Tag Archives: ups

Power efficiency of ATX PSU, power adapters and UPS

This article goes hand in hand with my previous one, since the findings I report in here are those that led me to update the power source of my server.

When you buy a PC, usually the PSU is the last of the worries: the more Watts it’s got, the better, as it can handle all the power hungry hardware you plug to it. Also, usually, this is “just right”, as we use our main PC for just a few hours per day (as long as you make sure you either use suspend for short pauses, or hibernation for longer ones, so you don’t waste energy while you’re not actually using it).

When it comes to servers tho, that are made to run 24/7, power efficiency is of paramount importance, because, make your own calculations, a single watt, year-wise, will be going to cost you something. Using my estimates, and living in Italy, for each watt of consumption of any always-on electric appliance, I will pay, after a year, about 1.75€. Not much in itself, but try and multiply it by 100.

Also, the more power you use, the more you end up polluting the environment, and raising the earth temperature.

So, of the 100, 150 or 200W-whatever that your PC drains, how many do you think are really needed to run the PC, and how many go wasted in the form of heat? For each fan you need inside you PC, you are raising one notch the waste-index of the computer.

Giving for granted the waste of power that goes into the hardware itself, I am going to talk about what regards power supply units, the PSU boxes that nawadays are very nonchalantly sold in the range of 500W-1000W. Do yourself a favour and buy a wattmeter (kill-a-watt or whatever you call it) and measure the power absorbed by your home computer, it will surely be way lower than the maximum rating of the PSU it’s using; the more the real load is distant from half the nominal power of the PSU, the more you’re wasting in heat, since the efficiency of a PSU is a gaussian curve that has its peak (be it 70% in the old fashioned PSU’s, or up to 86% in the newest ones) at 50% the maximum load. In other words, if the hardware in your PC drains 100W, you should get a 200W PSU, even if nowadays it’s hard to find one, so it’s still better a recent 86+ 5ooW one.

For a simple comparison: my current server, before I did the complete switch described in my previous post, was equipped with a 350W PSU, the only one I had available, and it drained little more than 42W in idle. One day, I had too much free time in my hands and went scavenging for other PSU’s, so I had the chance to test it with a 250W PSU, and the drain lowered to 39W, while, with a very old, and supposedly very inefficient, 120W PSU, the drain went even lower at 37W. As soon as I bought and installed a PicoPSU, and connected it to a PSU brick rated 12V@3A that I had from an old external disk, I got a surprising 29W drain while in idle, a total saving of 13W off the 350W PSU. Translates to roughly 23€ savings in a year… the amount it costed me to buy the PicoPSU from the US. A break-even after a year is a good break-even, I say.

Uninterruptible power supplies are another source of waste, even if you would never suspect it: my car-battery modded UPS drained an additional 10W in idle; thinking about it, though, it makes sense, as with a UPS you’re doing an additional conversion, from 220V to 12V, or from 12V again to 220V (in the PC it goes even one more time from 220V back to 12V), which is intrinsically inefficient. In a year, 10W would mean 17€, more than what I paid for a 120W 12V fanless power adapter from China; I already had the car battery, so again a one-year break-even by exchanging the UPS with a plain AC/DC 12V adapter, good!

Low power consumption server with integrated car battery UPS

I wrote in another post how I simply attached a car battery to a standard market UPS to give it even more juice; in that occasion, several users commented how I committed several shortcomings regarding the assembly.

Also taking into consideration that comments, this morning I rearranged the power supply of my home server to make it more silent and power efficient/independent.

On a side note, most comments were centered about me using thin cables to connect to the battery, with the reason that a car battery can deal a lot of amperes together, making a thin cable overheat, leading to possible fires. At first I credited them, but then I realized something, for which they should also get back to earth: we are not talking about shorting the battery leads for test purposes, but about using a day-to-day load, especially since it’s a low-power home server that together with accessories drain maximum 60W, which is 5amperes at 12V voltage.
Enter PicoPSU: you get a little toy able to give out up to 95W constant power (with my own model, but they make them up to 160W), which is 8amp at 12V. There is an 8 amperes current flowing through the cable running from the barrel connector of the PicoPSU… now please go and check the thickness of those cables. If those are enough for 8amps, how should 220v cable not be enough for 5 amps (tops, make it 4 on a regular basis)?

Back to us, my idea was to save the most possible on power waste, and have a silent PC that could sustain moderately long blackouts.


  1. Your favourite hardware configuration for home server/automation (I bought a D510MO from Intel, with a dualcore D510 64bit Atom, put 2GB of RAM on it, a PCI DVR card, and a 2TB Samsung disk, the cheapest I found)
  2. A picoPSU power supply or something similar (I bought for roughly 20€ off the US a PicoPSU80, rated for up to 95W max, but I’ll need way less than that)
  3. A decent 12V power adapter, preferably fanless, that can manage the load you’re going to put under it, plus some more (mine came from China, but it’s ISO and CE compliant, and apart from the build quality which looks sturdy, it can take up to a nominal 120W load, that is 10A@12V)
  4. A car battery, must not be new, but should be able to hold its charge for a while, otherwise it’s pointless to use it

Here goes.

server insides
This is how deserted the tower case (courtesy of someone who abandoned it near a garbage can) looks, but it's fine, as I have the space, and I want air to be able to circulate freely inside without needing fans.
12v barrel connector on tower case
Detail: I used a pre-existing screw hole used to hold in place the old PSU, enlarged it with a drill, and fitted the PicoPSU barrel connector inside. Notice the thin cables coming out of the connector (those are apparently perfectly safe for 8amperes).

NOTICE: Be ABSOLUTELY careful when handling the following step: never, Never, NEVER make it so the battery leads are shorted together, or you will be in for a GREAT amount of PAIN, including, but not exclusively limited to: electric shocks, fire, tools welded to other tools or to the battery or to rings (NEVER wear rings or similar metal things on you while doing this). Additional word of advice: when connecting a cable, make sure the other end is either not naked, or if you removed the coating already, put a strip of insulating tape around it, you don’t want it to make contact around; this is especially true when you have both cables connected, as the naked ends may make contact between them. When you are about to connect the battery to the load, uncover a cable end at a time (first the positive, then the negative), and not both together.

car battery negative lead
In the "car battery ups" post I wrote, several people criticized how I made the connections to the battery, both plug-wise and cable-wise. This time, I used cables 3 times larger, and since I didn't have a proper car battery clamp, nor wanted to buy one/disassemble from somewhere, I just curled the copper wire around the lead. Ugly, but in the end it's as much effective as a regular clamp (try and confute). I only had ground cables that big, so who cares, I just tagged them with coloured tape to tell them apart.
cabled car battery
Sexy. This is the final result: didn't have red insulating tape either, so I used black for negative, and white for positive.
cabled 12v 10A 120W power adapter
The cable panel for my adapter, large white cable is 220V mains input, ground cables are battery leads, and grey ones are the load (server + lcd monitor + modem/router + other stuff). Before you go and start bashing me because it's ugly: I don't care, it works and it's safe. Also, it's not visible in this picture, but each cable has its own white/black tape tags to tell if it's positive or negative lead.

Results were as following: I plugged the battery leads while the ac adapter was already powering the server, and no problem whatsoever popped up; after I did it, the wattmeter I was using immediately went down quite some figures, as the charged car battery was sharing the load together with the 12V PSU. With this setup, I can plug in/out the mains plug with no effect whatsoever on the server: load, PSU and battery are all in parallel, so it 220V inlet dies, the battery takes all the load with no hassle.

Currently, the wattemer is showing quite a lower load, since the battery was all charged: I expect it to slowly raise until full consumption, which should be little more than 42W (full stuff load, plus something more to keep the battery charged).


What would have you changed, and what do you like/dislike about this mone-morning project?

UPS and car battery, how to power up your emergency supply

UPDATE: there is a new article that expands and powers up this idea, you find it here.

For a while an old UPS of mine was abandoned in a corner because its battery was long dead and didn’t sustain the load of a PC for even a few seconds; and for a while I’ve been telling myself I needed to buy a spare battery off ebay or something, but never got around it because the prices were less than inviting; in the meantime, my little home server kept dying at each and every blackout or brownout.

The other day I noticed a car battery lying at home; we actually have a few car batteries here, unneeded but we keep them for the sake of it; none of them is new and they have been taken off old dismissed cars, they still work though.

car battery
When I saw this boulder I thought I may very well check if it was up and running, and hook it up to my abandoned UPS

Going by the theory, it’s still a 12V lead battery, so it should perfectly work with a normal computer UPS, yet I digged for information around several websites, and actually there weren’t any particular warnings (on the contrary, car batteries are made to sustain intense but short drains, followed by frequent charging, and that’s exactly the way an UPS is mostly used), but on the other hand, there could be, maybe, a little risk of production of toxic vapours, due to the acid contained in the battery; in other words, no problem whatsoever, but it’s recommended the setup be put inside an isolated, or well aired room; in the worst of cases you can keep the UPS by the PC, and use cables long enough to place the battery elsewhere; in this case the room I needed it into was quite isolated, so no problem at all.

After removing the old battery, I connected a couple of cables to the connectors (they should be thick enough), and I isolated everything with a hot melt glue gun (man's best friend)
ups battery cables outlet
I chose to make the cables come out of the front of the UPS, since the back was crammed with 220V outlets, so I made a little hole with a dremel and fixed everything with hot glue
car battery homemade contacts
I used a metal strip to fold into a couple of ring springs to connect to the end of the cables, isolated them with melt glue, and they fitted perfectly onto the battery poles
ups car battery setup
This is the final look of my own setup; the size of the UPS is roughly the same as the battery, so they fit together quite well; I could have found way longer cables (home AC cables are perfectly fine) to accomodate the battery farther away from the UPS, either in another room, or even outside

This setup has been tested to work perfectly, removing the plug activated the switch to battery power seamlessly, and I tested it for about one minute; I don’t know how long it could last, but probably could reach 30 or even 60 minutes, I didn’t test it thoroughly because I don’t really care, blackouts and brownouts last very short over here, and I don’t want to stress the battery for no particular reason, reducing its operative life.

A detail to note: when I wired the battery only, in a test setup, I couldn’t turn on the UPS, and I started thinking I failed somewhere, or even the UPS died for some reason, but it was just a coincidence, because my particular model of UPS is silly enough to refuse turning on if you try doing it without external power, in other words I needed to attach it to the mains plug as well, and I turned it on no problem.

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