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Some Context:

I have an old P4 machine that I am planning to use, for hosting my Java EE apps for demoing during interviews.


  • P4 1.9GHz
  • 40GB HDD
  • 2GB RAM
  • VIA chip-set
  • 300W power supply
  • Will be connected to KVM switch; no power for display needed during demo.

I need this server to run only during my interviews or whenever I demo my apps to other people. I will turn it on when I leave and turn it back off when I come back several hours later.

There are frequent power outages in my place so in the worst case, I need a UPS that can keep it running throughout the 5 or so hours of my commute + presentation.

The Question:

  • What kind of UPS should I buy? Right now, I only have the cheap basic desktop PC UPS model It barely keeps my own PC running for 10 mins, let alone another server. Obviously I need to buy a bigger better UPS. But I don't know what to look for when I shop. Any type/spec that I should look for? What should I ask for when buying?

  • Do I need external batteries? I've seen some people have a dozen of those connected in a room and it seems to power a handful of computers for several hours. How many do I need to get for my requirements? What is the technical name for those big black boxy things with a couple of points where you attach power cables? where can I learn how to set them up and connect them?

  • What other things should I need to know about before setting up this server and its power supply?

share|improve this question
"What should I ask for when buying?" -> Ask how long it will run your (300W) system. – slhck May 10 '11 at 10:45
why not just run them locally? you would probably get better, more impressive performance off a local server. – Journeyman Geek May 10 '11 at 10:53
Two things.. we are in a garage sale mood at the moment and I need an excuse to retain the old CPU :D.. also, my dev sys is also the only other machine in my home and many people use it. I have not control over them when I am at home, let alone when I am away :D Either way, the backup power question still applies right? – user57813 May 10 '11 at 11:02
up vote 1 down vote accepted

Most UPSs run on sealed lead-acid (SLA) batteries. The capacity of lead-acid batteries is typically rated in Amp-hours. The current draw of a 300 Watt load from a 12 V source is (very roughly) 300 / 12 = 25 A. To provide 25 A for 5 hours you need a bigger than 125 Ah battery plus a suitable circuit to charge it up and a suitable inverter. That's a largish marine or truck battery. Most PCs don't consume 300W continuously though.

As far as I know, most UPSs are designed for shorter durations and are used in conjunction with a back-up diesel generator for longer outages.

As other have said, consult APC or other reputable UPS manufacturers. Looks like a 1000 VA UPS with a couple of battery extenders - $2000?

share|improve this answer
+1 already.. but from your answer, I understand you can connect the battery(s), inverter and the UPS in a circuit. Where can I find a diagram of such circuit assuming I am using a 1 or a couple of batteries? – user57813 May 10 '11 at 14:21
Well.. forget it. After doing research and finding out about the possible hazards, I decided it is not safe for me to do any of this myself. I'd better leave this to a qualified electrician. – user57813 May 10 '11 at 14:45
A basic UPS is a battery + charger + inverter. For circuits ask in – RedGrittyBrick May 10 '11 at 14:47

I would suggest you check out APCs Product Selector as it will help you calculate your power needs and suggest a few products in the size you would need.

share|improve this answer
+1 for the product selector link. – user57813 May 10 '11 at 14:17

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