How to choose the right uninterruptible power supply for your data center

This tip discusses how to choose the right-sized uninterruptible power supply (UPS) for your data center. It also explains common mistakes in UPS sizing and why UPS power ratings can be so confusing.

When it comes to buying the right uninterruptible power supply (UPS) for your data center, size matters. This tip...

explains UPS sizing and capacity planning.

Child 1: "My dad's UPS is bigger than your dad's!"

Child 2: "But my dad's has more kVA per kilowatt!"

This imaginary children's banter is indicative of the confusion that has long surrounded UPS ratings. And it has become so common to oversize a UPS that "bigger is better" has usually been taken for granted. So how big should your UPS be, and what do those ratings really mean? I'll explain why Child 1's father may be wasting energy, and why Child 2's brag is actually a negative. If you've ever felt confused or misled by UPS ratings, you're not alone.

First we need to understand terminology.

Volts (V) x amps (A) = volt-amperes, or VA. (We'll get to watts momentarily.)

So 480 V x 250 A = 120,000 VA.

That's a big number, so we divide by 1,000 and get 120 kilovolt-amperes, or 120 kVA.

In my article on calculating the data center power load, I said that with alternating current (AC), VA does not equal watts, but I didn't say why. I also said we could probably ignore the error with today's servers. But for UPS ratings, the difference does matter. Let's discuss the reason.

For AC power, the complete formula is as follows:

Watts = volts x amperes x power factor, or W = V x A x pf.

Power factor is defined as the ratio between "real power" and "apparent power," but this is not an engineering article, so that's all we're going to say on that subject. Watts is the real power and volt-amperes is the apparent power, so VA is obviously something mysterious. But it's the watts figure that's important for today's data centers, so we can let the mystery be.

We do need to understand that this thing called power factor is rarely 1.0, except for incandescent light bulbs, heaters and toasters. It's usually less than 1.0, and never more, so watts are generally less than volt-amperes. Now let's look back at those servers, which today have power factors between 0.95 and 0.99.

120 V x 3.0 A = 360 VA x 0.95 pf = 342 W

That's a small difference between VA and watts. With better power factors it's even less, which is why we said the error doesn't matter much unless you have a lot of hardware.

Most UPS systems, on the other hand, are sold based on kVA ratings, but for years have been designed with power factors of 0.8. So a 100 kVA UPS with a 0.8 pf can deliver only 80 kW of real power. If you believed they were the same, you'd eventually find you had a 20,000 W shortfall. That's one reason many people have been surprised when their UPS said "98% capacity" but they were nowhere near the kVA rating they bought. Rule: If your UPS power factor is less than your computer hardware power factor, your actual UPS capacity will be its kW rating, not its kVA rating.

Since server power factors have gotten better, many UPSes are now designed with a 0.9 power factor, so a 100 kVA UPS will have 90 kW of capacity. And at least one manufacturer designs for unity, or 1.0 power factor, meaning that the kW and kVA ratings are the same. (With such a UPS, the load limit will be kVA, not kW, because your computer equipment is not perfect. In other words, a 100 kW/100 kVA UPS will probably max out at around 95 kW.) We won't discuss small UPSes that often have power factors around 0.7 -- they're specified in watts, so you will know.

How to size your uninterruptible power supply
When we know the kilowatt and kVA ratings, we can size our UPS. We previously showed how to estimate real load watts and explained why data center power is so often figured 40% to 60% high. Now we'll show how to "right size" the UPS. Start with the real estimated Day One data center load in kilowatts, then add some headroom. A good rule of thumb number is 125% (which is 80% loading). Then pick the next highest standard-size UPS. That provides some growth, as well as capacity for installing parallel systems during an upgrade.

That's good for a while, but it doesn't cover the long term. We also need to grow to the ultimate load we calculated, but we don't want to over-size the system in anticipation. At low loads, UPSes waste more of their power in heat, and are generally most efficient when running close to their rated capacities. Efficiencies vary widely, but many double conversion UPSes are 90-95% at 80-100% load, and then they go down. There are high-efficiency systems available today that go up to 98%, often using different technologies than we're used to, so to get real efficiency we may need to think about things differently than we have been. But let's look at more conventional system efficiencies as an industry norm.

  • 89% at 50% load;
  • 88% at 40% load;
  • 86% at 30% load;
  • 82% at 20% load.

That's lost energy, 24/7/365, which takes more power to cool.

A good consideration today is one of the modular or incrementally enabled systems. They let you plan for your maximum growth, but provide only the actual capacity you initially need. Modular systems let you plug in UPS capacity as you need it. Incrementally enabled systems provide the same end result, but are shipped with the extra capacity already installed, but disabled. It's activated via software or firmware when you're ready. These systems all grow differently, but the principles are the same -- add capacity and pay for it when you need it. Of course, there's an up-front premium for this flexibility, but it avoids the full initial capital expense while also saving energy, which probably means a good return on investment. Let's look at why.

A 1% efficiency loss on a 100 kW UPS is 1,000 W or 24 kilowatt-hours (kWh) every day of every year.

  • 1% x 100 kW = 8,760 kWh/year = $876 @ $0.10 and $1,226 @ $0.14 per kWh
  • 1% x 500 kW = 43,800 kWh/year = $4,380 @ $0.10 and $6,132 @ $0.14 per kWh
  • 1% x 1,000 kW = 87,600 kWh/year = $8,760 @ $0.10 and $12,264 @ $0.14 per kWh

A 5% efficiency loss is more dramatic, which is why right-sizing is even more important for redundant UPSes. Let's assume a 100 kW UPS and look at efficiency with N+1 and 2N redundancy at two different module or increment sizes.

With 50 kW modules, N+1 is actually a 150 kW system with 100 kW of usable capacity. (If any one of the three modules fails, the other two still maintain the system.) Eighty percent of 100 kW is 80 kW, which is only 53% of the actual 150 kW capacity. We've dropped less than 1% in efficiency -- not too bad, except that systems often run way below this level.

With 10 kW modules, N+1 is a 110 kW system, still with 100 kW of usable capacity. (If any one of the 11 modules fails, the other 10 still maintain the system.) Eighty percent of 100 kW is 80 kW, which is 73% of the actual 110 kW capacity. We're back in the maximum efficiency range, even at lower usage levels.

2N is just two 100 kW systems, regardless of module size or configuration, each running only half the 80 kW load. (If either system fails, the other picks up the total load.) Forty kilowatts is only 40% of design capacity, which is getting into the low-efficiency range, and below this the losses increase rapidly. This is why green design means carefully considering the level of redundancy we really need.

So to properly size a UPS, you must do the following:

  • Make realistic load estimates;
  • provide reasonable head room and short-term growth;
  • know both the kilowatt and kVA ratings;
  • buy with the capability of growing to long-term capacity but activate only what you need;
  • think carefully about the level of redundancy you really require;
  • check the efficiencies at the load level you will be running;
  • and look carefully at the various UPS options on the market today.

You may be surprised by the difference a careful choice can make.

About the author

What did you think of this feature? Write to's editors about your data center concerns at

This was first published in August 2009

Dig Deeper






Forgot Password?


Your password has been sent to: