Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm trying to put together a backup power system for our office and I need some guidance.

We frequently lose power for several hours at a time. I'm looking for a solution which would:

  1. Control power quality going into our devices (e.g. surges, voltage fluctuations etc.).
  2. During a power failure, allow us to continue working for several hours uninterrupted. Or, at the very least, allow our devices to be powered down safely.

On our LAN we have the following devices:

  • 1 x 1U Rack Server
  • 1 x Rack mounted 24-port network switch
  • 1 x Rack mounted PBX
  • 3 x Desktop Computers
  • 3 x LCD Monitors
  • 1 x DSL Modem
  • 1 x Multifunction Router/NAT/Firewall device

I'm pretty clueless on this topic and I was hoping you folks could help me with my confusion:

  1. How do I size a backup power device for our needs?

  2. Does there exist a single backup power device which can support all these devices at once? Or, will we require several smaller units?

  3. How do we scale our backup power system as we add more devices?

  4. Does there exist a battery based solution which will allow us to work for several hours after a power outage? Or will we need to rely on a gasoline generator?

  5. Do we need a UPS and an ATS? Can you use a UPS without an ATS?


share|improve this question
up vote 3 down vote accepted
  1. Sizing - For a rough estimate you can look-up the maximum power ratings of each device, assume they're running at 75% normally and calculate the power draw.

    If you want something more accurate, use a Kill-A-Watt type device. Some servers and computer have management information that can tell you how much power they're drawing. Many "accessory" devices such as monitors draw full power constantly.

    Offhanded guess, you're probably around 1-2kW.

  2. Assuming the 2kW guess is reasonable, you could get a single UPS that will run these devices for 10-15 minutes (multiple UPSs may be logistically more feasible). Which is enough time to shutdown. You can buy a portable generator that could run them for an extended amount of time. You'd probably want a permanent automatic generator if you want uninterrupted service, as it generally takes more than 10 minutes to get a portable generator running and all.

  3. Adding more UPSs for more devices is easy for the scale of your situation. Adding more generators is not so easy, you'd probably want to upgrade the generator as demand increases. See a proper electrician for "future-proofing" your plans, expect to pay a lot more for capacity that you don't need "right now".

  4. Batteries store much less power than most people think. You'd need a batter bank the size of a cubicle to run all that for "several hours". (Caution: some hyperbole present in the size estimation; but a system large enough for "several hours" is going to be rather large).

  5. What you "need" is up to you... USP is the only stopgap between power-out and generator. Generators are the only realistic way of keeping the power going for more than "minutes". An automatic generator and ATS (automatic transfer switch) is the only way to provide reasonable assurance that service isn't interrupted when the power goes out.

share|improve this answer
Right with ONE exception - he does not need a lot of batteries. I run a 2A average load on 3 phases 230V (about 2kw) and my Eaton 9355 shows me 400 minutes reserve time, also during power outages (which may last 10 to 15 minutes). – TomTom May 6 '13 at 15:24
You're right that it's not as big as a cubicle, that was a bit of exaggeration. It's still going to be fairly large, the 9355 is the size of a small server rack and would need to be half full of batteries to support 400minutes at 2kVA. – Chris S May 6 '13 at 15:44
Acutally no, it is NOT the size of a small server rack. It is about 1,60 meters high and maybe 40cm wide - that is VERY small as a rack. Yes, it is not trivial, but hey, one has to get what one has to get. – TomTom May 6 '13 at 16:45

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.