[arwineap]

Naive question, why run inverters?

Wouldn't it be more efficient to run direct DC appliances?

We have a building on our farm without power and it'd be ideal to be able to charge batteries and run lights at night

It seems to me that we would have to upsize batteries in order to make up for the loss in converting to AC

[belval]

> Dumb question, why run inverters?

Can't say for OP, but DC appliances are just difficult to find, usually more expensive due to economies of scale and not as uniform in voltage (12/24/48V) as AC appliances. If your battery is in a shed somewhere it's also much easier to run a smaller gauge AC wire than setup distribution for your DC power.

Most large system are also 48V so you need to get it down to 12/24V which adds components anyway, at which point you might as well just have an inverter and not worry about any of that.

[bruce511]

You also need some sort of battery management, and inverters typically have this built in as well.

So yeah, you can get by without an inverter, but then you need a battery charger manager so as not to overcharge, and (depending on the battery chemistry) something to cut off the battery as it runs down. (Lead acid for example shouldn't go below 50%).

[K0balt]

As a parallel answer, for small systems this can make sense, although in anything larger than a single small cabin or small boat it probably is borderline these days. Modern inverters are quite efficient, and DC distribution carries a lot of issues, and gets downright dangerous at higher voltages necessary for large appliances.

20 years or so ago I rebuilt a 60’ schooner, and even on that scale AC was by far the best choice. Just the wiring for a DC system was more expensive then double Refund inverters, and most of the appliances were actually less efficient, since they had their own inverters inside them (DC-DC converters). In all there just wasn’t any justification, and corrosion is another issue on boats, so we went all AC.

[K0balt]

To be clear, we are distributing power over about 600m of length, so it has to be high voltage. HVDC is very very dangerous, I would not want it in my house. Not only that, it’s hard to find things that are efficient and work on high voltage DC. Since our batteries are already 60V, our inverters already are very efficient, it’s simply not worth it. Panels are so cheap, it almost doesn’t matter how you are trying to save power, the answer is that it’s almost always cheaper just to buy more panels.

[vhanda]

Not who you're asking.

I really don't see why we're still using A/C inside our houses / apartments. I understand that the transmission loss is lower when sending A/C, so it makes sense, but then nearly every device in my house has their own AC to DC converter. Just have one AC-DC converter per building.

I'd like the future to just be USB-C sockets in my house. We have USB-C PD 3.1 which supports up to 48v, I imagine that would be good for all devices.

There are probably safety reasons why this future might be difficult.

[bityard]

That would have been a good argument to make a decade or two ago but these days switch-mode power supplies are very efficient, and GaN ones even more so.

I have a small mess of 12-ish volt computer/network equipment in the corner of my office and looked into running it all off of one $40 high-amp power supply to eliminate all the wall warts and bricks. By the time I figured out power distribution and termination, buck/boost converters for the things that aren't 12V, it all seemed like a lot of work compared to just spending a couple hours tidying up the cabling and hiding the wall warts.

You can live in the future now and install power outlets with USB-PD built right in, although a quick glance suggests they top out at 65W. Fine for phones and tablets, might not keep a gaming laptop charged while in use.

[K0balt]

A lot of appliances need >1kw.

This requires higher voltage and robust connectors.

That level of DC is quite dangerous compared to AC for many reasons.

Also, unless you want to have 60lb extension cords the size of bratwurst, you need to go high voltage. High voltage DC is its own kind of devil, and is something I would not want in my household except in very isolated, self contained places.

High voltage, high current DC is on yet another level of mortal threat, able to do cool tricks like making extensions cords burn from one end to the other like cartoon dynamite fuses. Also, absolutely the best for accidental electrocution, severe burns and flash blindness, and setting otherwise fire resistant structures thoroughly aflame.

[eternityforest]

At USB voltages, one or two volts of drop is significant. You need very heavy cables, and for every voltage USB-PD allows you'd need another set of cables.

Also, some devices run directly on AC, or need more than USB can do, even with EPR. Since we already need AC for that, why add more wires when USB chargers are cheap and efficient and reliable these days?

[wolrah]

I'm not an electrical specialist but there are three major reasons I'm aware of that AC "won" at normal household/commercial power levels:

1. Switching. If you go look at your favorite part supplier you can find a bunch of switches that are rated to switch 250 volts AC and pass 16 amps, enough for basically any standard household outlet anywhere in the world. Those same switches are only rated for 24 volts DC. Why? Because of arcing. AC voltage passes through zero twice a cycle, which means that any arc that may be formed will self-extinguish within a hundredth of a second. DC doesn't do that, so the arc potential has to be limited either by reducing the voltage or increasing the size/complexity/cost of the switch/relay/contactor itself. This also applies to any connectors that may be unplugged under power like wall outlets. If you want to do the same amount of work with DC as you do with AC you basically get the choice between doing it at lower voltage with thick expensive wires or doing it at higher voltage with expensive switches, relays, outlets, etc.

2. Motors. Synchronous AC motors are EVERYWHERE. They're simple, cheap, efficient, and as long as they're not overloaded they run at a consistent speed determined by the number of magnetic poles in the motor and the AC frequency. If you have an appliance or power tool that runs on mains power and does not offer motor speed control (or only offers two or three speed settings) it's likely one of these. Native DC motors are also cheap and simple but but have very different performance characteristics, no native mechanism for precise speed control, and flow current through the rotor which requires brushed contacts that wear out over time. "Brushless DC" motors are actually AC motors paired with a controller which is more or less a DC->AC inverter, adding cost and complexity that may not be otherwise necessary or beneficial to the application.

3. Voltage conversion. AC can use simple wound transformers to efficiently trade voltage for current or vice versa using nothing but wire and metal. You might have used or even built one in a middle-school era science class. DC voltage conversion on the other hand, the simple methods are inefficient and the efficient methods require high-frequency electronics which only became inexpensive enough to go mainstream in the last 50ish years.

None of these are insurmountable problems of course, especially these days when switch-mode power supplies, inverters, VFDs, etc. are cheaper than ever but they still make things more complicated and require going against in some cases multiple lifetimes of industry inertia to purchase equipment produced in much lower volumes which means higher costs, and especially for home applications where size and weight are not the biggest deals it can often be easier/cheaper to just run a larger solar/battery setup to counteract the efficiency losses.

In the RV and boat worlds where size and weight matter you'll find a lot more DC appliances, but those are also generally smaller capacity than a household equivalent.