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Calculating UPS output requirement
Any rules of thumb for wattage?

 2:52 pm on Sep 8, 2004 (gmt 0)

We've got lots of scheduled elec. board works in our area for the next two weeks. There's going to be power outages.

I'm taking the opportunity to invest in a UPS as I've been meaning to do it for a while now.

I have no idea what our power requirement is going to be. I can add up all the wattage ratings of the PC's, but they're all 400/500 watt supplies, but that's max output(?) and none of them will be drawing anything like that(?).

They are all pretty much standard tower case PC's from PIII's to PIV's with one hard drive and a CD-Rom.

Can anyone give me some indication? The monitors are easy to do as the power rating will be actual rather than maximum PS rating.

I've got 5 of these PC's all with monitors.

Ignoring the monitors and other hardware which is easily calculated what do you think would be an approximate rating per PC? I appreciate it would vary, but I'm only looking for an average. The power outages are scheduled to happen at night, so they won't be in use at the time, I just want to keep them alive (turning them off at night is not an easy option).

I found a lot of resources that tell me approximately how long the UPS will hold out for on it's batteries for a given wattage, but can't find any ballpark wattage for your average common-or-garden PC.





 3:22 pm on Sep 8, 2004 (gmt 0)

You can buy a meter at your local hardware store to measure the current. The in-line ones are plug and go, but require you to break the circuit to insert the meter, while the inductive "Amp Clamp" meters require you to clip the circular clamp around one lead of the two-or-three-wire AC power. This will give an actual reading in amperes. Multiply by voltage to yield VA (apparent power). Then you must add a safety margin -- and this is the tricky part. It is best to make sure the machines are very busy when you make the measurement(s); then your guessed safety margin can be lower and more accurate.



 3:29 pm on Sep 8, 2004 (gmt 0)

Thanks Jim - such a obvious answer when you think about it - I hadn't even considered the option of measuring it :-)

As for keeping them busy, I could always run scanddisk/defrag on them all when taking a measurement to get some HD activity. I'd probably stick 25% on in any event.


I've just noticed that my existing electricity meter already gives me watt-hours. If I take reading (a) and then exactly one hour later reading (b), I presume my average watt consumption is b-a?

Having turned everything off bar the PC's first.




 4:40 pm on Sep 8, 2004 (gmt 0)

I suppose that might be worth a try, but considering that you're about to invest in a UPS that can keep five machines running for hours, the expense of a meter is likely to be insignificant. Just a wild guess, but it will cost about $500 per hour per PC with a single medium-sized UPS, and more if you go with individual units.

The off-the-shelf UPS units that you can carry out of a store are designed to give you 5 to 20 minutes to shut down your PC in an orderly way after a main-power failure. When you start extending the run-time to hours, the required units get big and expensive. You might want to look at a 1000VA regulating UPS in conjunction with a rented generator. For this solution, you'll likely need a licensed electrician and a permit.



 4:44 pm on Sep 8, 2004 (gmt 0)

The electricty board reported that outages shouldn't be for more than a few seconds. I don't really need hours - I'm in Central London. Continuous power is taken very seriously here. I just know from experience what can happen to a hard disk with a sudden power loss if it was in the middle of writing ;-)

I'd be happy with 20 minutes cover.



 10:40 pm on Sep 8, 2004 (gmt 0)

If you only need a few minutes of power - as you have described - you will not need to power the monitors.

This will save you many $$s (or pounds seeing you are in London) because you don't need to SEE anything. Ask yourself the question of what is essential to keep running and what is a nice to have. Monitors may be a nice to have.

As you will be buying a UPS and keeping it for a while, consider going a bit bigger than your estimate. Remember to maintain your UPS by testing it regularly and every few years, the batteries do need to be replaced (depending on the model). As the batteries get older, their capacity also goes down - another reason to go a bit bigger.

The way I have always sized UPS is to go the manufacturers site and looked up their usage. This only works with name brand servers and not white-label boxes.


 10:49 pm on Sep 8, 2004 (gmt 0)

Thanks Warren, I'm shopping around now and I'm looking at brandnames.

I figured if I'm going to do it, I may as well have the capability to power the monitors as, if we have an unscheduled power cut in the middle of the day, it's useful to have the facility whereby people can save their work before shutting the PC down.

The monitors are all LCD - only 70 watts a piece anyway.



 11:47 pm on Sep 8, 2004 (gmt 0)

Of course, the other option is to give them all laptops - when the power goes out (as it did to our office only yesterday) all of the laptop users were able to save their work.

You could tell a laptop user from a desktop user - the laptop users had this warm glow to their faces as they were basking in the LCD light saving their work, whilst the desktop users were the ones swearing at blank monitors in the dark.

For the record, I was one of the laptop users ;-)


 10:59 am on Sep 9, 2004 (gmt 0)

We have a couple of laptops too, but need the tower PC's unfortunately (backend hardware running through PCI cards).

You're absolutely right, if we didn't need the PCI slots, we'd all go the laptop route.

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