Server with spare power supply

I am trying to work out the power consumption in my server room.
We have few physical servers with dual power supply.
Does both PSU draw the current ?
We have PE2950 with 790W PSU. Do i have to add both PSU's current?  my understanding is one power supply is standby this case it should not draw the current.
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These can typically be configured with one as an inactive standby or with both operating all of the time.  In any case, I'd measure the current of both.  Even in standby the power supply may draw a small amount of power, though not enough to matter.
Mal OsborneAlpha GeekCommented:
A server with 2 PSUs usually as one for redundancy, however if heavily configured with the less powerful PSU option, this may not be the case.

In any case, power consumption is usually WAY below the PSU rating, the server with a 790W PSU is probably drawing 200 or so .

Below is a link to HPs online Power Adviser (there is also a downloadable version). Although your server is a Dell, if you plug in the same values for CPU, RAM, HDD etc, for a comparable model it should be close enough.

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I may have misunderstood "do I have to add both PSU's current?".  I was assuming that you are going to actually measure the current (or power) independently of the 790W maximum rating.  That rating doesn't tell you much about the actual draw.

It's inexpensive and easy to get a reasonable measurement of the power draw.  For example:

These will allow you to read the power draw for $20-30.  The accuracy is likely well within what you need.

You can use them to measure energy use over time which is important as the power draw may vary with use.

Don't overlook the fact that virtually all of this energy goes into heat which you'll likely have to deal with.
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Mal OsborneAlpha GeekCommented:
Also, the management software for any half decent UPS will show you power consumption.
The watt rating of a PSU is about how much the device can deliver to the server's components, not about how much the PSU itself burns up to work. The server itself will only draw as much power as it's components (disks, RAM, CPU, Mainboard, fans etc.) need. Those components won't suddenly need double the power just because there is a redundant PSU attached. Of course each PSU does burn up some power itself, but that is a fraction of what it can deliver.

So what you should be measuring is not how much watts each of your PSU's may be able to output, but rather what each of your servers need. For example find which CPU's are used, and what each of those needs. Check what and how many harddisks you have and what each of those uses etc. A server that isn't fully filled with all it can take (all disk bays full, all CPU sockets used etc.), will not come close to what it's power supply can deliver, so it will draw much less power from it. Besides that, it will draw more power when powering up than when it is operating normally.
If you are looking at a computer that you don't have on hand, going through the calculations of what each device can draw is reasonable.  On the other hand, if you already have the computer, just measure the actual draw.  It is inexpensive, probably more accurate, and very quick.  The biggest difficulty is that you may have to shut the computer down to insert the meter.

The other advantage of measuring is that you can see how the load changes with use.  For example, it may be significantly higher as you turn on the computer.  This won't affect your average power requirements significantly but may affect the peaks for which you need to plan.
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