Backup Power Series: Sizing a UPS

Welcome back to our series on everything backup power for your IT environment. If you’re seeing this series for the first time, hop on back to the first two posts: Why You Want A Well-Maintained UPS and Meet the Uninterruptible Power Supply (UPS).

Sizing a UPS is possibly the most important of all the power exercises. Unfortunately, it can be a mix of science and educated guesses. For starters, you must know how UPS sizes are even measured, and what the heck a VA is.

UPS models are commonly referred to simply by a VA number, 1500 VA (Volt-Amp) or 1.5 kVA (kilo Volt-Amp) but there are several numbers that matter: VA, wattage, and runtime. So how do these numbers interplay, and how do we use them for sizing?

VAs, Watts and Runtime

Volt-Amps are one of the simplest numbers you can imagine: the voltage of the device multiplied by the amperage draw of the device. In other words, if a device runs on 120 V and can pull up to 5 amps, it is 120*5 = 600 VA.

Wattage is similar, but more confusing. You may be reading this and thinking, “Wait, but I thought that was how you got wattage” and you’re not exactly wrong … some of the time. Volt-Amps are used for sizing circuits, UPS units and panels. Wattage is a measure of the actual usable draw of a device. Getting deeper into these definitions gets complicated, so suffice it to say that sometimes you get 200 W of energy efficiently, and sometimes you don’t. If it’s efficient, the VA and the watts are the same number. If it’s not, the VA may be much higher than the wattage.

If you want to dig deeper into how these calculations shake out (and much more, like all the different types of plugs and all kinds of fun electrical stuff), download our full whitepaper.

To that end, UPS units list both numbers in their output rating. For example, a lower-end 1500 VA UPS may have outputs of 900 W and 1.44 kVA. You can’t exceed either number. However, a higher-end UPS may be 8000 W and 8000 VA, again, not being able to exceed either number. Well, that’s annoyingly complicated. Sorry.

What about runtime? Runtime is almost directly proportional to the batteries in the UPS and the draw on the UPS. More batteries give you more runtime. Similarly, less draw also gives you more runtime. Therefore, more batteries and less draw together give you even more runtime.

How to Size Your New UPS

So how do we combine these three disparate numbers to get our actual required UPS size? If you’re lucky, you can simply log into a UI on your existing UPS unit(s) and see the VA, wattage and runtime of the current one(s). If you can see that, this is a great starting point for sizing your replacement, but don’t just check it once! Check it several times at various loads. Better yet, put it in your normal server monitoring program (like PRTG or SolarWinds) to get long-term, granular trending.

If your current UPS units don’t let you get that information, you can do some extrapolation and guesswork from the little “load” lights in the front. Another and better way is to log into the devices connected to the UPSes and see what they are showing their power draw to be (modern switches, iLO and iDRAC all have this information). If you have multiple UPS units and you want to consolidate into one or two larger ones, just add this information together.

You must also consider what your future is going to look like, knowing that most people replace UPS every five or six years. Make sure you add enough capacity in your UPS to accommodate future additions like new servers or switches.

Finally, look at how you are going to plug things into the UPS. What type and number of connections do you need? Do you need Power Distribution Units (PDUs)? What kind? How do those PDUs plug into the UPS? Do we have the right plugs on the right circuits on the UPS to connect the PDU?

If all this sounds extremely confusing … that’s normal.

Check out our next installment on using and maintaining your UPS.

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