Mar 26 - Sorry, maintenence underway, things will be fragile for a bit
Meauring Power Consumption
I manage a computing cluster for the Center for Statistical Genetics. As the cluster has grown and power becomes more and more critical, I had lots of problems trying to balance the load on the circuits and UPS units.
Most of our gear is from Dell and I found that subscribing to the Dell User Mailinglists has been very useful. I asked my questions on that list and was led to these Dell web pages which were very helpful in calculating the power consumption of our nodes. Playing with the configurations, I was able to determine that disk drives require about 10 watts and each GB of memory requires about 12 watts (remember YMMV).
Others told me one should only load a circuit to about 80% (e.g. you only get 24 amps from a 30 amp circuit). Because the various UPS units can only support fixed increments of power (and the bias is to not load them 'too heavily'), most UPS units only have 2-3 machines plugged in (depends on size, of course). This means for every 2 or 3U of machines, one might use 2U of UPS - meaning a third or more of a rack is dedicated to UPS units.
Still, this wasn't enough - more accurately, I didn't trust that I could answer the questions well enough to get a completely accurate estimate. Eventually some kind soul whose name I have lost/forgotten led me to the Kill-A-Watt Electric Usage Monitor (cost $35-40). Just plug this residential-grade device into a three-prong grounded wall outlet, plug in your device and get easy access to the real-time values for your device.
I put the machines under load for a period and got values that I think are accurate - of course the trick is to get the load correct. Still, I can plug the Kill-A-Watt into the UPS and the machine into it and let it sit for some hours and have some confidence I know what the real load is over a period of time. Here are the numbers I've found for the machines in our clusters:
In addition I've measured the power consumption of our RAID units. As you might expect the more drives, the more power, but compared to our servers, the RAIDs draw little power for the space they require.