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What to do when UPS sizing goes wrong

UPS sizing is hard, with confusing terminology and an ever-shifting IT load. If you get the calculations wrong, don't worry about a Michael Bay movie scene in the data center. Here's what to do when UPS is too wimpy or too beefy for the task.

The differences between kW and kVA ratings have confused people for years and caused some major misunderstandings of uninterruptible power supply capacity.

What problems will I see if I size my UPS wrong?

Nothing's going to blow up if you miscalculate data center UPS sizing.

If you under-size uninterruptible power supply (UPS) systems, the problem is obvious. When the data center overloads the available power, the UPS shuts down and should go into bypass mode. The IT load then operates on unfiltered utility power, which is OK most of the time. Modern computer equipment isn't as sensitive as prior generations, which is also why economy-mode UPSes are becoming popular.

However, a UPS in bypass mode affords no protection from a power failure or brown-out, or other significant power anomaly. The data center power design should specify a surge protection device on the bypass to handle any serious transients while you plan a UPS upgrade or replacement.

Oversizing the UPS results in wasted money: the additional capital spend, the never-ending bill for unneeded power and the money spent on cooling a hot, inefficient UPS. When you operate a UPS at 40% load or less, efficiency drops quickly.

A UPS should always be slightly larger than necessary to handle normal growth. Oversizing 20% to 30% is normal and acceptable -- it provides headroom for growth as well as capacity peaks and the short-term parallel operation of new IT systems during an equipment refresh. But, when you oversize by three or four times in anticipation of significant growth, it's wasteful.

If growth is hard to predict, consider some form of modular UPS. That allows you to add units in fairly small increments, keeping capacity in the high efficiency part of the power curve.

The real concern about miscalculating UPS size is with redundant systems. A 100 kW, N+1 UPS, made up of three 50 kW modules, starts out with 33% extra capacity. The extra is only there for redundancy. If you then oversize by 30% (meaning you are running a 70 kW load), you're operating at 70% of the 100 kW design capacity, but only 47% of the actual 150 kW capacity.

Redundant systems are a great use case for modular UPS. With relatively small modular systems, a 100 kW N+1 system built of 10 kW modules is actually 110 kW, bringing efficiency up to 64% with a 70 kW load.

Sizing problems get worse with 2N redundancy, where uninterruptable power systems share the load 50/50. Now that 70 kW load is only 35 kW on each 100 kW UPS, or only 35% of rated capacity. That's typical with full redundancy and a major reason to go modular -- to avoid even greater oversizing. Adding 50% capacity to a 2N UPS means operating at only 25% of the actual capacity on each system -- a terrible spot on the efficiency curve.

This was first published in August 2014

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