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One supermicro enclosure comes with "3 SP382-TS 380W Power Modules for a total of 760W".

2x380W = 760W, okay, but there are 3! So where is the missing third 380W equalling to 1140W?? Is it hot-sparing, kicking in when one dies?

  • What wattage is supplied at any given time, 380W, 760W?
  • Or are all running at-once, with one having the ability to die leaving the server with 760W? (Is that wasting power, given that PSUs perform best at ~70 load?)
  • Or is one waiting for action?
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I'm not sure I've seen a common chassis that supports three PSUs. Are you sure that it's not a typo on their site and that it's two PSUs? –  MDMarra Dec 13 '11 at 20:13
    
i have an older 4U supermicro chassis with 3 PSU's. they all spin up when it boots, but i believe the idea is that your available capacity is two psu's worth, and the third is just idle capacity until needed. i measured the current draw with a kill-a-watt, and there was only a few watts difference between having two psu's plugged in and three. –  anastrophe Dec 13 '11 at 20:22
    
It's a Supermicro SC933. For the 3 PSUs: Have a look at serverfault.com/a/97419/102280 - this indicates the third one is idling. Do these Ablecom/Supermicro PSU have some logic inside that switches PSUs once once dies? –  isync Dec 13 '11 at 21:23
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2 Answers

The concept is called n+1 redundancy. The reason three PSUs are commonly used is that it is more efficient to have three 380 W PSUs operating at 50% load and 90% efficiency than have two 760W PSUs at 37,5% load and less than 85% efficiency.

All that said you should consider that the PSU rating is for rough orientation only. For one thing, your system power consumption will typically be way below the peak rating. For the other, the value is usually just a sum of all available power bus ratings (there are plenty in a PC) and thus not useful as such for any kind of precise calculation.

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Indeed, a replacing a 100 watt PSU with a 700 watt PSU won't increase your energy usage 7-fold. Much like a car with a 500HP engine, you're only using that power if the throttle is all the way open. Usually you're just using a portion of it. A PSU's wattage rating is what it is capable of. –  Jeff Ferland Dec 13 '11 at 21:47
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I'm assuming your chassis has an Ablecom SP-762. It's about the only setup that uses 3 SP382-TS power supplies, and that's what Supermicro uses.

This is a power supply enclosure that holds three small power supplies, each with their own AC connector. It has a total output capacity of 760W. It can provide the full 760W even if one of its smaller power supplies fails, since each supply can output up to 380W.

There is nothing to equal 1140W. The power supply cannot put out 1140W. At any time, the power supply can produce up to 760W, so long as at least two of its modules are working properly.

All are running at once, with one having the ability to die leaving the server with 760W. No power is wasted. These PSUs are specifically designed to operate in this way. (PSUs are typically designed for 70% load because that's what they typically see. Redundant power supplies are an exception and are designed appropriately.)

This review on Tom's hardware shows the power supply.

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I think you are right with the Ablecom as it's a Supermicro SC933 chassis. Thanks for the explanation. And: is that really true that no (or very low) power is wasted as these PSUs switch over intelligently in case one dies? Would IPMI show me this PSU unit like in serverfault.com/a/97419/102280 ? –  isync Dec 13 '11 at 21:26
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