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All the laptops I know about operate on ~19V, in consuming up to 150W.

All the non-gaming desktops I know about take 110v/220v, convert into 12V & 5.5V, consuming up 400W.

To provide desktops with a good power, uninterruptible power supplies take 110v/220v AC and output the same AC, though they have, to my knowledge, 20V batteries inside.

There must be a good reason companies do so, rather than combining UPS & PSU into a single unit.


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closed as off topic by Zoredache, Wesley, Tom O'Connor, Chopper3, Chris S May 4 '12 at 12:37

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Those are lead-acid batteries, which are 1.2v. LiIon have the highest voltage of common batteries at 3.7v. – Chris S May 4 '12 at 12:41

There are a couple of competing interests in this space.

Ease of Integration

By supplying power that looks just like utility main-feed, it greatly eases the task of distributing the UPS-conditioned power to a variety of equipment. Only one output voltage need to be produced (sometimes two), and stepping to the device power is handled on the spot by the PSU. A system like this is dead easy to put together.


Stepping voltage down from utility main-feed (which can be 3-phase 440v as it enters the UPS) down to 120v or 220v incurs an efficiency loss. Which only gets worse if there is an AC -> DC -> AC conversion along the way. A further step down from what a Power Supply consumes to produces (another AC -> DC conversion and usually a lot of 12v, but specialist equipment can do darned near anything) also produces a further loss of efficiency.

If instead the UPS provides a constant DC feed instead of an AC feed, the efficiency loss from stepping down the voltage is reduced, possibly a lot. It gets even better if the UPS is supplying the voltage needed by the devices since a step-down stage is not needed. A system like this makes more efficient use of power than the easier to put together version.

Ease nearly always trumps efficiency in this space, unless the datacenter in question is a specialist for efficiency, or only has one tenant.

There are datacenter systems available now for building with DC power distribution instead of AC. These system are generally 48v DC, and mainly for Telecoms systems not server systems. Ideally, 12v would be best as that's what used internally, but I've only ever heard of demonstration systems using that. Some equipment vendors do offer DC power-supplies as a build option, but not all of them do.

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(update) As far as the original question goes...

or, its direct page:

The catch is, it's probably useless for any modern PC, because it's only 200 watts & appears to lack ATX12V power.

Here's another. Unfortunately, like the first, it too lacks ATX12v and has only 200 watts: (kind of wimpy, though)

...and my personal favorite that I'm probably going to burn my weekend trying to find somebody on earth who actually sells, because it explicitly advertises 45-65hz tolerance for input power and generator-compatibility. It's not built into the ATX power supply (it's built into a box that fills a 5-1/4" drive bay), but unlike the first two, it would actually be useful in a modern PC:

insofar as another answer mentioned...

They are designed to allow switch over to a generator

Actually, 99% of the non-Enterprise sub-$1,000 UPSes on earth are completely dysfunctional when sitting between a computer and a generator... and most of the Enterprise-grade ones, too. There are plenty of war stories about people who breathed a sigh of relief when their new generator kicked in... then had their day ruined when the UPS(es) refused to switch over, and stubbornly remained on battery power until they unceremoniously ran out.

The main thing that keeps UPSes from working with generators is the frequency of the power they make. As a practical matter, the frequency of power from all non inverter-type generators is going to drift as the load changes. With small and cheap generators, it lurches and thrashes around. With large (100-kilowatt or more) generators, it drifts and wobbles. As far as most UPS logic boards are concerned, none of them are good enough.

Making matters worse, the UPS problem gets lots of people to junk a perfectly good (or at least adequate) generator and buy a new inverter-type generator, only to discover that inverter-type generators bring an entire host of NEW problems with them.

Most cheap inverter generators don't output sinusoidal power -- they approximate it with stepped square waves. And most UPSes won't tolerate it, and will reject it as firmly as they rejected the frequency-unstable power from the conventional generator that came before it. The sad irony is that the pseudo-sinusoidal power put out by cheap inverter-type generators is actually superior to the dirty square-wave TOTALLY non-sinusoidal power put out by most UPSes.

Anyway, back to the drama. Frustrated with generator #2, you junk it, and buy a $3,000 pure sine inverter-type generator... then discover that it doesn't handle radical load changes (like when the UPS kicks in or out) well, and ends up in yet another death spiral battle to battery death with the UPS. The UPS kicks in, and the generator fires up. The UPS sees line power, and switches the load over to it. Hit with an instant surge of several hundred watts, the total harmonic distortion of the inverter's power goes through the roof for a fraction of a second... which, to the UPS, looks like power that's deviating from 50/60hz. So the UPS kicks in, and the cycle begins anew.

Stir, rinse, repeat, and you'll see why everyone who has to try and make a UPS play nicely with a generator ends up getting frustrated. In the end, it all keeps coming back to the complete intolerance of most UPSes for deviant line frequency.

The sad thing is, there's NO TECHNICAL REASON why it has to be this way.

UPS makers say they fail over to battery power when the frequency is out of spec because they can't fix it otherwise, then mumble something about AC motors not working well when the frequency deviates. The problem with that logic is that 99.9% of the computer hardware likely to be plugged into a real-world UPS doesn't actually care (much) what the exact frequency is or how stable it is from second to second, as long as it's somewhere between ~45 and 65 hertz.

It's a classic case of optimizing the wrong thing for the wrong reason just because it's easy to do, and might have made sense at some point in the past. It's easy for a UPS to determine whether or not the line power is within 1hz of 50 or 60hz. All you have to do is trigger interrupts on the zero-crossing, and compare the time since the last one to your estimate of how long it should have taken. Twenty years ago, before UPSes had microcontrollers with faster CPUs and more ram than a Commodore 64, it might have even made sense. But now, it's just plain stupid, counterproductive, and causes more grief and harm than it prevents. We need to get UPS manufactuers (APC, are you listening?!?) to give us an option to have it just disregard the line frequency, and sync up to whatever it finds (if it finds anything at all) as long as the voltage is within some reasonable range.

Most of APC's UPSes (even their lower-end ones) actually have a setting that reduces its sensitivity to voltage spikes & surges, including models whose official free monitoring app doesn't include the option to change the setting. Just connect at 2400/8N1 and send two characters with a brief pause between them: 's' and 'L/M/H' (L=low, M=medium, H=high). Make sure the PC you're using isn't actually running from the UPS at the time you connect, because some models will interpret DTR's change as a legacy "shutdown" command. However, this setting won't do you the least bit of good in most cases, because the generator's frequency will still keep it from working.

This is purely anecdotal, but from what I've read, TrippLite and Cyberpower are the most rigidly-intolerant of frequency-deviance. APC will tolerate +/- 1hz, but no more. According to the official specs, the Ultra (CompUSA/Tiger Direct) 1000AP will tolerate +/-10%, which means (in theory) it should tolerate anything between 54 and 66hz. I own that particular UPS, but I haven't been able to verify it or its generator-friendliness. It appears to be a rebadged Powercom BNT-600A, but the BNT-600A officially tolerates +/- 5%. Then again, I believe frequency-tolerance is entirely a function of firmware, so it's quite possible that Ultra might have had Powercom tweak the firmware to make it twice as tolerant as their own model.

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-1 While this is interesting, This is a Q&A site. Answers are expected to answer the question. Your post has nothing to do with answering this question. You seem to be purely focused on replying to another answer. – Zoredache May 4 '12 at 3:57
Updated, to show a few products that are theoretically what he's looking for. I say 'theoretically', because I have yet to find one that's physically available for purchase today anywhere in North America. The generator response itself is important, because most people don't have any idea what they're getting themselves into when they try to use a UPS with a generator. People need to know that their likelihood of getting ANY remotely affordable UPS to work with ANY generator is very, very low. You can spend a HUGE amount of money before realizing it's borderline-futile to even try. – Bitbang3r May 4 '12 at 4:30
This is a Q and A site. We do not do shopping Questions or Anwsers. – t1nt1n May 4 '12 at 6:01

A UPS uses a inverter to convert the 24V DC into 240AC. If you included this into a PC PSU it would make for one heavy PC. A UPS is more than just a battery. It also filters power so it is a pure wave at 50hz (or the variant for the AC you are using) some countries use 60hz.

Also UPS's aren’t designed to power a computer. They are designed to allow switch over to a generator or clean shutdown of a system. A Laptop battery is designed to give runtime on the move (as a laptop has lower current demands)

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If it was in the PSU then it wouldn't need to up-convert if it only powered the computer. And they don't always do a good job of filtering, and generating a pure sine wave is hard. – Ignacio Vazquez-Abrams Mar 5 '12 at 22:23
If you buy DataCenter grade products they do a good job ;) – t1nt1n Mar 5 '12 at 22:24
They'd better do a good job... – Ignacio Vazquez-Abrams Mar 5 '12 at 22:26

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