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I would like to buy UPS, but the review I read:,2785-12.html

make me alert of one thing -- the noise of UPS. So, I understand when powering computer (from batteries) UPS fans kick in, making some noise. OK.

But in normal mode, when there is no blackout, and UPS is working in passive mode (not from batteries), does UPS make any noise (I think about its fans)? In other words -- do they work (spin) all the time?

Just in case, I have CyberPower CP900EPFCLCD or APC Smart-UPS 750VA (SMT750I) in mind particularly.


I bought APC Smart-UPS 750VA (SMT750I) and it is noisy all the time -- the noise is similar to grasshopper or older models of HDDs. When I close the windows (so all the street traffic is muted) the noise of the UPS is a bit annoying, but since fans in computer are louder I don't blame UPS for discomfort. Also I find that constant hum of the fan is more disturbing, that the ticking of UPS (as I said, it is similar to grasshopper).

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this site does not entertain shopping considerations – Shekhar Sep 16 '11 at 6:49
I am not asking for recommendation, but how things work -- from FAQ "Super User is for computer enthusiasts and power users. If you have a question about computer hardware"... And I have -- do UPS fans work in passive mode. – greenoldman Sep 16 '11 at 6:54
Loading a battery generates heat. Whether a UPS in question is able to dissipate the head passively (i.e., without fans) when just doing refresh loads is up to the model in question. (I don't own a UPS myself, but I assume they will use their fans when loading up after a power outage.) Best ask the manufacturer. – DevSolar Sep 16 '11 at 8:17
up vote 3 down vote accepted

The only sensible answer is: It depends.

All UPSes I've seen so far don't produce any noise when they aren't supplying power to a computer, but then again none of them have fans. Also a thing to consider is that internal electronics (not fans) may produce noise while UPS is powering the computer and that some UPSes have buzzers which can get quite annoying, especially if there's no way to turn them off. Also you may hear noise from relays when UPS switches between its internal power source and external power source. In some cases that can get quite annoying, especially if it so happens that your mains voltage is near UPS's tripping point. This can also lead to reduced relay lifetime.

UPSes usually use lead-acid batteries that are often sealed and they don't produce much heat when being charged and the charging circuitry itself, if it's slow enough, will not produce much heat.

So the bottom line is: I wouldn't expect a UPS to require fans when charging batteries or providing power to the computer from an external power source, but if you want to be safe, ask the manufacturer.

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Thank you for detailed explanation. I asked, because I really hate all that noise with modern PCs, and last thing I want is another source of noise. And also in reviews of some APCs I read that when switched to batteries they buzz like there would small engine inside. – greenoldman Sep 16 '11 at 8:30
@macias Well the main problem here to consider is how often do you get power problems. Normally an UPS shouldn't produce much noise or any while your power is good. Also note that UPS devices are designed to provide power for relatively short amount of time. If the noise is bigger problem than a sudden power loss, then fine, but keep in mind that UPS will allow you to safely turn off your computer and after that you can turn off the UPS too and then eliminate the noise coming from it. – AndrejaKo Sep 16 '11 at 8:34
That is why I asked about noise in passive mode not when UPS is active, because on average I have two 1 minute shortage of power (by surprise!) per year (the main problem here is loss of data, due to cold shutdown of HDDs). So for me having UPS is having an emergency plan (just in case), and since I would not use it very often, the crucial issue that it should not bother me at all during ~363 days in year. – greenoldman Sep 16 '11 at 9:44
@macias Well then to be sure you could get a fanless UPS and be safe. – AndrejaKo Sep 16 '11 at 11:34

There are three UPS "topologies": standby (aka offline), line-interactive, and double-conversion (online). The simplest/cheapest ones are standby, which kick in only when the voltage is bad (zero is bad). Otherwise they're charging the battery. I have a few with low volt-amp ratings that don't even have vent holes; they just get slightly warm.

On the other end, double-conversion units are always doing AC->DC->AC conversion. The one such unit in your linked review, the Opti-UPS, is the really loud one. I don't have any personal experience with them, but it makes sense the fan is on all the time. If anything, when the power goes out, there is actually less work to do.

In the middle are the line-interactive, which do Automatic Voltage Regulation (AVR). According to this APC knowledge base article, fans were included in Smart-UPS units rated at 1400 VA or higher. They were on only in certain conditions, including temps above 140F. Later generations all have two-speed fans that are always on low, and go high in similar conditions, but the trigger temperature has gone down to 104F (internal). They probably did this to improve reliability. I have some older low-VA models with plenty of vent holes and get warm, but I've never heard any fans (haven't looked inside).

However, the higher-VA models in the review all seem to have active fans at all times. At 25dB, the CyberPower is pretty quiet. Their general FAQ says that the fan may not be on, but this might not accurate for that particular model/series.

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Just as a tip, I had a SmartUPS 1500 that spun its fans 24/7 even without load, and it turned out the batteries had developed some issue where one of them internally shorted out, and started heating up and eventually vented into the house. Vaporized mist acid is not a very great thing to contain in a house, and I had to drag this (slowly) smoldering pile out to the garage at 3AM one morning. The next time my UPS starts running fans 24/7, I'm unplugging it and rebuilding the pack.

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The ups buzzes because it is charging,swiching from ac to dc, or it is in normal mode. I have a APC back-ups rs 1500 in my bedroom and makes very little noise in normal mode. Also the fan turns on only when it is in battery mode.

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