How do I calculate how much power I need for my servers / switches / routers? This is needed when moving into a datacenter.

I figure I should just add up the wattage on all the power supplies even though all that power is not used most of the time, is that right? If there are redundant power supplies, should I count just one or both of them?

3 Answers 3


APC has a pretty decent power calculator here. It's certainly better than pen and paper calculations.

  • 1
    Ya, saw that one this question, serverfault.com/questions/48833/calculate-ups-power, but I would still like to know if and why I would count redundant supplies ... Oct 2, 2009 at 15:48
  • Are you sure the APC tool just blindly doubles power requirements if you specify redundant PS in your server configuration? If it doesn't then you'll be all right there. Oct 2, 2009 at 17:26

You will grossly overestimate using this method. We found dual socket servers with 600W P/S draw steady-state around 180W and dual quad-core SLI graphics workstations with 1000W P/S draw less than 300W most of the time. Properly designed redundant power supplies add very low overhead, just a few percent.

Buy a $30 power meter such as the Kill A Watt, leave it plugged in for 24 hours each for a representative device, and know instead of guess.

  • Have one of those, but can't unplug the servers :-/ Oct 2, 2009 at 16:33
  • You can get a clamp ammeter, but you have to clamp it around a single conductor; that is, only hot or neutral, not the whole cable. You should be able to split the cable open to get to a single conductor if you're very careful. You might also be able to get to the wires behind the panel in your circuit breaker box.
    – wfaulk
    Oct 2, 2009 at 16:49
  • 1
    That's a problem, but you only need unplug and test one representative server of each type.
    – kmarsh
    Oct 6, 2009 at 19:13

See this related question/almost dupe.

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