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As discussed in the section on UPS capacity and run time (which you should read and understand before reading this page) a UPS has a limit on the loads it can power. In fact, it has two limits: a maximum apparent power load specification (in VA) and a maximum actual power load specification (in W). Sometimes the actual power load limit is not explicitly provided but must be computed. If the wattage is not listed specifically, it is safe to assume that it is 60% of the VA number. However, you should read the specifications carefully, because some UPSes use a different ratio.
The rule for sizing a UPS is simple in concept, and difficult in application: the loads you attach to the UPS cannot exceed either of its limits. Let's use a couple of examples to see how the two limits work: Let's say we have a 650 VA UPS. The wattage limit is not specified, so to be safe, we use 60%, which yields a limit of 390W. This means any load we run cannot exceed either 650 VA or 390 W. Some sample power supply loads:
Now that we understand what the limits are on the UPS side, let's take a look at the load side. To figure out what a load will demand of the UPS, we need to know both its VA and W ratings. Unfortunately, many manufacturers do not provide the information required to make a knowledgeable decision. An upper bound of the VA requirements of a unit can be obtained by looking for its current rating, which almost all devices have on a label or plaque near their power cord. However, these numbers are usually rather conservative. If a device has a 2A rating, then its nominal VA requirement is 2*115=230 VA. However, in practice, it may never actually require more than, say, 175 VA. There's no way to know for sure unless you know how to test the device electrically, so if you want to be absolutely certain, you must be conservative and use the maximum figure of 230.
PC power supplies introduce an added level of confusion. :^) When people talk about power supplies, they usually say something like "that's a 300 watt power supply". It's important to realize that the number "300" in that designation is the approximate, total output DC power of the power supply. That number is absolutely meaningless when it comes to sizing a UPS! It doesn't represent either the VA or W requirements of the supply. Most power supplies will have a current specification that you can use to calculate the VA maximum of the supply as in the previous paragraph, but if you use this you will probably over-purchase your UPS, because the output rating of the power supply is itself a maximum and only represents what will be required if the power supply is completely maxed-out. A 300 W power supply on a typical PC isn't actually putting out anywhere near that total. See here for more details.
Fine, fine, so now I have told you everything you shouldn't do to size a UPS. What are you actually supposed to do? Unfortunately, ideally sizing a UPS isn't easy to do. You often have to be conservative with your numbers and purchase a bit more UPS than you really need, to make sure you have your bases covered. Here are some tools and techniques you can use to help you with sizing:
Once you know your UPS has enough capacity to meet the requirements of your hardware, the next matter is one of run time: how much do you need? The most important goal of a UPS is to provide enough power to let you gracefully shut down your system; anything beyond that is "gravy", so to speak, that lets you keep working through an outage for a while. To get enough run time to actually work through anything but the shortest blackouts, you will usually have to pay substantially more money. To lengthen your run time, don't plug unnecessary gadgets into your UPS's backed-up receptacles. For example, most peripherals don't need to be plugged into a UPS. Most UPS manufacturers provide charts that show theoretical run-time for various sizes of load, such as the one shown on this page.
One item that you should consider having plugged into your UPS is your monitor. Having the monitor shut off suddenly in a blackout won't cause loss of data, for example, so it isn't mandatory to have it connected. However, if you have a blackout and your PC stays up but your monitor goes down, how are you going to see what you are doing when you try to save your data and shut down your applications? :^) Monitors, particularly those with large CRTs, can draw a substantial amount of power, so keep this in mind when doing your sizing.
Next: Power Conditioning