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.

December 14, 2012

5 Min Read
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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 CorrectionI mentioned Power Factor Correction (PFC) in a previous posting, but I didn't explain it. If you can understand Power Factor and Power Factor Correction, you can really build some credibility with the electrical engineers. It is not an easy thing to understand, so don't get frustrated.

If you think of a car burning a gallon of gas, efficiency is similar to miles per gallon. In other words, how efficient the car is at converting the gallon of gasoline into miles traveled. Power factor is analogous to a leaky fuel line: It's the fuel that never reaches the engine. This isn't measured in the computer; it is only evident upstream. Using the car analogy, the car will calculate the same MPG, but you will only notice the gas leaking from your tank if you check the gallons supplied at the pump compared to the gallons used by the car.

We can see the PFC feature in some power supplies in the DoE and LBL graph above. "Efficiency" above shows how watt-hours consumed are wasted by the power supply itself. PFC is about getting more watt-hours out of your existing power entering the power supply.

As discussed, the conversion of VoltAmps to Watts includes a Power Factor. The Power Factor describes how "reactive" the load is by how far the load lags the voltage waveform. In English, this means that an ideal load would match voltage wave perfectly. Any deviation from ideal creates two problems: stranded capacity and harmonics. PFC is important to deal with both of these problems.

The first problem of a leading or lagging power factor is that it reduces the amount of power that can be used on a power circuit. For example, if you assume an IT power supply is on a 20 amp breaker (rated 16 amps continuous load) at 120 volts, which operates at a 0.80 power factor (very poor), the power supply can only consume 1.536 kWh in an hour (16A*120V*0.8PF). At a 1.0 power factor, the power supply can consume 1.920 kWh per hour (12*120*1.0); an increase of 25%. The 384 watt-hours is the stranded power capacity. The electrical infrastructure is present in the data center to support the full 1.920 kW, but your power supply can't extract the energy. A crude explanation of the limitation is that power supply is lagging and can't take advantage of the peak voltage. When the power supplies provide PFC, they take much better advantage of the peak voltage.

Another problem to understand when discussing PFC power supplies with the data center staff is harmonics. When the power supplies lag or lead the voltage, they create "echoes" on the power circuits. These echoes can resonate at specific frequencies and are called harmonics. (like feedback on an audio system). The details aren't important, but the best way to think about harmonics is that it is "noise" on the electrical system.

Power harmonics can cause many types failures, such as overheating conductors, damaging power supplies or causing logic chip failures due to voltage conditions outside specifications. You can track the historical progression of this issue by reviewing the increasing specification by engineers for Z-rated or "Harmonic Mitigating" transformers.

Why should IT care? Not only can you correct a major source of the harmonics by using Power Factor Correction power supplies, you get more out of your electrical infrastructure without any upgrades. By breaking into the data center code, you can show how IT use of PFC power supplies leverages existing investments in data center facilities while reducing noise on the electrical system inside the data center.

Ken Miller is data center architect with the IT Infrastructure and Operation Services division of Midwest ISO, developing mission-critical facilities.

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