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~750VA UPS for ~250W power usage from a ~600W power supply - is anybody doing this?

midix

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#1
If you are using an UPS rated as 750VA or even less with an APFC PSU rated at 600W or more and it has been working just fine when switching to battery, and it continues working for a year at least, then please, please, let me know the brand and model of your UPS.


The long story.

I wanted a silent UPS (no fans when working from mains) for my workstation / media PC / occasional gaming PC.

So, I got my kill-a-watt measured the real max power use of the system during various stress tests (3D Mark Demo, Cinebench...). It turned out to be always less than 250W, which is expected for a mediocre non-gaming system with i7-7700 CPU and GTX 960.

The problem is that my PSU is an active PFC unit rated at 620W (an overkill for my case but I thought that a PSU with some redundancy is a better choice - more powerful components etc.). And I've heard that APFC units can trigger high inrush current when a UPS is switching to battery, especially a UPS with simulated sine.

It usually is not a problem for UPSes that are rated well above the max power of the PSU. For example, 1000VA, 1500VA might work just fine. Although not suggested, but still I might be fine with a simulated wave unit.

But there's a catch. In my country the only reputable UPS brands available are Eaton and APC. I have heard lots of complaints about APC Back-UPS (and Pro) units lately (dying too soon, someone bought 15 of them and only 8 survived after warranty). I was looking at BR900GI and reading reports of its premature deaths on APC forums, and APC response was that it would be a mistake to use Back-UPS with an APFC PSU, and that might be the cause of the damage. I'm not so sure if this is true or just an excuse. Anyway, after reading those stories I'm afraid to buy BR900GI (or any other of the same series).

So, considering APCs response, I should buy a SMT UPS with a sinewave. And calculating the full power of the PSU (620W) I would need to buy at least SMT1000I, which is very expensive.

I really would like to get by with SMT750I (and even that would be an overkill because I don't need those "smart" features), but I'm not sure if SMT750I also would not suffer from overload caused by the 620W PSU, even if the real consumption is just 250W max.

My other options are Eaton Ellipse Pro (simulated sine) and 5SC (pure sine). They have generally good reviews ... except complaints about fan noise in their 1000VA and up models since 2016, when Eaton made the fan always spinning, even when idle. So - no solid, silent UPS from Eaton at this time, unfortunately. Some people voided their warranty and replaced the fan with Noctua, but I'm not quite ready for that.

I also tried an AEG Protect B 1000 (true sine) and it worked fine on my system, but I had to return it because of pretty high non-stop fan noise. And also I read some service reports about the consumer grade UPSes from AEG - they are saving costs on battery charger transistors, picking ones that have just 10% reserve, and as a result, services often see these units coming in with burnt charging transistors. And APC lower end is no better.
And I have no true sine Cyberpower units in my country.

So, my current only choice seems to be SMT750I, but I'm not sure if it's worth trying and that's why I wanted to know is somebody else has done that and if it works reliably. Of course, you can never guess and every UPS-PSU combination might behave differently, but still it would be encouraging to know that at least some people have no issues. I just don't want to buy a 1000VA UPS for a system that is never going to normally consume more than 250W.
 
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#2
If you are using an UPS rated as 750VA or even less with an APFC PSU rated at 600W
First, great idea you are considering using a UPS. But bad idea about the size.

When it comes to computer power, the conversion formula from volt amps to watts is NOT a simple 1:1 conversion. You must factor in the "power factor" (PF) too.

UPS makers typically rate UPS in VoltAmps but you will find most of your hardware will be rated in Watts. So, you need to convert one to the other. In simple DC to DC conversions, 1 watt = 1 volt x 1 amp, or 1W = 1VA. But in dealing with AC to DC conversions and power supplies, including uninterruptible power supplies, you have to factor in the "power factor".

Watts = VoltAmps x Power Factor (W = VA x PF)

VoltAmps = Watts/Power Factor (VA = W/PF).​

Most supplies have a PF of .6 to .8. If you don't know the specific PFs for your supplies, use .65. If it is not easily determined, I just use .65 anyway because that builds in a little buffer - and that is always good for longer uptimes, and future expansion.

Example:
To convert 750 VoltAmps to watts, 750 x .65 = 487.5W​

That clearly shows your 600W power supply could easily overload that 750VA UPS. And if you also run your monitor, network gear and other devices with that UPS, you create an even greater overload scenario. If the power fails at just the wrong moment and your computer is demanding more than the UPS can provide, the UPS will immediately shutdown and your computer will crash, potentially destroying your data. Not good.

While your system may typically only consume 250W, you have to factor in the highest "peak" demands that may be possible when all the components inside your computer are demanding maximum power at that same instant - even if that possibility is rare.

I recommend you plug your hardware into the eXtreme Power Supply Calculator. Plan ahead and plug in all the hardware you think you might have in 2 or 3 years. This might include extra hard drives, a bigger or 2nd video card, more RAM, etc.

While this is a PSU calculator, note it will also give you recommendations for UPS size. And if you will be protecting your monitor(s) and network gear too, add their power requirements in there too.

Also note while pure sinewave UPS are nice, and for sure, if the price is right, go for one. But don't fall for the marketing hype that you need one. Any quality power supply can easily support and deal with the stepped approximation or simulated sinewave output of most UPS. Rather than repeat myself again, please see this, and note what both EVGA and Seasonic say about their power supplies and simulated sinewaves.

And remember, the biggest advantage of a "good" UPS with AVR is the AVR (automatic voltage regulation) and that has nothing to do with the waveform when running on battery. AVR is functioning 24/7. Battery backup support is typically a few minutes total per year and really only needed long enough to "gracefully" save any open documents and shutdown the computer.
 

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#3
The power supply's capacity has nothing to do with how big of a UPS you need. The actual amount of power your computer uses is what matters. if you are only using 250w, then you don't need a UPS that can do the 620w your power supply is rated for. Obviously, the bigger the UPS the better.

I run my server, which has a 850w PSU, on a 850VA/510W UPS. It works just fine, because the server runs at ~200w load constantly(mainly due to the 12 hard drives).

Also, the small peaks, especially the inrush after the switch to battery to recharge the caps in the PSU, is not a major issue if you get a decent brand UPS. They are all designed to handle peaks above what they are rated for for a few seconds.
 

midix

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#4
While your system may typically only consume 250W, you have to factor in the highest "peak" demands that may be possible when all the components inside your computer are demanding maximum power at that same instant - even if that possibility is rare.
I did as many active stress tests as I could find to totally load the CPU to 100% and also load GPU - for example, at first running WinRar benchmark, and I saw in task manager the CPU running at 100% on all cores, and then I also launched 3D Mark Demo that heavily utilized my GPU. I set kill-a-watt device in High detection mode (so it showed not just current value, but the max value detected during entire session) and it did not exceed 250W. It is expected, since GTX 960 is mediocre graphics card. A more serious system in a heavy game might consume about 350W (I've seen some measurements done in some videos with GTX 980).

SMT750I is rated at
Output power capacity 500Watts
so, in theory it should cover my 250W with ease. Except that I'm not sure about those inrush currents from the 620W power supply.

That's why I wanted to hear some experience stories from people who have been using weaker UPSes with stronger PSUs while clearly knowing that their system is always consuming less.

Thanks, newtekie1 for sharing your experience.

May I know what are the models of your UPS and PSU?
 

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#5
May I know what are the models of your UPS and PSU?
The UPS is a Cyberpower 850VA AVR and the PSU is a Corsair HX850.
 
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#6
if you are only using 250w, then you don't need a UPS that can do the 620w your power supply is rated for.
For the record, I was not suggesting the UPS must support the maximum capability of the supply. But it must support the maximum load the computer might put on the supply, plus any additional components (monitor, network gear, external drives, etc.) that might also be connected to the UPS.

Kill-o-watt devices are nice, but only if the connected devices actually present their peak demand. And unless a stress test is conducted, that likely did not happen. And of course, as midix notes, in-rush (or "switch-on" current can present a very high demand too as electronics generally demand much more when starting up than when running normally. And the problem there is every component is demanding a lot at the same time when the computer is switched on.

@midix What else will be connected to the UPS? Most users like to connect the monitor too, so they can see what they are doing when the power goes out. And for sure, I would recommend all your network gear (modem, router, switches, etc.) be connected too to keep your network alive so you can, maybe, use a wireless device to contact the outside world.
 
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#7
Some UPS manufacturers list the VA rating for peak loads, while the continuous load may be less than 60% of it (or in some cases less than half).
For example, my old 800VA Sven Reserve was giving me an overload alert and 110% load on LCD with an overclocked X5650 + GTX750Ti, which barely added-up to 300W on my old rig.
So, just to be on the safe side assume that you can't exceed 375W total.
 

midix

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#8
Thanks for the warning, @silentbogo. I'll make sure I'll look for continuous load specifications.

@Bill_Bright - yes, I measured with the monitor attached.
My kill-a-watt device actually has a built in peak detection, which is really useful in such situation because I could run every torture test I could find and I saw how some of the tests caused the captured wattage to jump up a few watts and never going down again. After half an hour I ran out of ideas for any more stress tests, and the kill-a-watt showed 251W highest detected power consumption.
 
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#9
Okay. The only other issue is, and this is sad but common, the better UPS tend to be the higher capacity models. And by better, I mean they may have faster cut-over times, better regulation, LCD display panel, more outlets, etc. At any rate, while battery run times are just the bonus, higher capacity models do provide longer run times too.
 
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#10
You did it the right way ... at 250 watts, (250 x 1.5 rule of thumb - 375) you should be fine. The GTX 960 pops out about 125 watts on its worse day and the CPU 90 ... so 225 is right on target since ya didnt say anything about overclocking .... unless you have a coffee pot in there or something. And yes, with the "new" APC ... it's never their fault.

Ideally, it'd be pure sine wave for the PFC PSU but AVR should serve as well. Oddly enough Cyberpower's "Sinewave" series is AVR not PS. And yes, especially post Schneider Electric's takeover of APC, the Backup UPS should be avoided... They keep changing the series names and I can't keep up with what is what today since we stopped using them.
 

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#11
Actually, AVR can be combined with pure sine. AVR is a feature of line-interactive topology, it means that the UPS has a transformer to deal with brownouts more efficiently than so called off-line UPS. Off-line UPS will always switch to battery when it detects that the mains power is out of normal range, while line-interactive UPS (which means - AVR) will first attempt to boost or buck the incoming voltage without switching to the battery.

Pure sinewave can be formed in the battery output circuit, no matter if the UPS is offline, line-interactive (AVR) or online (very expensive ones, they always go through full voltage conversion without any switching to/from battery). But, as far as I know, pure sinewave is usually a higher level feature, thus it is uncommon to see it in offline UPSes.
 

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#12
I'm running a cyberpower 1200VA/720W unit with *three* desktops running off it.

1. Main gaming PC + monitor
2. Server/HTPC/NAS (tower only)
3. VR machine (tower only)
4. Dlink router (wifi bridge)

With all of those at desktop levels i get 25 minutes of battery, around 10 minutes with one machine gaming

Pure sine would be great, but simulated sine isnt having any issues with my rather odd setup either
 
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#13
Ideally, it'd be pure sine wave for the PFC PSU but AVR should serve as well. Oddly enough Cyberpower's "Sinewave" series is AVR not PS.
Sorry but this really makes no sense. AVR is a totally separate feature and literally means "automatic voltage regulation". It does not suggest or imply anything about PFC or whether the output is simulated, stepped, PWM, modified, or pure.
Actually, AVR can be combined with pure sine. AVR is a feature of line-interactive topology
Right and right.
it means that the UPS has a transformer to deal with brownouts more efficiently than so called off-line UPS.
Ummm, kinda-sorta.

The transformer is but one small part used by the AVR circuits. AVR implies there is intelligent circuitry that monitors incoming voltages in real-time and makes slight adjustments when they deviate from the norm without causing the UPS to kick-over to battery back up. It attenuates the voltage when necessary and boosts it when necessary and will even use the batteries to uphold the boost for extended periods (several seconds or voltage "sags", not hours or extended "brownouts"), again without totally kicking over to battery backup. It is only when those deviations turn into abnormal events or anomalies (either extreme high or extreme low) when the UPS will actually kick over to full battery backup.

That's the main feature that makes a "good" UPS with AVR so special - that regulation. But that has absolutely nothing to with the UPS having a stepped approximation or a pure sinewave waveform output. The shape of the waveform is controlled separately from the amplitude (voltage) of the waveform.

That's why this CyberPower can be pure sinewave with AVR, and so can this APC while this Cyberpower uses a stepped approximation (simulated) sinewave with AVR.

Pure sine would be great, but simulated sine isnt having any issues with my rather odd setup either
And didn't for the 25-30 years simulated were commonly used all the time on regular PCs as well as mission critical servers.

The "need" for pure sinewave UPS is really just exaggerated marketing hype by those makers trying to corner the UPS market. The only real need for a pure sinewave UPS might be on critical care medical monitoring and life support equipment. But even those devices use very robust, and quality power supplies.
 

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#14
The "need" for pure sinewave UPS is really just exaggerated marketing hype by those makers trying to corner the UPS market. The only real need for a pure sinewave UPS might be on critical care medical monitoring and life support equipment. But even those devices use very robust, and quality power supplies.
Yes, majority of modern switching power supplies seem to be just fine with simulated sinewave. Still, I've heard some worrying reports that some PSUs don't like it and start emitting weird buzzing sounds or completely turn themselves off rendering UPS useless. Buzzing sound alone wouldn't be a tragedy, but it often means that some component is being stressed more than it normally should, which could reduce PSU lifespan.

Anyway, I'll try to find some way to try out some 750VA unit from a shop that accepts returns. Unfortunately, that is not that easy task in the small town where I live, and also considering that UPSes are rarely available in small shops just because most modern "average" people use laptops.
 

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#15
Yes, majority of modern switching power supplies seem to be just fine with simulated sinewave. Still, I've heard some worrying reports that some PSUs don't like it and start emitting weird buzzing sounds or completely turn themselves off rendering UPS useless. Buzzing sound alone wouldn't be a tragedy, but it often means that some component is being stressed more than it normally should, which could reduce PSU lifespan.

Anyway, I'll try to find some way to try out some 750VA unit from a shop that accepts returns. Unfortunately, that is not that easy task in the small town where I live, and also considering that UPSes are rarely available in small shops just because most modern "average" people use laptops.
that buzzing likely only comes out when you're on battery mode, when the UPS has gone into high fan speed screeching mode anyway
 
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#16
Yes, majority of modern switching power supplies seem to be just fine with simulated sinewave.
The reason why most are fine is that most can even be run with high voltage DC input: http://power-topics.blogspot.com/2007/09/using-ac-dc-input-power-supply-with.html I've had to put systems together for work that ran on high voltage DC and every power supply manufacturer guaranteed DC compatibility; which should make the whole stepped / pure sinewave feature pure marketing unless you're unlucky enough or too cheap and bought a power supply that requires an analog AC input. Even most of the monitor / router / whatever else you plug in should be able to handle DC (unless it has a linear power supply).
 
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#17
Mussels is right - IF the PSU is going to buzz, it will only happen when running off the battery and ideally, that never happens, or if it does, it is just for a couple minutes every few months. If you have frequent full power outages that last longer than a few minutes, and your computer is "mission essential" you should probably be looking into a backup generator too. This is what we had on all our critical communications facilities. The idea being, if you have a full power outage, the UPS would immediately kick over to battery backup for "uninterrupted" power. Then, while the UPS was keeping the mission essential equipment up and running, then the generators would fire-up and when their power output stabilized, the entire facility would kick over to generator power. The UPS would then flip back to line power and the AVR feature of the UPS would ensure the connected equipment received clean power.

That said, like power supplies and UPSs, don't get a cheap generator. A cheap generator may be fine to power the lights or even your refrigerator or freezer as those devices are less picky about the quality of the power on their inputs. But a "good" UPS may see the power from the generator as too "dirty" or too far out off 50/60Hz for the "AVR" to compensate for and refuse to run - out of an abundance of caution to protect the connected equipment. And if the power is that dirty, I sure would not run my sensitive computer and network gear directly from the generator.

Still, I've heard some worrying reports that some PSUs don't like it
That's true, but those are likely cheap PSUs you should avoid anyway.

I live in Tornado Alley where we frequently have severe weather. Total power outages are not everyday, but they are not rare either - especially in my 100 year old neighborhood where the utilities are not buried. Even when the weather is nice, it is not uncommon for a squirrel to forget to let go of one wire before grabbing the other, becoming an instant "crispy critter" blowing the transformer fuse. I have a "good" UPS with AVR on all 6 of my computers in the house, and my home theater audio equipment and big screen TV. I also have one on my garage door opener and even one in the bedroom for the alarm clock and electric blanket! And none are pure sine wave and none have caused damage (or buzz) with any of the power supplies.

While some PSU makers "recommend" the use of pure sinewave UPSs, they do not say to avoid them or that they are required. Note the following from two makers of quality power supplies,

Seasonic FAQ
What type of UPS does Seasonic recommend?

Seasonic recommends a UPS with a pure sinewave output; however, a high quality simulated sinewave UPS from a reputable manufacturer could also be a possible solution to be used with our power supplies.
EVGA Knowledgebase
Question / Issue
Do EVGA Power Supplies support UPS backup devices that support Line-Interactive AVR UPS which uses a simulated/artificial sine wave?

Answer / Solution Yes, all EVGA power supplies support UPS backup devices.
And just as a reference, according to my PowerChute (APC's UPS monitoring program), in the last 24 weeks, the APC Backups XS 1500 I have supporting this computer has kicked over to backup power 8 times - but ran on batteries for a grand total of just 1 minute, 10 seconds. So like I said above, it is the AVR that makes a "good" UPS so valuable. Backup power during a full outage is just a bonus and just to give you time to save your work and "gracefully" shutdown the computer.
Ummm, "analog"? As opposed to what? If something is not analog, it is digital.

And sorry, but when it comes to ATX compliant computer power supplies, the rest of your comment is inaccurate too. Your 2007 article is very generalized, and does not apply to ATX PSUs used in computers - the subject of this thread.

The facts are, ATX computer power supply manufactures do NOT specify or guarantee DC input compatibility (if you have an example, please post the link). For example, note the specs for EVGA's top of the line 850W. No DC input capability. Same with Seasonic top of the line models as seen (or not seen) here - no DC input support.

So to suggest people are unlucky (or "too cheap" :() is just nonsense, and frankly, indicates a lack of understanding about "computer" power supplies. I would suggest you do your homework read up on the official ATX Form Factor PSU Design Guide - the "industry standard" for ATX form factor power supplies (which are required to be "switching" PSUs, BTW) and note there is nothing there about DC inputs.

And, of course, virtually all modems, routers, monitors and even notebooks require DC inputs so they don't apply.

As for your comment about linear power supplies, because they are very heavy and typically only used for "low" power applications, computers don't use them. A 600W linear PSU for a PC computer could easily weigh 20lbs or more!
 
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#18
Ummm, "analog"? As opposed to what? If something is not analog, it is digital.
As opposed to a stepped / digital approximation of an analog sine wave. And yes, I have actually used 270 VDC myself and powered the server, PXI Express chassis, RF up and downconverters, etc. with DC. The manufacturers of the supplies themselves guaranteed correct and warranted behavior when using 270 VDC.

And furthermore, linear power supplies are still in use in lab supplies. That's why I added the note that there are some scenarios where high voltage DC can't always be used if you don't know the nature of the supply it is powering.

Edit: How about this then, can you explain to me how a switched power supply would perform fine with a voltage that fluctuates from +169.7 to -169.7 V yet not with a constant input DC voltage? Because that's what they are doing when run on 120 VAC (120 * 2^.5). In our case we had to use 270 VDC which corresponds to ~191 VAC rms.
 
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#19
I never said linear supplies were no longer in use. But they are not used in PCs, except, maybe in some lab scenario.

And I am afraid you still don't understand the what analog means, or apparently digital. So please do some home work. You really only have 10 options (and that's not ten).
 
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#21
Edit: How about this then, can you explain to me how a switched power supply would perform fine with a voltage that fluctuates from +169.7 to -169.7 V yet not with a constant input DC voltage? Because that's what they are doing when run on 120 VAC (120 * 2^.5). In our case we had to use 270 VDC which corresponds to ~191 VAC rms.
It may work with a passive PFC or no PFC system, but I'd worry about how active PFC would deal with straight DC longterm, at least.
 
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#22
Sorry but this really makes no sense. AVR is a totally separate feature and literally means "automatic voltage regulation". It does not suggest or imply anything about PFC or whether the output is simulated, stepped, PWM, modified, or pure.
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That was in the response from my inquiry, you welcome to argue with them.
 
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