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I have a couple servers in my garage running ESXi. Each server has a 1000W gold rated PSU which I bought second hand from a miner. They work great and were pretty cheap. These are not super powerful machines, but they each have 4 DIMMs, 4-5 HDDs, and an old (somewhat power hungry) AMD GPUs (5850 and 7780 IIRC).

I am now looking to purchase a UPS to protect these servers in case of a power outage.

Please understand that I am not looking for shopping advice, my questions are not related to a particular model/brand. With that in mind, I have two related questions:

  1. Given that each server has 1000W PSUs, but run on average much lower (300-400W, saw it peak once at 500W, but should not be more). What is a good size for the UPS? I am not interested in keeping the servers running in case of an outage and 5 minutes should be plenty of time to stop everything.

  2. I have sometimes observed that the fuse was tripping (this is the garage, so the electrical installation is not very good) when both servers are rebooted at once. I assume this is because the PSU is running at peak for a few seconds when powered on (is that possible/true?). If so, is that a concern for my UPS? For instance, if I get a 1200VA UPS, will it get in trouble when the power returns and both servers come back up at once? If so, what would be a solution? I do not feel like purchasing a super expensive UPS, so would it be an option to simply get lower wattage PSUs? Any better (i.e. less expensive) options?

Thanks in advance for your help!

  • Your first question, "What is a good UPS?", is outside of our scope here at Superuser. In general get a UPS that support a 500 Watt load for the period of time you will need to in order to manually turn it off. "For instance, if I get a 1200VA UPS, will it get in trouble when the power returns and both servers come back up at once?" - You should call out an electrician and ask them what they would suggest to prevent this problem. – Ramhound Jan 15 '17 at 12:34
  • You are asking an off-topic question (hardware shopping). Please read On-Topic, How do I ask a good question? and What types of questions should I avoid asking?. Try Hardware Recommendations. but please first read What is required for a question to be 'high quality'?". – DavidPostill Jan 15 '17 at 12:44
  • The question was not asking about a particular model or brand, but asking for sizing advice. I have edited the question to reflect that better. I repeat: I am not expecting shopping advice, but sizing advice (as per the question title). – user510159 Jan 15 '17 at 12:44
  • OMG you guys are really pedantic :-( Again, I do not care for a specific brand/model. – user510159 Jan 15 '17 at 12:50
  • The PSU capacity is irrelevant. The actual usage is what counts. Base the UPS wattage on the maximum load it will ever see, or the load will pop the UPS breaker, which will be the same as no UPS at all. Compare pricing on multiple smaller units vs. a single big one. If the servers shut down because of handshaking with the UPS when the power goes out, you typically restart them manually. So you wouldn't need the UPS to handle simultaneous restarts. A single UPS could be sized for the combined operating power plus the startup excess for one. (cont'd) – fixer1234 Jan 16 '17 at 9:13
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Spend $20 or so on a "Kill-A-Watt" or similar plug-in power meter. A cheap investment in an expensive enterprise.

Shutdown your systems, plug in the meter, set it to read "VA" (volt-amperes, what really matters when sizing a UPS) and power them up. Perhaps be all modern and video the screen of the meter, or just watch it carefully for the peak values. Now you know how many VA your UPS needs to be to start your systems without overloading. In most cases this will mean you should get 5 minutes (when already running) out of it, until the batteries crap out and you get less than a second, and you need more batteries (I mentioned that it's an expensive exercise.)

Most modern systems deal gracefully enough with a power outage that you need a really good reason to spend what it takes to buy and maintain a UPS for them.

  • The VA reading is pretty useless; you need watts to size a UPS (based on the UPS watt rating). The UPS VA rating is a BS marketing number based on a ridiculous power factor that isn't relevant to computers. The computer VA draw is virtually watts. The UPS VA rating is a ridiculously inflated number, if you size the UPS based on its VA rating, it won't handle the computer's watts. – fixer1234 Jan 16 '17 at 8:58

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