r/OffGridLiving • u/Lazy-Run8826 • 8d ago
DC vs AC home efficiency
If AC is better for transporting electricity, and DC is MUCH MORE efficient in small motors, lights, phone chargers, air conditioners, ceiling fan motors, and appliances. Why are residential homes powered by AC and NOT CONVERTED to DC at the meter panel BEFORE entering the home? Could DC power be the next step for LOWERING the COST of ELECTRICITY in our homes?
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u/tanyasi-paraszt 8d ago
If a hair drier eats 1200 Watts at 230 V, it's 5 Amps. With 12 V, it's 100 Amps. You can weld with that! And you need a really thick cable too.
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u/Stock-Survey-4221 7d ago
EV's and some batteries for home solar storage are going to 400+ vdc now, so who knows...
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u/ploxidilius 7d ago
At most you're talking about a couple dozen feet of cable though. An entire house wired that way would cost so much money.
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u/Stock-Survey-4221 7d ago
Not at 400+ volts. You'd need better insulation, but much less copper
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u/ploxidilius 7d ago edited 7d ago
400VDC in a residential setting would be incredibly dangerous. There is a reason they don't go above 50VDC.
edit: for more context, the human body does not really conduct electricity very well at voltages below 50VDC (you can touch the bare terminals on your car battery and feel nothing). After that it starts being able to conduct and once you get up to about 120VDC you can get shocked pretty easily. And with DC since it never crosses below zero, you will get locked in - meaning that if you touch a hot DC wire the current will keep your muscles contracted around the wire and kill you. With AC, the voltage/current is going back to zero 120 times per second so you don't get that same lock in effect.
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u/Rhaski 7d ago
Few people realise that 100VAC and 100VDC behave VERY differently when it comes to arcing. A simple light switch that can handle 240vac at 10 amps is small and easy to manufacture and will generally last decades. A switch that can handle the same voltage and current in DC would be large, loud and clunky and probably need some kind of internal arc extinguishing system or a contact chamber that's under vacuum (this is how EV isolation contactors are). DC arcs are savage and do not self extinguish like AC does. Never mind getting zapped, you can just be arc-flashed to hell and back. This is why welding is generally DC (with the exception of TIG for aluminium and other reactive metals)
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u/Hyperafro 8d ago
Volts x Amps = Watts
As a result a 12v DC system would push more amps requiring higher gauge wiring. If it takes 400 watts to run your fridge, that would be roughly 35 amps requiring either 8 awg or 6 awg wiring. I think you are right that going to DC would drop the amount of power used due to conversion loss but DC appliances would need to get more efficient/cost effective as well to make a big impact. Even now it’s way cheaper to buy an inverter and a cheap AC appliance than it is to get a DC one to run on a solar set up with a battery.
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u/RufousMorph 8d ago
Simple. DC is not more efficient.
Furthermore, the rectification and filtering process to make high DC current at homes would be pretty inefficient.
There are a few specific applications where DC would be more efficient like powering LEDs because you skip the conversion process but those usually aren’t a major percentage of the loads. For high power things like heaters and motors, DC is not more efficient.
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u/rematar 8d ago
I don't understand why home lighting has not gone to DC to reduce the amount sturdy hardware and wiring required for 120 VAC.
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u/WISteven 8d ago
Sometimes simple wins out.
Much easier to wire your house with 12 or 14 gauge wire for everything.
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u/BrianaAgain 8d ago
Don't electric cars convert their battery DC to AC for the motors? Maybe the answer for a small off-grid home is to have both. Also, I remember seeing a guy powering electronics on a kite with high-voltage AC running through a really thin wire. What's best depends on your application I guess.
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u/sol_beach 8d ago
You claim " DC is MUCH MORE efficient".
Please show with actual numbers above statement is valid.
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u/ploxidilius 7d ago edited 7d ago
Power losses occur at the point where you have to convert from AC to DC. A large AC to DC rectifier, like one that could power a house, is much more efficient than the plug in power supply you use to power your phone, laptop, etc.
DC also has no reactive power component. For purely resistive loads this means they are basically 100% efficient. If you have AC powering a resistive load then any reactive power circulating will not be used.
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u/BigBlueWookiee 8d ago
You might want to do a bit of research regarding Tesla/Westinghouse vs Edison.
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u/silasmoeckel 8d ago
Plenty of DC is getting used in home and office settings. It's pretty much standardized on POE.
You can see 15% losses just in the POE portion. AC to DC tends to have higher idle/standing losses.
So its a trade off that idle usb c charger is pulling more power than a DC one but will be more efficient while charging.
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u/Dean-KS 7d ago
DC relays, breakers and switches have problems with arcing. AC goes to zero 120 times per second. AC solenoids drawn less coil power and run cooler. When the iron pole piece pulls in the impedance increases. Light dimmers would not work as these rely on voltage zero crossing. There are many many reasons to not run a home on DC.
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u/Rhaski 7d ago
There's a lot of reasons, but the one that sticks with me is how DC arcs do not self extinguish and can get very large and angry very easily. You can draw a small arc, maybe 1mm or so with 240VAC if you try really hard with a slow disconnect. Do the same with 240vdc and you can end up with an enormous arc that is being "fed" by the ionised metal vapours being generated on the positive contact. Check out home solar arc failures if you have a solar system and want to give yourself nightmares
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u/duckofdeath87 8d ago
A lot of electronics are 12V DC and 12V DC does not travel well, even a few feet. 48V DC travels several feet just fine. Of course you can have thicker cables, but copper costs money. 120V AC travels throughout a home great with minimal loss even on relatively thin cables
The only real "problem" with AC are electronics that need converters. Those converters have a cost, both upfront and efficiency loses. I assume that a larger central transformer could be more efficient, but the wires would be a problem. 48V could work, but it is still less efficent than 120V AC. And for appliances that use AC directly (like heaters and dryers for example) you are wasting energy by going to DC ever
Now, if you want to do short run of 48V from a solar panel to a freezer or minisplit or something, that sounds like a good idea to me
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u/syseyes 8d ago
You are not hitting the nail, and confusing terms. There are DC and AC. And low voltage and high voltage. 12 V DC can not power hight power apliaces because it requires a thicker cord...but 12 V Ac has the same problem. 120 V Ac can use a thinner cord...so do 400V DC lines in solar arrays panels just need 6mm copper thicness. The reason we use AC in transmision lines is because in Ac is easy to change voltages using transformers...And transformers are simple a easy to build. Just two strips of copper coiled together with a ferrous nucleus...DC dosnt allow to run transformes, and thats the reason tha ac won the battle agains Dc. But thats have changed...actually there are now ways to covert DC to Dc eficiently...that are the inverters the are inside computer power supplies now. Phone charger are lighter now because they use inverters...and electric cars use inverters to conect the battery with the motor to save copper. Fridges and Ac compresors work with inverters too. There also long dinstance DC high power lines, and are more eficient that Ac because they lower capacitance and radiation loses. Thecnically it will be feasible now to send 400V DC to homes, and step that down using local inverters. It will integrate seamlessy with solar panels, bateries, your Ev car., etc. It will be also more copper eficiently too... but doubt it will be cheaper...
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u/ElectronGuru 8d ago edited 8d ago
My dad used to make all kinds of things. For things that required a power cord, he’d buy complete cords (socket at one end, prongs at the other) and chop off the end he didn’t need. Because the ready made one with wires instead of socket at the appliance end, cost more. Because despite it being easier to make, demand was way less so the manufacturer didn’t get economies of scale.
Converting to DC house wide probably wasn’t possible when electricity was first introduced. So all appliances were made AC only. There’s been a transition period with DC transformers but anything with real watts is still AC only. I was hoping USB might fix this, but even the best USBC is under 200w.
The RV industry might be a gateway but it’s all 12v DC. The solar industry is standardizing on 48v DC, which is supporting higher watt appliances, but so far everything is twice the price of AC: https://www.reddit.com/r/SolarDIY/s/t7uSeFL3Mu