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I am not that of an electricity knowledgable person, so I'll try to put as much info as possible.

I have a Rack Mount UPS of 1800 Watts Model: PRP 3050 RM (in total I have 2 of these - talking about one specifically)

I had to change it's batteries today, and that led me to think if it can support the hardware that are connected to it.

*I will add the specs I was able to find on each manufacturer site with links to PDF's Currently connected to it are:

  • Netgear 1100 specs out of production
  • QNAP U-859 RP+ specs *no amps indication
  • Dell R610 spec
  • Dell 1950 spec
  • Dell 2850 spec
  • Screen L1710S *minimal screen watts - < 0.5A

I have created on Dell's Power calculator ESSA, a map of the hardware I have:

Map of Rack Tower from Dell ESSA Site

  • Total AMPS used is 5.8
  • All three Dell's each have a redundant PSU
  • according to Dell's site - that means that each PSU uses half of the said/needed Watt's.
  • Each Dell is connected to 2 UPS's (both are PRP 3050 )


  • My UPS's specs says it has a 8.5Ah - does that mean I can connect more appliances until I reach the MAX?
  • What can I learn from this information that I have provided?

adding link to power calculator that was suggested by @amotzg http://www.jobsite-generators.com/power_calculators.html

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This FAQ entry might help: superuser.com/questions/9946/… –  amotzg Aug 5 '12 at 12:20
If you add up the Watts used by your hardware and the sum is larger than the watts provided by your UPS, that's usually not good. –  Oliver Salzburg Aug 5 '12 at 12:24
Technically it depends on the specific UPS design but I would not generally recommended it. Though you can work near it's maximum and reduce consumption when triggered to stretch the up time. –  amotzg Aug 5 '12 at 12:37
Expanding on UPS design: There are a kind of passive UPSes. They detect power failure and quickly switch to batteries supplied power. And active UPSes which always provide power from batteries while continuously charging. –  Hennes Aug 5 '12 at 13:03
On a different note: Can it support extra hardware today (with fresh batteries) or can it support it over a year or two (with batteries near replacement again). Plan for the last case. –  Hennes Aug 5 '12 at 13:06
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migrated from superuser.com Aug 10 '12 at 4:24


2 Answers

up vote 3 down vote accepted

If you have 8.5 Ah and draw 8.5 amps, you can do it for one hour. Conversely you can draw 5.8 Amps for 32% longer or approximately 81 minutes. Really you should only try to draw 80% of your max rating. Batteries get HOT under 100% load.

I would be nervous about operating so close to the stated maximum wattage of the UPS's. You should upgrade as soon as possible to the next available size (wattage).

The formula for wattage is very simple. It Volts (electrical pressure) time Amps (electrical current). So a 120 Volt 5 Amp (maximum current draw) device would need a 600 Watt power supply.

What would all of this information provide you? Figure out the total in kilowatts, and then multiply that by the number of hours it's on per day (daily kilowatt hours). Then multiply that by the cost per kilowatt, and you now know what this rack of equipment costs you in electricity every day.

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First thing to do is to ignore the nameplate and actually measure the current draw. Get an electrician to do that if you don't have the gear for it. Make measurements at the heaviest load your servers normally experience. It's surprising how different (and generally lower) real-life figures are compared to the server specs.

Next ensure your calculated figure is no more that half of the UPS rating. While you could theoretically draw the full Ah rating of the battery for an hour, drawing anything more than half that figure will result in a drastic reduction of the battery life. Ideally even stay below one quarter of the rating.

Of course your UPS should also state the maximum current it can supply, so be sure to factor that in as well.

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If you're using an on-line UPS also remember that the UPS has to be able to supply your expected startup load. Powering up a rack of servers (spinning up the hard drives and fans) often draws even more wattage than "peak load" for a machine that's running. You need to plan for this by either allowing extra UPS capacity or having a specific power-up sequence to avoid overloading the UPS. –  voretaq7 Aug 10 '12 at 16:42
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