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How do you size Uninterrupted Power Supply units?
Old 10-17-2007, 08:29 PM   #1 (permalink)
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With the upcoming winter storms season here my main powerlines get flaky with the occasional glitch, blip and outright outage happening. I want to get a UPS that will do the job but there are so many out there with different power ratings and battery lifes. How do I know what the minimum should be for my own situation?

I've got the main box itself, 2 monitors, a printer, a scanner, a wireless router and a couple of 5.25in. external drives (with their own power plugs)...so I'm looking at safeguarding quite a few things.

Anyone got some ideas on this?
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Re: How do you size Uninterrupted Power Supply units?
Old 10-17-2007, 10:14 PM   #2 (permalink)
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UPSs are usually advertised by their VA rating: 650VA, 1500VA, 3000VA, etc. The VA stands for VoltAmps. VoltAmps are the amount of power you'd get out of a basic power equation if there weren't any losses or inefficiencies in the system. In a perfect system, VoltAmps would equal watts because there wouldn't be any losses. But typically there is about a 30% loss in efficiency (called the power factor). Long story short, if you take the VA rating and multiply by .7 you'll get a good idea of the amount of power (watts) a UPS can support. So a 1500VA UPS can support a number of devices that have a power draw of ~1050 watts. (More expensive units are usually more efficient, read the fine print).

Find out what the power draw is for each of your devices. Some manufacturer's will list it in actual watts, for others you'll have to multiply the voltage (typically 110 in a U.S. home) by the number of amps it draws. For instance, my 21 inch CRT monitor draws 1.4 amps at 110 volts, for a power draw of 154 watts per hour. Add up the power draw for your devices and then pick a UPS that can supply that many watts.

None of the above has anything to do with *how long* your devices will run on a UPS. That's a function of the number of batteries in the UPS, their Amp-Hour rating, how they are wired and the amount of energy your stuff is pulling out of them. Many UPS companies will have charts listing the run times of their units at 1/2 and full draw. Find a unit that has the necessary VA rating (or higher) and the run-time you need.

Here's a runtime chart from American Power Conversion (APC) listing a bunch of their units: http://www.apc.com/products/runtime_...?upsfamily=165

If you want to draw 600 watts for an hour (5.5 amps @ 110 volts), you need something like a 3000VA UPS (~$1200).

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Re: How do you size Uninterrupted Power Supply units?
Old 10-18-2007, 02:53 AM   #3 (permalink)
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That's a lot of information but I'll see if I can figure it all out, thanks.

I'm not so concerned with the battery life but I do want to ensure that I have enough power for my needs. Of course there's always the old 'buy more than you need' to cover any upgrading of equipment...but at a cost.

I just don't want to underpower myself.
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Re: How do you size Uninterrupted Power Supply units?
Old 10-18-2007, 08:29 AM   #4 (permalink)
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You would probably want 2 UPSs so that you have enough power sockets for everything (8-ish devices by my count). Something like a pair of 900VA units would probably suit you just fine and run around $300 for both. If the monitors are LCDs, put the computer, monitors, and router on one and everything else on the other. If your monitors are CRTs then I'd put the computer, main monitor, and router on one, and everything else on the other. You could expect something around 10-15 minutes of run time.

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Re: How do you size Uninterrupted Power Supply units?
Old 10-19-2007, 12:08 AM   #5 (permalink)
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The other factor not discussed yet, is what you envision the purpose of UPS protection being. My vision, is not that it let's me keep working, but rather that it lets me safely save my work and then shut down the computer. If that's the case, then you don't need as powerful a unit. If you are just protecting from brownouts (the power goes off for a second and then comes back on), then a less expensive unit may do just fine. The only time you "really" need the bigger units is if you continue to work on your computer after the power is off and it may stay off for an extended period of time. But if this is the case, you are now in an unsafe situation as you may then have the unit run out of power in the middle of something. So basically I'm saying that probably for 99% of the people the less expensive less powerful units will be just fine.
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Re: How do you size Uninterrupted Power Supply units?
Old 10-19-2007, 09:09 AM   #6 (permalink)
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I tried to convey that in the 3rd. paragraph of my original post. But you really do need to determine the power draw of all your devices when sizing a unit. At one time I had (2) 21" CRT's, a laser printer, the computer and a couple of external SCSI enclosures that needed protection. Collectively they drew about 900 watts, and a smaller UPS would have shut down almost immediately had I been running the laser printer when the power cut out.

Ink jet printers, LCD monitors, and today's hard-drives draw far less power than older equipment does. But at a bare minimum you need to make sure that the UPS you buy can supply enough power to run your equipment, and you do that by totaling up the number of watts your equipment needs and comparing that to what the unit can supply.
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Re: How do you size Uninterrupted Power Supply units?
Old 10-19-2007, 11:42 PM   #7 (permalink)
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My point, sort of, is that usually you don't care if a printer job is interuppted, since you can just start the job again later when power is resumed. What you don't want is to lose the data which means you protect the computer with the UPS and the monitor and that's about it. You only need to protect enough of the equipment so that you can safely shut down and not lose data. But if you have brownout problems, and you want to be sure that jobs will continue and finish, then you're right, you need a unit that will support all of the equipment attached.
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