't there camping fridges/freezers? Do they use AC as well? Are they more or less efficient?) \$\endgroup\$ – Martin Thoma Feb 4 '19 at 19:00
  • \$\begingroup\$ My stove/oven is gas, electric plug looks all the others. Only my washer (Bosch) and table saw use 220V and have different plugs. Would need a big-a__ AC/DC converter to make DC for all the appliances - and then at what voltage? Makes more sense to keep single AC supply, and then let individual things change it as needed. Or not, if they can use 120VAV directly. \$\endgroup\$ – CrossRoads Feb 4 '19 at 19:00

Within your house, electricity is still transported.

The typical voltage loss on the average household cable of 1,5mm² cross section with an average length of 50m (forth and back) from the meter cabinet to your applicance is about 1V per 1A.

  • 1V isn't much for 230V.
  • 1V is much for lower voltages.
  • For lower voltages, you need more current for the same power, so the voltage loss on the cable gets worse.

That's why you want to have 230V at your electrical outlets. But your question was about DC. Why not DC?

Because, until lately, it was highly impractical to turn one DC voltage into another. You always need electronics to do that which have some sort of DC-AC-DC inverter inside.

AC on the other hand can be transformed to another voltage by a simple electrical component. No electronics. Copper on an iron core. A transformer.

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  • \$\begingroup\$ DC is also harder to switch. Mechanical switches that work fine for 230VAC would arc/fuse if asked to break a DC current. \$\endgroup\$ – caf Feb 5 '19 at 3:23
  • \$\begingroup\$ DC switches in households existed 100 years ago, when 110V DC installations were still common. It's those spring-loaded turning-knob types. Also, a single light switch per room made this viable. \$\endgroup\$ – Janka Feb 5 '19 at 8:17

The power company uses AC in the distribution network so that it can easily use transformers to move power between high voltage segments (for long distance lines) and low voltage segments (for supplying individual customers).

To supply you with DC, they'd have to add additional equipment near your home to produce the DC from their existing AC network. That would cost them money, so obviously they don't want to do that.

Alternately, you could take their AC and buy your own converter to produce DC for distribution around your home. But then you'd be faced with a fairly complicated optimization problem.

  • What voltage should you use? Large loads like motors would want higher voltages to reduce wire cost, smaller loads like smart phone chargers would want lower voltages.

  • How big a converter should you buy? Do you know how many appliances you'll have in 20 years and how much current they'll need?

  • Should you use DC for all loads, making the wiring simpler? Or should you stick to AC for (incandescent) lighting and heating loads, to reduce the size (and cost) of the AC-DC converter you need?

Currently we distribute AC around the house and use a separate converter for each appliance that wants DC. The benefit of this is it lets us optimize the DC converter for each load that needs it.

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  • \$\begingroup\$ @Hearth, I agree I left that unsaid, but I think that's mainly the reason the power company's network doesn't get upgraded, while the focus of this question was on the customer's wiring. \$\endgroup\$ – The Photon Feb 4 '19 at 19:34
  • \$\begingroup\$ Oh! I misread the question, it seems. Never mind, then. \$\endgroup\$ – Hearth Feb 4 '19 at 20:01

Because converting to DC is not necessary and costly for your entire house.

Most of the devices you named can use AC without an issue.

Converting your whole house to DC would be expensive and inefficient. You would have to buy a large and expensive converter to convert the whole house. This converter would use energy as it was converting and would generate a good deal of heat. Your electricity bill would go up 30% + whatever extra your air conditioning would use to counteract the heat generated. The converter would break every 10 years requiring expensive repairs or replacement, meanwhile rendering your house without power.

Buying small converters for devices the require DC is acceptable. DC only devices are a small percentage of the overall usage of the house. Small converters are cheap and easily replaceable.

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  • \$\begingroup\$ What if I only used solar energy? Then I would not need a converter, right? \$\endgroup\$ – Martin Thoma Feb 4 '19 at 19:57
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    \$\begingroup\$ @MartinThoma You would need a converter, because PV panels don't output a constant voltage. \$\endgroup\$ – Hearth Feb 4 '19 at 20:07

If DC is used instead of AC the copper drop issues are still there so large loads would be best on a reasonably high voltage .100VDC was normal before Tesla won the AC/DC battle over a century ago.Arcing and its associated fire hazard is much worse with DC because there are no zero crossings for the arc to go out .Hence DC switchgear is much larger and more expensive than AC switchgear at high power.

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I understand that for transporting electric energy it makes way more sense to have AC. But within my house, I don't think that argument holds any more.

You'd have several issues.

  1. Generation of the DC. This would require a large AC-DC PSU in the building.
  2. Choice of DC voltage: 12 V for car stuff, 19 V for your laptop, 5 V for you USB devices, 60 V for a particular LED light fitting? Which one would you standarise on? When you've done that everything else will require DC-DC converters.
  3. Size of cable required to run around the house to keep voltage drop to an acceptable level for the higher currents drawn at low voltage.
  4. DC circuit breakers.

I suspect that many off-grid residents trying to run on DC power banks are well aware of the advantages of a constant voltage AC supply.

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  • \$\begingroup\$ Agree - but I think you should emphasize part 2. It's easy to say, "provide DC", but once you do that there's no easy way to change levels, and that will impact all of the DC devices. If you provide a common AC level, any AC/DC converter can simply pick the right transformer winding to get the proper DC level, although transformerless switching converters can be a bit tricky. \$\endgroup\$ – WhatRoughBeast Feb 4 '19 at 20:20
  • \$\begingroup\$ On reading it again I agree. Fixed. \$\endgroup\$ – Transistor Feb 4 '19 at 20:30

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