Question about UPS backups

[Oniya]

My current UPS died recently, and I'm looking to replace it.  It was a Belkin 550VA with 8 outlets.

I'm in the process of comparing/contrasting replacements, and I'm wondering what the effect of higher/lower 'VA' numbers are.  I'm guessing that bigger is 'better', but what should I reasonably expect to need for a single computer/monitor/router setup?

[Shiny]

First, I have never seen a UPS in my life  :-[ , so caution is advised, but I think I can still be of help.

The number in front of VA/Watt stands for the power the UPS can provide safely.
If more power than specified is drawn from it, either the power collapses (i.e. your computer, monitor, router shuts off) or an internal fuse blows (since the energy coursing through the UPS might damage it, otherwise). There should be a bit of an power reserve, but I don't now if these things heat up if they keep running for a longer time.

The most precise way would be to look for the power requirements of your computer, monitor and router. They should be printed somewhere on the casing (usually somewhere near the power cable of the device) or if the device's individual power supply unit is external, somewhere on that one ('power brick'). If there is one where there's no value of output power i.e. W/VA, but only output voltage [V] and current [A] written on it, take those two and multiply them.
Add those three values up and you have your minimum wattage needed to provide power to your setup. You might get away with less, since the power supply unit of an computer might be a bit bigger than needed, but everything over the calculated value should be safe.

[edit] minor tweaks. 'output', important word  :-)
[edit2] wrong and misleading, sorry.

[Geraint]

I’ve been using UPS for my entire career, so know a decent amount about them.  I have four of varying sizes (450, 550, 1000, 1400 VA).

The Long Confusing Answer

Yes, bigger is generally better, but there are other considerations.  UPSs with high VA ratings usually also have more battery capacity but one does not necessarily equate to the other.

As a rule of thumb I usually total up the peak wattage of everything to be connected to the UPS, and use that as a starting point.  Peak wattage for a computer can vary a lot depending on what you are using it for, with the greatest variable generally being the graphics card (if you’re a gamer), or the CPU if you’re not.  A single high end graphics card can draw upwards of 400 watts and a high end CPU almost as much.  But if you don’t know where to look for that info, then a suitable substitute is look at the wattage of the power supply on your computer (knowing that it will almost always be higher than the total peak wattage expected to be running in the computer.

Everything else should be able to pick off of the equipment labels.  Flat screens draw a lot of power also, especially the big ones.  Mine take about 200 watts each.


Anyway, once you have your total load I would advise you to look for something up to twice that amount in VA.  There’s a simple reason for that; the more capacity generally means longer run time.

For instance a 550 VA UPS might supporting a load of 550 watts   might only run for less than five minutes with the power out, while it might run for 15 or 20 minutes at half load.  The manufacture should be able to supply you with run times at half and full loads, so that you can compare.  If you are there to shut down reasonably quickly, run time isn’t much of an issue.  But if you walk away from your computer leaving it on, it may very well be. 

Or, if you are like me and run a cable or DSL modem and a router, a substantial run time can keep both of those going for quite a while, with the main computers shut down and you accessing the internet from a laptop.


If you do rely on your UPS for more than a few minutes, you should know that VA does not equal watts for alternating current (AC) except for short bursts.  So you should make sure that your UPSs actual W rating (which is more or less average wattage over time) is higher than the load you are putting on it.  As a guess, I would say that your 550 VA UPS was only rated at 350 W.  (mine is 550VA / 330 W)

But don’t just look at VA or W.  Compare run times also.

That being said, you can probably run a single computer, 20” flat screen, modem, router for like 550-600 or so VA, if you don’t have a high end graphics card that you use for games, and expect to shut down quickly after a power failure, 1000-1500 if you have high end graphics, play high-res shooters, and use bigger flat screen(s) at high resolution, or want longer run time.

At the same time the price differential between 600 and 1000 VA may not be all that great and, in that range, bigger is better. The last time I looked, 1000 VA was the sweet spot in the market in terms of value for dollar spent, but that may not be the case now.



Caveat:  my UPS experience is entirely with APC

[Oniya]

Thanks - the long confusing answer actually helped.  I'm pretty sure that the reason my old one died is because - while I normally shut it down quickly after the power goes out, there have been a number of times where the power has gone out without my knowledge.  While other people have been up during those times, they apparently didn't hear the loud, obnoxious beeping, and allowed the UPS to run all the way down.

[Geraint]

Thanks - the long confusing answer actually helped.  I'm pretty sure that the reason my old one died is because - while I normally shut it down quickly after the power goes out, there have been a number of times where the power has gone out without my knowledge.  While other people have been up during those times, they apparently didn't hear the loud, obnoxious beeping, and allowed the UPS to run all the way down.

Well it should recharge itself afterward, of course, but yes that is very hard on the battery.  They use lead-acid batteries (like cars do) and are subject to the same kind of problems.

One thing you might consider is that with most UPS units, you can replace the battery (which generally lasts about 3-4 years) if you are happy with the UPS itself.  The cost of a battery replacement could be like half the cost of a new UPS.

[Geraint]

One other thought specific to your situation. 

With APC at least, the higher capacity UPS units generally also have internal logic and USB connections (as well as software for older operating systems that don't already have it) that allows the UPS to safely shut down your computer itself in case of a power failure, and some of them have the separate capabilty to shut off power to selected peripherals any time the computer is shut down.   

[Vekseid]

The VA rating is maximum power flow... which, for devices normally expecting AC input, is not going to be the same thing as a Watt.

See APC's guide to the difference between volt-amps (VA) and watts here.

The peak-to-trough voltage of a typical US outlet is not 110 volts, but rather about 170. The RMS ('average') voltage is 110, and that's what gets listed. Because of this, you need a VA rating somewhat higher than the watt rating to be safe. Multiplying VA by .65 is typical, the link suggests .6, which is probably safer.

I honestly don't like going over .2-.3 or so, however, because the bigger the drain on the battery, the faster it gets exhausted, and the ratio is not linear. A UPS that lasts 10 minutes at full load may last forty at half load, for example.