Data Center Power Draw Calculation

Question

I have a difficult time calculating the power draw of our upcoming server (Dell R710) and what that means in terms of how a data center will calculate power costs. Hopefully you can help shine some light on this!

1. I've looked at Dell R710 Power measurements at spec.org which tells me that the server is drawing between 64 (idle) and 236W (full load). Our server's specs are slightly below the measured server, so I expect the draw to be even less.

2. I understand that Amp are calculated by Watts / Voltage. In the United States, the outlet voltage is 120 V. That means, the server is drawing between 0.5 and 1.9 Amp depending on load.

3. Most data centers I've looked at say they're including 1.5 Amp with a 2U space. Is this based on average draw? When is it typically measured? What happens when the server draws more power?

4. The smallest R710 PSU is a "570W" PSU - but it looks like it will never, ever, require 570W. How does the PSU relate to the power draw?

This is kind of confusing, please help me understand how this typically works a bit better. I have a feeling I'll run into power overcharges otherwise!

Edit: Even more confusing, Dell's Energy Advisor calculates 3.1 amp for my server (Dual E5620, 24 GB RAM, 4 SAS HDDs). I'm getting lost.

Answer 1

On the Dell R710 (and many other makes/models) you can monitor the power usage yourself with this command:

# ipmitool sdr list | grep Watts
System Level     | 84 Watts          | ok

That's the linux version, but there are Windows equivalents. Graphing that in your favorite tool is left as an exercise for the reader. It should be noted that this gives the power draw into the motherboard. Power supplies are not ever 100% efficient, so add about 15% to that number to get the input into the power supply. Or connect it to a watt meter and measure the efficiency yourself. PSUs are most efficient in the middle of their stated range, somewhere about 50-60% of the rated capacity.

If you are concerned about power usage you might consider using an L-series processor.

What happens when you draw more? That depends on the provider. You'll likely just get a warning (if they even notice at all.) And that's also the scary part. What if everyone draws just over and the circuit breaker trips? How closely do they monitor those circuits? Is it active monitoring or passive monitoring (is there a meter on the circuit or do the building engineers do spot checks with a clamp on meter?) If there is a meter is it per power port or per circuit?

Overall, it's just best to monitor the draw yourself.

How do you know before you order the server? Well, that's a guessing game. Unless you're really cranking on the HW you won't get near the peak.

Answer 2

The number of Watts on a PSU is a maximum rating, and is generally substantially more than the system will ever pull. Most vendors do not customize PSUs for every load. After all, your power draw will depend on the exact configuration you choose, your applications, etc.

1.5A gives you about 312W/VA (assuming active PFC) which would be pushing it if you upgraded that server any. Of course, that's going full blast, which most servers don't do very much.

My advice: contact the sales team at the data center you're considering. My guess is that they allocate 1.5A/2U space when speccing out UPSes/generators, and on average, the servers consume a lot less than that. Cut-rate data centers may do worse, but that server should be fine in any decently run DC.

Answer 3

What a data center does when you reach capacity really depends on the data center and the equipment they connect it to. I mostly deal with full circuits, say 20A, and for those it's not uncommon that if you exceed it by too much for too long the circuit shuts down, just like would happen at home if you run the microwave and toaster at the same time. :-) But at a 1.5A loading, it'll depend on what the equipment you connect to can do, and how loaded the circuit is.

Give them the specs of the server you are looking at and ask them if you need to reduce your server power draw or they're ok with it mostly being under that. It's really up to their policies and procedures, but expect that they've seen people come in with over-powered servers before, saying "we'll only use 0.5A usually" but then a process gets stuck and they start using 2A all the time.

As far as the power supply specs, most power supplies are at their peak efficiency around 50% of utilization. Meaning they convert most of their input power into output power. So that 570W power supply will only produce around 175W to 200W of usable power to the components at that 236W peak load you are talking about.