[Nut-upsuser] Recommended UPS to buy for use with NUT?

Peter Selinger selinger at mathstat.dal.ca
Sun Jun 4 19:54:45 UTC 2006

Hi Arjen,

I understand your technical points, but I think you are overkilling
this user with information. He asked a simple question about what UPS
he should buy. For reference, here is his original question:

Clay Barnes wrote:

> I am running a home box (600watt total load, may expand to 800-900) and
> I need a UPS that can sustain power to that for at least 20 min, but
> I want as much time as I can get.
> Does anyone on the list have a similar setup with a UPS that was easy to
> set up with NUT?
> Generally speaking, what mfrs. and models are very well supported?
> Which should I avoid at all costs?
> I know this seems like something that would be in the archives, but my
> searches thus far have been fruitless.

Many PC's nowadays ship with power supplies rated 3A or similar (for
example, I have such a PC). This is of course overengineered, as no PC
will actually draw this much power, but I guess consumers are happy
with the idea that they could put, I don't know, ten motherboards into
their computer without changing the power supply. The downside is that
a UPS's nominal output power should ideally match the nominal power
supply of the PC, regardless whether that power is actually utilized.

So when I got my PC, I multiplied 3A by 120V to obtain 360VA. To this,
I added some more VA for the monitor and other miscellaneous stuff.
As a consumer, I didn't know much about the difference between Watts
and VA, so I may have easily confused one with the other. Fortunately,
this didn't matter since my power supply was rated in Amps anyway, so
when I multiplied that by 120 Volts I got the correct answer.  Perhaps
Clay did a similar calculation. (In which case he would, indeed, be
speaking of maximum, not average, load).

So to answer his question: pretty much any store-bought UPS will do in
your situation. My personal advice: avoid Belkin; their hardware and
software is crap. Look at the compatibility list at
http://www.networkupstools.org/compat/, and get a UPS that is listed
there, if possible. On the box, it is usually written (in big letters)
how much runtime you can expect at a given load, so just buy one that
says you get at least 20 minutes for some reasonable load.

-- Peter

Arjen de Korte wrote:
> Peter Selinger wrote:
> >>>> I am running a home box (600watt total load, may expand to 800-900) and
> >>>> I need a UPS that can sustain power to that for at least 20 min, but
> >>>> I want as much time as I can get.
> >>> Is that rated load, or actual load? That seems like an awful lot of
> > 600 watt (or 600VA) does not strike me as an unduely large load rating
> > for a UPS.
> Please don't mix up VA and Watt ratings, they are very different things.
> > My smallest UPS has 800 VA.
> This is irrelevant when it comes to runtime. You can have an 800 VA UPS
> with a Watt rating of just 10 Watts and a runtime of 1 second for
> instance. Not very useful, but it is possible. The VA rating of a UPS
> says nothing about the runtime.
> > What did you have in mind Arjen?
> As a summary from my previous post, you should consider both peak load
> and average load when selecting a UPS. Just considering peak load, will
> mean that you will select a way too big UPS if the average load is much
> smaller, which is usually the case for computers. I have yet to see a
> home computer system that draws more than about 50% of the full load on
> average (and usually this figure is even lower).
> > Also, 20 minutes runtime is not too much to ask.
> Probably not, but at a load of 900W, conversion efficiency of 80% and
> lifetime derating (due to aging of the battery) to 80%, this will
> require a 1400 Wh of battery capacity (which is about 120Ah @ 12V, which
> is a very large battery). You won't see many consumer grade UPSes with
> that amount of battery storage installed (if there are any). Therefor my
> question, if this is rated or actual (or better put, average) load?
> > It's true that the system can shut down faster than that, but a human might
> > appreciate a chance to save his work.
> Of course. I'm not questioning the legitimacy of the need for a 20 min
> runtime, it's the amount of power drawn that looks excessive.
> Regards, Arjen

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