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Comment by PaulHoule | Hacker News Reader

I recently bought an APC UPC to protect my DSL modem and phone when the power goes out. I have enough battery powered devices that I can be in communication as much as I need.

Those two devices draw about 8 watts, but the UPC I bought can run for only about 10 hours on that load. It could run a lot longer except that the inverter is scaled to handle a larger load so efficiency is bad. And why the hell is it converting AC to DC, then converting to AC, then converting it to DC in some awful wall-warts.

Since you don't run the inverter for really long times, there is a pressure to put in a cheap, inefficient inverter. However, if you took out the inefficiencies, you could afford to use lithium batteries and produce a much better product for this market. Hell, maybe the thing supplies power to the phone and modem through USB-C and it is just a super-powerbank.

Basically a "micro-powerwall". Safety issues still concern me but smaller scale reduces the odds.

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Something like this 50Ah battery pack?

https://www.amazon.com/Laptops-External-50000mAh-Portable-No...

Google around for external laptop batteries, they make some really big ones.

That is close. The single 12V port packs enough amperage that I could wire the two devices in parallel. I'd still have to make cables to connect to the devices. (The phone in question is a cordless phone connected to a landline. It probably should take usb-c but it does not.)

Mike Gyver sells some interesting batteries: http://mikegyver.com/

If you only need to backup power 12/24/48V devices, then you can skip the mains voltage inverter and use a battery-backed power supply. MeanWell comes to mind — they make these power supplies for security and telecom systems.

It's further down my list, but I am starting to investigate adding DC circuits to my house. Between solar and battery backups it would make sense to run as much as possible off of 12V DC.

I had the pleasure of working with one of the best electrical (motor) companies in the states, and decided to sit in on the journeyman electrician course (because sysadmins want to know everything), and I was surprised to find out that DC is what is used for the large high-power long distance transmission lines. I think there are probably quite a few applications where DC might be more appropriate than AC that haven't been realized yet.

I'm little enough of a hardware person that I'm just interested for the reasons the OP mentioned; constantly switching between AC and DC and AC and DC is obviously inefficient, and batteries are expensive enough that it's probably cheaper to add a DC circuit than to double or triple the battery capacity.

Tesla's powerwall claims 90% round-trip efficiency (AC in vs. AC out) and most SMPS power supplies are also 90%+efficient as well so you are losing <20% capacity (not 100-200%).

Numerically, I'm worried about ~50% power loss. If you are converting your solar from DC to AC there's another loss.

There still might not be enough power loss to matter, but even with 70-80% efficiency you're still talking an extra 25-50% batteries over the ideal.

90% is going to be in ideal conditions; probably not quite at full load, but not much below.

If you are using it to power a very light load, the efficiency will be much worse, as it takes a minimum amount of energy to run the inverter.

AC powerlines start suffering unacceptable dielectric losses above 750kV. DC power lines can go to about twice that. I don't want anything near 750kV anywhere near me.

DC power lines are also better underground or undersea, they don't suffer from the capacitance to the surrounding material.

The AC-DC conversion equipment at each end has a cost, but for 300 mile aerial lines or 30 mile underground lines it is currently cheaper to do DC.

DC works for transmission because it operates at incredibly high voltages. Power is voltage * current, but resistive losses are proportional to current only. So the theory that makes it work is since P=I*V, energize the line at incredibly high voltages and get very little actual current flowing to minimize resistive losses for a given power rating. UHVDC is something like 800kV.

DC is much less workable at low voltages like 12v or 5v common in household application -- the P=IV equation is more dominated by current, so with more current the resistive losses add up quickly even in a run across your house (it'll be down to 10 or 8v by the time the wire reaches the far end of your house). Not to say some people haven't suggested wiring homes with 12v lines... data centers have implemented (or at least experimented with) DC distribution due to the density of DC devices.

That's all true for AC as well though.

The benefit of HVDC transmission lines is that you don't have the skin effect at all which makes a difference even at 60 Hz when the cable is as thick as your arm. If you've ever seen one of those large transmission lines where each phase has three conductors all very close to each other but kept slightly apart that's done because of the skin effect.

Also there's always capacitance which will cause some loss for AC but of course capacitance won't affect a DC transmission line. HVDC also makes combining different electrical grids extremely simple as there's no phase difference or frequency difference to worry about. DC also requires lower peak voltages as AC is going to be sqrt(2) times higher than the rms voltage.

What I've learned powering an offgrid cabin…

Modern inverters and efficient walwarts (thank you EU) will be more efficient than 12v DC because of the power lost to the cables for anything but trivial loads.

The problem is some squared math functions which come and bite you. Your 12V DC system will pull 10 times the current as a 120V AC system. That's going to mean bigger wire. But then the kicker comes in, each volt you drop means 10 times the energy loss because 11/12 is a lot worse than 119/120, so you'll have even bigger wire. My 24V system with the batteries in a shed has $900 of copper wire between there and the house.

Then consider that all your loads probably already have a voltage converter in them, either an AC-DC external converter or something internal. Most of them won't take 11-14VDC inputs with spikes on it, so you'll need DC-DC converters on them anyway, so you aren't saving the conversion losses on the device end.

In summary, with modern high efficiency inverters you will probably find that your dollar is better spent on solar panels and batteries than masses of copper wire. If I were starting in 2018 I would not have run the DC lines.

DC-DC converters don't have the hellacious design constraints that AC inverters have.

If an AC inverter doesn't provide a sine wave, some equipment will fail. Making the sine wave is a serious burden, drives up the cost, and doesn't make efficiency any easier. In my case the DC cables are a few feet long.

Have you considered using aluminium wire instead of copper?

It might have been cheaper, but my conduit is full and won't handle the extra size. I trenched and buried that in the rock years before I learned to always oversize any conduit, especially ones that I have to trench and bury.

From your experience it sounds like I should do the math carefully before spending any money.

Have you had to replace your batteries yet? I’d assume batteries are a recurring (if with a long period) expense, while wire is a one-time cost. Does the downward trend in battery prices still kill any consideration in that direction?

I’m on my third set of lead acid batteries. I got 8 years out of the last ones and 7 from the first. This is on the long end of lead acid battery life. Capacity was down at the end for both, but I limped along. These are my last lead acid batteries. In 6 years I’m sure I will use something lithium based. The location is remote, requires small boats and hand carrying and the batteries are about 1000 pounds. A lithium replacement system will weigh a fraction of that, last longer, and won’t rupture and spill acid all over the place when I accidentally freeze them in the winter while the cabin is inaccessible and in telemetry only mode. (I’m two for two on this, both sets I’ve misjudged just how small their capacity had become and not gotten into emergency power conservation mode fast enough.)

You can buy 'super-powerbanks' for a few hundred dollars. Depending on the voltage and power requirement of the DSL modem you could probably run it, and your phone, right from there.

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