Tip

Server power consumption: Determining UPS runtimes

Runtime refers to the number of minutes of power that the batteries in your uninterruptible power supply (UPS) can provide. Calculating this value is important because all the capacity in the world won't do you much good if your UPS battery doesn't hold out long enough to get you through the blackout (or at least long enough to gracefully power down the computers attached to it).

The first thing you need to know about UPS runtime is this: Take the claims on the box with a grain of salt. I recently had to replace one of my UPSs. The new one was rated at 1500 VA/865 Watts. The box it came in claimed the UPS could provide 110 minutes of runtime. Although this claim is not exactly false advertising, the UPS does not provide anywhere near 110 minutes of runtime under a full load. In fact, when it's supporting a full load of 865 Watts, runtime is a mere four minutes.

So what happened to the UPS being able to provide 110 minutes of runtime? Lead acid batteries, such as the ones used in UPSs, will typically support a small load for a long time or a heavier load for a very short time. The UPS I recently bought will support a workload of 50 Watts for almost two hours (118 minutes to be exact). However, if you double the workload to 100 Watts (still way less power than what most computers consume), the runtime decreases to 65 minutes. The workload doubled and the runtime was cut almost in half.

Unfortunately, you can't just assume that when the workload doubles, the runtime

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is cut in half. Although it worked out that way in this example, it was just a coincidence. Typically there is a huge drop in UPS runtime as the first 200-300 Watts of power are applied. After that, the runtime starts dropping off more gradually as the workload increases. For example, in the table below are the stated runtimes at various wattages for the UPS that I have been talking about.

Table: UPS runtimes

Watts

Run Time in Minutes

50 118
100 65
200 32
300 20
400 14
500 10
600 8
700 6
800 4
900 0

As you can see in the table above, there is a huge drop in runtime as the first 200W of power are applied to the UPS. When the load is increased from 200W to 300W, there's another big drop in runtime, but not nearly as dramatic as the drop from 100W to 200W. Above 300W, the runtime continues to decrease, but at a much less dramatic rate. For wattages of 500 or above, there is only a decrease in runtime of two minutes for every 100W that are applied.

The Internet is littered with inaccurate information regarding UPS runtimes. In researching this article, I tried to find a mathematical formula that can be used to calculate UPS runtime based on power consumption. But the ones I found were nowhere close to accurate.

So my advice is not to buy a UPS based on the runtime stated on the box. Instead, prior to making a purchase, check out the manufacturer's Web site. Reputable manufacturers will provide a chart listing the approximate runtime at various load levels.

Once you have this information, you can use the techniques discussed in previous articles to calculate the approximate load the UPS will be supporting. You can then plot the estimated workload onto the chart and get a good idea of how long the UPS is going to hold out during a power failure.

About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server, Exchange Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. He writes regularly for SearchWinComputing.com and other TechTarget sites.

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This was first published in December 2006

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