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Asked: 2011-12-14 12:11:16 +0800 CST

How much power draws a redundant PSU?

  • 772

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?
power-supply-unit redundancy

3 Answers

  1. 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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  2. 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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  3. The load is shared on all 3 power supplies, but one is for redundancy. When one drops, you have 380+380=760Watt hence the rating. Its designed to keep running when one drops, that's why they don't rate it for 1140W.

    On a dual P/S configuration of the same power, they would include 2x 760W PSU's. Again the load is shared until one fails. But since its designed to keep going after the failure, its rated for 760W not 1520W.

    The-Wabbit, is spot on about how loading and efficiency is different on a dual vs triple PSU. PSU's run under 20% are not very efficient. (newest titanium PSU's are nice however)

    Say your system idles at 250W and peaks at 450W on your 760W rated system.

    Dual power supplies sharing the load, your just under 16.5% load at idle, not good at all for efficiency (250/1520). Triple PSU's sharing the load, it idles at 22% which is a better number. And in either configuration when one PSU fails you still have 760W of rating.

    However their are some benefits to having dual vs. triple. With dual, you can easily run them on two UPS backups, on two circuits, on two phases. Each decreasing chances of the system going down, but attempting to do that with triple is a bit much.

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