• 12/14/2012
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How to Speak Data Center: IT Power Supplies

Knowledge of IT equipment power use and efficiency are essential in the data center. Here’s how to get started.
In my first two posts on how IT pros can improve their interactions with data center facility pros, I covered the difference between power and energy, and how to covert between kWh and BTUs. This post will address power supply ratings and power supply efficiency.

IT and data center professionals need to understand how much power IT equipment uses. You may have heard of a quick-and-dirty option to use the "nameplate" power rating to estimate energy use. The nameplate is a safety label that comes from the Underwriters Laboratory which was formed in the year 1894 for the independent evaluation of electrical products for safety.

Despite the nameplate's function as a safety label, it can provide a clue as to the device's energy use. It's a gross rule of thumb that most IT power supplies typically sustain a load that's less than half of the nameplate rating.

However, I do not recommend relying on this method. Get the right tools to measure actual energy consumption. For instance, power distribution equipment often includes reporting that you can use. You can also get tools such as a multimeter or "amp clamp" for external verification. Don't touch the power without proper training and safety equipment!

Another unwritten rule in the data center is to pay attention to the power supply efficiency rating, such as the Energy Star External Power Supply (EPS) v2.0 metrics adopted by the Department of Energy. In conjunction with recent European Union efficiency standards, the bottom end of the power supply market has been increasing efficiency, but your choice of efficiency should still depend on your business requirements.

Power Supply Efficiency

You can see in the DoE and LBL diagram above that most power supplies are less than 75% efficient. If you also consider power supply redundancy, splitting the load across multiple power supplies means efficiency can reach below the 30% load. This can take a mediocre 75% efficient power supply and drop it to 60% efficiency. This means 40% of every watt-hour consumed is just turned into waste heat.

Why should you care? Every watt-hour of energy lost to a 40%-inefficient power supply is a watt-hour that you can't use for another computer. Although you can't get 100% efficient power supplies, what would you give for 10% to 20% increase in your UPS capacity? Every watt-hour saved is liberated power capacity on your UPS. Highly efficient power supplies can give you that capacity boost at a fraction of the cost of a new UPS.

Next page: Power Factor Correction

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