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Here in the United States the electricity grid is AC. I have heard that AC allows transmission of power at greater distances with less loss. However, with the advent of solar panels, it would seem that one could generate DC power directly and power the home this way. There are no great distances involved.

Why is this not done? As far as I know, solar panels feed back into the main electricity grid. This means they convert dc to ac, presumably at some loss. Could you power your whole house using DC? Assuming you lived in a sunny area and had sufficient roof space, could you power everything (fridge air cond. etc), perhaps storing the power in batteries for use at night time? I assume you'd need all new appliances that work with DC?

It would seem a small price to pay to be energy independent. Could you reuse your existing house wires? I've never heard of this, so I assume there are major obstacles. Could someone give me the layman's explanation as to why it's a bad idea, or impossible, or just not done?

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AC doesn't inherently allow transmission with less loss, high voltage does. AC is just much greatly easier to turn into high voltage with 1900s technology. If they could change DC voltages just as easily back then, our electric grid could have been DC. –  whatsisname Jul 30 at 16:59
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FYI, this was a big debate in the 1880's, called the War of Currents. AC won. Also note that the phone network does distribute DC power. –  The Photon Jul 30 at 20:47
    
You might be surprised to learn that many electric and hybrid vehicles (which rely on DC batteries) use AC motors. –  nispio Aug 2 at 22:30

8 Answers 8

up vote 2 down vote accepted

It's not impossible, it's just more complicated and expensive. Everything in your house is designed to run from AC. Many smaller products do take DC in but they come with an AC adapter because that's the only available source of continuous, inexpensive power nearly everywhere. The voltage required can be different for each device. The closest thing to a standard for DC power is probably USB 5.0V, which only offers enough current for small gadgets and not anything larger.

The way a solar powered house works is roughly: solar panel to battery charger to battery, to DC-AC inverter to wall outlets, plus another power regulator & meter if feeding extra energy back to the grid, which isn't a requirement. One could power a house directly with unregulated DC from the battery if the appliances were designed to run from it, but most aren't. If the battery voltage had to be regulated before distribution to the house, all you'd really be doing is swapping the DC-AC inverter for a DC-DC regulator, basically a different box with similar cost.

Due to the small size of the market for DC appliances (at the moment), they'd be harder to find and possibly more expensive than AC units. If a time comes when nearly every house has solar on the roof, they might be just as easy to purchase and maintain.

As to reusing wiring, a wire is just a strip of copper and doesn't care whether you put AC or DC on it, IF you stay within its capabilities. If you had to put a lot more current through the wire due to lower voltage, you might need thicker wires, different safety features in the wiring boxes, higher rated fuses and so on. You'd want different plugs on the outlets so nobody made a mistake of plugging an AC device into an outlet providing DC.

Overall, it's cheaper and simpler to put a DC-AC inverter at the battery than it is to gut the entire electrical system of the house and rebuild it, plus buying all new appliances, plus still needing small DC-AC inverter in each room for the devices which can't be repurchased to run from DC - which at the moment is nearly every gadget. You might think of the AC inverter as providing "backward compatibility" with the previous hundred years of electrical devices.

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You can feed your house DC, however the issue remains that while most devices rectify AC to DC, they are designed for an AC input. This is why you need an inverter, even if it's at some loss, you feed your electronics what they were designed for. Even then, grid tie-in solar systems that you speak of only help boost the grid's power. You need quite a bit of solar panels and buffering (batteries) to make yourself completely separate from the grid, and even then, your capacity is limited to your setup, as opposed to being able to dynamically pull from the grid when needed. Getting more opinion based, I wouldn't think it would be worth the trouble and you miss a lot of the benefits. For example, say 50% of the population gets solar panels, not enough to fulfill their power needs individually. However, together with a grid tie-in based setup and inverters, they can reduce the load on the power generation company itself. Though, I also wonder about the safety of DC with current wiring standards. Maybe someone more experienced could chime in, however since AC isn't at peak voltage all the time (returns to 0V), it gives a bit of a cooling headroom. DC on the other hand is constant.

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I guess you could power a SMPS powered AC electronic device rated at 220 V with 310 V DC at no risk. –  Cornelius Jul 30 at 16:44
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One problem with high-voltage DC is that it's harder to switch. Switches and relays have to be (generally) highly derated. With AC, the zero-crossing will extinguish any arcing between the newly-opened contacts, while with Dc, the arcing can continue and damage or destroy the contacts. –  DoxyLover Jul 30 at 16:48
    
@DiegoCNascimento - Nevermind - somehow, reading Cornelius' original comment, I completely skipped over "SMPS" and thought he was talking about line-voltage equipment in general. I'll delete my previous comments. –  DoxyLover Aug 1 at 17:06

AC allows transmission of power at greater distances with less loss

Not exactly, heat losses are minimized by transmitting at high voltage as heating losses are generally proportional to the square of the current multiplied by resistance, allowing for inexpensive conductors and long distances.

...solar panels... [w]hy is this not done?

Look up the price of purchasing and maintaining enough solar panels to power your house.

Could you power your whole house using DC?

Yes. Most digital appliances will run fine on DC. Appliances like refrigerators and washers require AC to run timers and AC motors.

Could you reuse your existing house wires?

Yes, copper is copper.

This is not done because it is much more expensive. In the long run, however, there is a return on investment.

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Some modern appliances use 3-phase BLDC motors and have a HV DC bus, so they could, in principle, operate from HV DC with minimal modifications. –  Spehro Pefhany Jul 30 at 18:23

When I first started looking into solar powered homes in the early 1990's (U.S.), things were in a state of flux.

The old way of doing things was as the questioner suggested: run the house on 12V, with 12V lights, 12V refrigerator, etc. Some people advocated 24V instead of 12V for better efficiency of transmission within the home. I 12V was more common because there was already a market for things powered off of 12V car batteries.

But inverters were becoming more efficient and less expensive, so the more modern advice (in the early 1990's) was to get an inverter and use standard lights, appliances, etc. Instead of sinking money into non-standard, more-expensive 12V appliances, buy some extra PV panels.

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We're headed toward the cost of installation dominating the cost of panels & inverter. Getting rid of the inverter not a big cost savings. energy.gov/articles/… –  Matt B. Aug 3 at 5:32

Hi: When sizing wires for use in a house there are two considerations (1) the ampacity rating of the wire must be larger than the maximum current it will carry. This allows enough current to flow to blow the breaker if the wire is overloaded and (2) the wire size should be large enough so that the voltage drop at maximum load is less than 2%.

If you try to wire your house for 12 VDC you'll find that you need buss bar (you can't buy wire large enough in diameter) to carry the same amount of power you get from a 120 AC 15 amp circuit. Even if you only power low current loads the cost of the copper will be very high.

But powering a house from solar panels doesn't make economic sense because of the cost of the batteries. Much better to use a utility intertie inverter and just pump power back into the AC grid. Even in this case most inverters are very close to the panels and output 240 VAC to minimize the copper cost of getting the power from the panels to the nearest 240 VAC outlet in the house.

I have a 200W solar panel driving a small inverter plugged into a 120 VAC outlet after going through a Kill-A-Watt so I can see how much power it's making. The panel is not aimed and so getting 60 Watts is the peak for a summer day.

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You COULD do this. If you appliances were built to use DC. Which they aren't. Since houses are wired for AC, the appliance manufacturers design and build to use AC. That's the main thing holding you back.

There are standards in different parts of the world for what AC power should look like (120 Volts @ 60 Hz for US, 220 Volts @ 50 Hz for Europe, as examples) and light bulbs, vacuum cleaners, TVs, computers, etc. are manufactured to those standards. So far as I know, there is no internationally-recognized standard for DC. Ergo, good luck finding appliances which will use DC power distribution. There are a few which use 12 Volts DC, meant to be used in vehicles and boats, but they're pretty limited.

I've long thought it would be ideal to wire a house for 500 Volts DC and have point-of-use inverters which could produce whatever you want. 500 Volts would allow you to supply any of your existing loads with the same wiring (wire cross-section limits the amps; higher voltage = lower amps for a given load so the wires could handle AT LEAST as much as before). 500 VDC is also the maximum specification for electric vehicle quick-charging that I'm aware of.

If you were supplying 500 VDC through the house, a PWM circuit, an IGBT and an H-bridge would be enough to invert it into any AC voltage < 353 Volts. If we create AC at the point-of-use, for one plugin, not for the whole house, the components for that could be much smaller-scale and cheaper. Yes, you'd be putting one or two of these in each socket, which would drive up the total cost. But it would be possible to plug in that made-for-the-USA stereo next to the made-for-Europe lamp (or vice versa). Or, a variation on that device in the socket could supply the DC your laptop, flat-screen TV, etc. needs, directly, without converting DC -> AC -> back to DC again with the power brick. Arguably, converting high-voltage DC to low-voltage DC would be more efficient than that process. And "efficient" is the name of the game when you're running off photovoltaic panels or a battery backup.

Some years ago, I was reading an article by someone who dual-wired their house for the usual 120 VAC @ 60 Hz (USA) and 48 VDC. He was off-grid, routinely adding more loads and was trying to avoid spending money for a new, higher-capacity inverter, more batteries and more solar panels. He selected 48 VDC because he could get simple, resistor-based step-down converters for other DC devices. His answering machine ran off stepped-down DC, instead of a "wall wart" plugged into AC. Ditto for his laptop. His motion-detecting security lights used both; the motion detector was wired to stepped-down DC (yes, he had to crack the case and modify it himself) and the lighting used AC. Switching various things to DC was efficient enough that his existing battery pack lasted significantly longer and he was able to stay with his existing inverter and solar array. The resulting system, while more complex, was more efficient. This sounds like the sort of thing you're asking about.

Houses use AC because it was easier to make step-up/down transformers for AC back when all this infrastructure started to be built. At least one person has referenced the War of the Currents. Westinghouse and Tesla (proponents of AC) won over Edison (proponent of DC) because the ease of AC voltage step-up/down made it efficient and comparatively cheap to build a few power plants and distribute high-voltage power all over creation, then step it down to usable levels closer to point-of-use. DC required that the power be produced very locally, as stepping it up/down was difficult. Back then, stepping DC up meant you had a low-voltage motor turning a high-voltage generator. They didn't have semiconductor-based, solid-state switching in those days.

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Forgive me if I've misunderstood something. However, PWM does not itself create an inverter. PWM can create a sinusoidal DC waveform, but it does not create an AC waveform. Though, similar to most chopper circuits in SMPS's that would at least prevent coil/induction saturation, allowing for the proper functioning of items with transformers at the first stage of their power supplies. –  Jarrod Christman Jul 30 at 22:29
    
Updated my answer, being a little clearer about the circuitry needed at point-of-use. Better? –  Meower68 Jul 31 at 14:31
    
Ah yes, much clearer, just didn't want people to get the wrong idea. –  Jarrod Christman Jul 31 at 16:04
    
@JarrodChristman Two (or three) out of phase sinusoids constitute AC however. –  Chris Stratton Aug 1 at 0:04
    
Oh, my point was that his original post made it sound like you can get AC from a PWM DC signal.... Though that just creates an offset DC, since there is no zero crossing without additional circuitry (as far as I'm aware). PWM DC does provide the fluctuating current required to prevent saturation of induction components though, but strictly speaking, it's not AC. –  Jarrod Christman Aug 1 at 0:16

I disagree with many of the other answers. The more advanced our appliances get, the easier your proposal would be to implement -- as long as we're talking about DC voltages at bus level (150-170V in the US). Almost everything we use runs off DC, so there's power conversion going on anyway -- and luckily, almost all relatively new appliances use SMPS for the AC-DC conversion; these power converters have no problem accepting DC inputs (since the first thing they do with the input is rectify it into DC anyway). The only appliances you'd have to watch out for are things with big electric motors -- though many new appliances use BLDC and 3-phase motors driven by controllers which -- again -- run off DC. Also, anything with a transformer input -- think hi-fi stereo receivers -- is going to have issues running on DC. For these appliances, you'd need an inverter (which others have mentioned).

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About SMPS, without any modification you can have some troubles running it on DC.

The DC BUS rectifier. Only 2 diodes (considering a full-bridge rectifier) will be conducting, if they are close dimensioned (without safety margin) they can pose a problem. (For this just wire the DC directly to the DC-bus or replace with a higher current diode).

The PFC. Depending on how the PFC is implemented this can become a problem. Some controllers expected zero-crossings to create a sinusoidal representation of the current to compare and correct the device current waveform. In this cases where a boost type PFC is used, the voltage at the DC-bus is higher, so solving this is possible, but not so easy as feeding DC to the device without modifications.

About other things, there's some devices that control the power applied to the device by phase-angle control. Under DC they will simple latch.

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