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Capacity and Run Time
One of the most important features of a UPS is its capacity. Another is its run time. While these two concepts are related, they are not the same. It's important to understand the difference between them, as these numbers play a critical role in UPS sizing.
The run time that a UPS provides is the number of minutes that the UPS will be able to provide power from the battery for a given load level. Since driving more equipment requires more power to be taken from the battery, driving more load will result in shorter run time. A UPS with a larger battery, or with an add-on battery pack, will provide longer run time for a given load size. Run time is critically important because it tells you how long a power outage the UPS can handle before the battery runs out. Most UPS manufacturers will provide tables that show the typical run times for a model based on the loads it has to handle. Here's a sample table for an APC Back-UPS Pro 650 model:
In the chart I have highlighted the run time values for 100, 200 and 400 VA. Notice that when you double the load, the run time is reduced not by half, but by more than half. One key reason for this is that lead-acid batteries discharge more quickly when they are drawing more load. See the discussion of the UPS battery for a thorough explanation of battery capacity and the effect of discharge rate.
After looking at that chart, a natural question might be: "If it will run a 200 VA load for 22 minutes, and a 400 VA for 9.3 minutes, how long will it run an 800 VA load for? 4 minutes? 2 minutes?" Unfortunately, the answer to this is probably "0 minutes". This is where capacity enters into the picture. The UPS has a nominal maximum capacity, usually indicated by the number in its model name. In this example, it is 650 VA. The UPS will probably handle a load slightly above this figure--much the way your car doesn't stop dead when the fuel gauge hits "E"--but going to 800 VA will probably cause the UPS to overload and shutdown when it goes on battery. The reason for a maximum capacity figure is not just the batteries. It is also a function of the ability of the UPS's circuitry and even its wires to handle a particular size of load. It's also a safety issue, as exceeding the current limit of a circuit is unsafe.
While the UPS's capacity is rated in terms of its nominal load limit in, there is another limit that must be observed. To understand this other limit we must understand the difference between apparent power, which is the power provided as input to a device, and actual power, which is the power the device actually uses. For most complex devices, these are not the same. The ratio of actual power to apparent power of a device is its power factor. For PC power supplies it is often around 60%. (These concepts are explained in this Power Basics section.) The load on a UPS cannot exceed its apparent power load limit in VA, nor can the load exceed its actual power load limit in Watts. Many UPS manufacturers do not publish a specific wattage limit, but instead assume that the power factor of its loads will be about 60%. If you are driving a load that has a power factor of 60%, then the apparent and actual power limits are the same. But if the loads you are driving have a power factor higher than 60%, you cannot use the total VA limit because your watt usage will be over the wattage limit. If all this seems a bit confusing, you're probably right. I explain further, with examples, in the section on UPS sizing.
Many larger UPS models offer the ability to increase run time by adding on expansion battery packs. These expandable units are great, especially if you aren't sure of your exact load requirements when you purchase the unit. Expanding the UPS saves money over buying another since you are only paying for extra battery and not duplicating the UPS's other components.